MITIGATING THE EFFECTS OF ARCS IN M.V. SWITCHGEAR

686
MITIGATING THE EFFECTS OF ARCS
IN M.V. SWITCHGEAR
WORKING GROUP
B3.37
APRIL 2017

Contributing members
D. Fulchiron, Convener FR G. Kachelrieβ, Secretary DE
A. Brandt DE L. Del Rio Etayo ES
J. Douchin FR T. Du Plessis ZA
A. Gardner US M. Grote DE
T. Hintzen DE J Kjønås NO
J. Meehan CA I. Ndiaye US
M. Palazzo CH S. Singh DE
P. Skryten NO Y. Tits BE
K. Tsuchiya JP
WG B3.37
Copyright © 2017
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responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and
MITIGATING THE EFFECTS OF
ARCS IN M.V. SWITCHGEAR
ISBN : 978-2-85873-389-7

MITIGATING THE EFFECTS OF ARCS IN M.V. SWITCHGEAR
3
EXECUTIVE SUMMARY
SCOPE
This Technical Brochure addresses functions and pieces of equipment which are specifically added in
order to mitigate the effects of internal arc fault events in medium voltage a.c. enclosed switchgear
(> 1 kV, ≤ 52 kV) . It includes functions and equipment beyond what is already covered by the
Internal Arc Classification (IAC) introduced in IEC 62271-200 "High voltage switchgear; a.c. metal-
enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV"
[1]1 or by the internal arc performance criteria in IEEE C37.20.7 " IEEE Guide for Testing Metal-
Enclosed Switchgear Rated Up to 38 kV for Internal Arcing Faults" [2].
Pieces of equipment which are part of the protection plan (over-current relays, fuses, fault-current
limiters, etc.) are not considered, even if some of them also contribute to limiting arc effects.
A review of existing commercially available solutions is provided in Chapter 2.
Users of this document are responsible for determining the appropriate safety, security,
environmental, and health practices or regulatory requirements and should rely upon their
professional judgment in the exercise of reasonable care in all circumstances, or seek the advice of a
competent and experienced professional.
BACKGROUND
Several systems in the market place incorporate internal arc fault limiting functions. These functions
generally operate by short-circuiting and hence diverting the arc fault using very fast detection system
and a fast fault-making device, most often to earth.
The international medium voltage switchgear assemblies standard IEC 62271-200 [1] (valid for air-
insulated and gas-insulated switchgear assemblies) and the IEEE guide for testing medium voltage
metal-enclosed switchgear for internal arcing faults C 37.20.7 [2] acknowledge such devices as
supplementary protective measures, but with little information. The standard IEC 62271-200 [1] also
states that, in general, arc limiting devices are out of its scope. They are not described nor specified
and no guidance is given for the related nameplate rating of the switchgear.
The IEC Subcommittee SC17C requested CIGRE to carry out a technical review to give
recommendations to support an extension of the current IEC standard to cover such a function or
piece of equipment and to provide assessment of the same.
Medium voltage switchgear incorporates many features designed to prevent arc faults and relative to
the number of units in service, failures are very rare. Limited sources for medium voltage metal-
enclosed arc fault data include:
A 1976 IEC world-wide study [no reference available] estimated a switchgear failure rate of 0,001 per
cell x year;
The survey launched by the working group for this Brochure reported an average event rate of
0,00013 per installation x year (the questionnaire was on experience over the last five years).
The two studies report significant difference in the magnitude of the rate of arcing fault events, but
are not actually comparable due to the fact that the survey bases are not the same (cells versus
installations) and the survey period was not similar (absolute versus last five years). Results of the last
survey are presented in Annex A.
IEEE Standard 493 "IEEE Recommended Practice for the Design of Reliable Industrial and Commercial
Power Systems" [3] contains failure rates for switchgear sub-assemblies. However, the reference
period is already rather old (for a publication in 1997) and it seems some progress has been achieved
which should call for a revision.
Arcing faults are not only a concern for metal-enclosed switchgear, and the standard
IEC 62271-200 [1] in its Table 102 "Locations, causes and examples of measures to decrease the
probability of internal arc faults" provides a list of possible causes for faults within switchgear, as well
1 Numbers between square brackets refer to Bibliography

4
as some guidance for reducing the probability of their occurrence. This Brochure focuses on the
consequences of faults, and on the advantages/drawbacks which come with the various arc mitigation
systems commercially available. It also considers how such systems could be described, specified and
tested.
Such systems shall be understood as acting in parallel with and in addition to a standard protection
system, basically based on over-current protection function(s), which is always supposed to trip an
upstream breaker clearing the arcing fault after a given duration, or to be able to do so.
This assumption is implicitly made when assigning a rated arc duration, for instance 1 s; otherwise,
the arc fault supplied by a network would not extinguish at all. The internal arc validation as per IEC
or IEEE assumes the timely reaction of a fault clearing system.

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CONTENTS
EXECUTIVE SUMMARY ............................................................................................................................... 3
SCOPE................................................................................................................................................................................................. 3
BACKGROUND................................................................................................................................................................................. 3
CONTENTS................................................................................................................................................... 5
DEFINITIONS................................................................................................................................................ 9
1. POSSIBLE EFFECTS.........................................................................................................................11
1.1 GENERAL ............................................................................................................................................................................11
1.2 EFFECTS CONSIDERED.....................................................................................................................................................11
1.2.1 Main phenomenon: the gas flow..........................................................................................................................11
1.2.2 Effects within the switchgear.................................................................................................................................14
1.2.3 Within the switching room......................................................................................................................................17
1.2.4 People safety: the internal arc classification approach .................................................................................18
1.2.5 For the distribution system.....................................................................................................................................18
2. ARC EFFECTS MITIGATION STRATEGIES...................................................................................19
2.1 GENERAL ............................................................................................................................................................................19
2.2 PASSIVE MITIGATION OF INTERNAL ARC AND ITS EFFECTS.................................................................................19
2.2.1 General .....................................................................................................................................................................19
2.2.2 Enclosure design to cope with overpressure (GIS design) ..............................................................................19
2.2.3 Bursting disc or pressure relief devices (GIS design).......................................................................................20
2.2.4 Enclosure design to relieve overpressure ...........................................................................................................20
2.2.5 Exhaust ducts (design and installation) ...............................................................................................................21
2.2.6 Cooling Systems for the hot gases (design).......................................................................................................23
2.2.7 Control of the electric arc (design)......................................................................................................................23
2.2.8 Single phase designs ..............................................................................................................................................23
2.2.9 Passive systems implemented at the building level (installation) ..................................................................24
2.3 ACTIVE ARC MITIGATION STRATEGIES.......................................................................................................................25
2.3.1 General .....................................................................................................................................................................25
2.3.2 Arc detection by overcurrent sensing..................................................................................................................25
2.3.3 Arc detection by light sensing...............................................................................................................................25
2.3.4 Arc detection by pressure sensing and mitigating effects..............................................................................26
2.3.5 Arc detection by sensing sound signals...............................................................................................................27
2.3.6 Arc detection by sensing mechanical deformation...........................................................................................29
2.3.7 Arc detection by temperature sensor..................................................................................................................29
2.3.8 Processing..................................................................................................................................................................29
2.3.9 Short-circuiting devices...........................................................................................................................................31
2.3.10 Acceleration of (existing) protection relays.......................................................................................................34
3. BENEFITS AND DRAWBACKS DUE TO ARC EFFECT MITIGATION SYSTEMS.....................35
3.1 GENERAL ............................................................................................................................................................................35
3.2 ACTIVE ARC MITIGATION SYSTEMS............................................................................................................................35
3.2.1 Benefits ......................................................................................................................................................................35
3.2.2 Limitations and drawbacks....................................................................................................................................36
3.3 PASSIVE ARC EFFECT MITIGATION SYSTEMS............................................................................................................37
3.3.1 Benefits ......................................................................................................................................................................37
3.3.2 Limitations and drawbacks....................................................................................................................................38

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4. POSITION ACCORDING TO AVAILABLE STANDARDS AND CUSTOMER'S
SPECIFICATIONS.......................................................................................................................................39
4.1 GENERAL ............................................................................................................................................................................39
4.2 IEC STANDARDS ...............................................................................................................................................................39
4.3 OTHER STANDARDS AND REGULATIONS..................................................................................................................41
4.3.1 Standards..................................................................................................................................................................41
4.3.2 Regulations................................................................................................................................................................42
4.4 MANUFACTURERS' DOCUMENTATION QUOTES (CLAIMED PERFORMANCES, DEMONSTRATION...).........42
4.4.1 Passive mitigation....................................................................................................................................................42
4.4.2 Dedicated relays.....................................................................................................................................................43
4.4.3 Fast acting short-circuiting systems ......................................................................................................................43
4.5 USERS' SPECIFICATIONS, AND OTHER USER'S REQUIREMENTS............................................................................43
5. PERFORMANCE ASSESSMENT ....................................................................................................47
5.1 PERFORMANCE INDICATORS AND RELATED TESTS.................................................................................................47
5.2 SYNTHESIS OF POSSIBLE VALIDATION APPROACHES ...........................................................................................51
6. GUIDANCE FOR USERS................................................................................................................53
6.1 GENERAL ............................................................................................................................................................................53
6.2 OVERVIEW OF CONCEPTS AND BENEFITS................................................................................................................54
7. CONCLUSION................................................................................................................................57
8. BIBLIOGRAPHY ..............................................................................................................................59
A.1. GENERAL ............................................................................................................................................................................61
A.2. COMMENTS FROM THE WG.........................................................................................................................................61
FIGURES AND ILLUSTRATIONS
Figure 1.1 – Exhaust of a gas duct during an internal arc event 13
Figure 1.2 – Arc power curve and pressure development during an internal arc 15
Figure 1.3 – Plastic deformation of switchgear after an internal arc test 15
Figure 1.4 – Typical burnthrough in an enclosure caused by an arc 16
Figure 1.5 – Cable compartment before and after an arc fault test 16
Figure 1.6 – Pollution within switchgear after arc fault test 17
Figure 1.7 – Damaged outdoor substation after an arc event 17
Figure 2.1 – Deformation of structure after an internal arc test in the SF6-filled switch 19
Figure 2.2 – Simulation of the distortion of metal enclosures due to pressure 20
Figure 2.3 – Bursting disc operating 20
Figure 2.4 – Enclosure design to relieve overpressure 21
Figure 2.5 – Oscillograms of vented Internal Arc Test (overpressure and currents) 21
Figure 2.6 – Exhaust ducts 22
Figure 2.7 – Isovalues of pressure (Pa) – Case of AIS switchgear, 55 ms after arc ignition (50 kA) 22

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Figure 2.8 – Example of installation of a duct 23
Figure 2.9 – Cooling system for escaping gases using a metal labyrinth 23
Figure 2.10 – Typical oscillogram of an arc quenching operation, triggered by optical method, showing
the very short detection duration 26
Figure 2.11 – Arc detection by Pressure method 26
Figure 2.12 – Air-insulated and gas-insulated switchgear with arc mitigation systems using pressure
methods 27
Figure 2.13 – Arc detection from light and sound signals 28
Figure 2.14 – Example of sensor placement in 2-high design switchgear, side view 28
Figure 2.15 – Example of light and pressure sensor technology 29
Figure 2.16 – Mechanical switch schematic 31
Figure 2.17 – Event sequence of an arc extinguisher (AE) 32
Figure 2.18 – Current limitation by an arc extinguisher (AE) 32
Figure 5.1 – Example for a “testbox”: 48
Figure 5.2 – Example of available test container 49
Figure A.1: Respondent’s country and quantity of respondents in each country 61
Figure A.2: Total quantity of MV switchgear reported by respondents 62
Figure A.3: Respondent’s country answered “Always” 63
Figure A.4: Respondent’s country answered “Site specific” 64
Figure A.5: Respondent’s country answered “Never” 64
Figure A.6: Respondent’s country answered “IEC Type A or IEEE” 65
Figure A.7: Respondent’s country answered “IEC Type B” 65
Figure A.8: Respondent’s country answered “Other” 66
Figure A.9: Respondent’s country answered “IEC F or IEEE type 1 (front access only)” 67
Figure A.10: Respondent’s country answered “IEC F or IEEE type 1 (front access only)” 67
Figure A.11: Respondent’s country answered “Other” 68
Figure A.12: Respondent’s country answered “Yes” 69
Tables
Table 2.1: Applicability of single phase tests according to IEC 62271-200 24
Table 5.1: Criteria for possible extension of validity of test results 50
Table 5.2: Synthesis of possible validation approaches 51
Table 6.1: Expected benefits with an non arc-resistant switchgear 54
Table 6.2: Expected benefits with an arc-resistant switchgear, 55
Table 6.3: Drawbacks and limitations of various categories of systems 56
Table A.1 Location of each respondent’s MV switchgear in each country 62
Table A.2 Percentage of each respondent’s installed arc-resistant MV switchgear in each country 63
Table A.3 Arc-resistance rating in each country 66
Table A.5 Summary of Internal Arc (IA) events 70

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Table A.6 Time for getting the affected installation back into service after IA event(s) 71
Table A.7 Shift the protection relay settings faster and reduce the PPE level 72
Table A.8 Safety practice change 73
Table A.9 Recognition and practical use of AAEMS 74
Table A.10 Sensor for AAEMS 74
Table A.11 Safety practice change 75

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DEFINITIONS
Medium voltage (m.v.)
any voltage above 1 kV and up to and including 52 kV
Arc fault
fault in an electrical system with an arc between conductive parts normally insulated one from
the other
Internal arc fault
arc fault occurring within an enclosed switchgear assembly
Internal arc fault detection system
set of elements (sensors, relays, wiring, etc.) providing an output signal in response to the
occurrence of an arc fault
Active arc mitigation system
system dedicated to react to internal arc fault conditions and decrease the arc energy using
some detection system (more than over-current)
NOTE: Examples are short-circuiting devices with dedicated tripping system (even mechanical) as an add-on device
or embedded in the switchgear, or acceleration of an existing protection scheme, and thereby also mitigating the arc
effects
Passive arc effect mitigation element or system
element or set of elements dedicated to control the arc effects and not requiring a ny dedicated
detection system, which cannot be switched to an inactive mode
Self tripping arc effect mitigation mechanical system
mechanical system which take its tripping energy from the arc fault itself
Stored energy
amount of energy stored and sufficient to complete a function under predetermined conditions
(provides independence from any external power supply, at the time of operation)
Arc quenching device
arc extinguishing device, transferring the fault current to metallic conductive parts by
establishing a short-circuit (either between phases or/and between phases and earth)
NOTE: This could be self-tripping or not
Auxiliary power supply
external power supply, either a.c. or d.c. that powers the device through dedicated terminals
separated from the measurement inputs of the device
NOTE: refer to the scope of IEC TC85
Auxiliary equipment
any equipment built into an auxiliary circuit (refer to IEC 62271-1 [18], subclause 3.5.4)

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1. POSSIBLE EFFECTS
1.1 GENERAL
High-current arc faults mostly develop from low-current arc faults. Basic arc fault causes are
maloperation, exploding fuses, foreign material, negligent cable assembly, voltage transformer
insulation faults, contamination and switching device failures in medium voltage applications.
Operating the switchgear beyond its designed ratings could also initiate an internal fault, e.g. transient
overvoltages beyond the rated insulation level, sustained overload or exceeding the rated temperature
range.
The occurrence of an internal arc fault in an electrical installation is very unlikely, but when it occurs it
may seriously damage the electrical equipment, the switchgear buildings and endanger personnel.
Answers to the survey made by the working group reported an average occurrence rate of 1,3 per
10 000 switchgear assembly x years. The physical results of an internal arc fault are significant,
showing the following major physical impacts:
– the energy released from an electrical arc heats the gas or the air within the switchgear enclosure,
resulting in a pressure rise;
– overpressure generated during the internal arc fault causes mechanical stress on the switchgear
enclosures and on switchgear room walls;
– the arc can burn on a surface of the metallic enclosure, melting and puncturing it (burn-through).
Hot gases may then stream out of the affected compartment;
– the resultant force of expelled gases following rupture may cause debris to be catapulted at high
speed away from the arc fault affected compartment;
– sound levels may exceed the human pain threshold resulting in permanent hearing damage;
– very bright light emissions might occasionally blind operators.
Such effects are also described in the CIGRE Brochure 602, published December 2014, chapter 1.1
[4].
The effects listed above may be mitigated by the design of the switchgear itself and/or the switching
room. More details are provided in subclauses below.
1.2 EFFECTS CONSIDERED
1.2.1 Main phenomenon: the gas flow
An arc burns in air with a temperature in the order of 10 000 K - 15 000 K, (15 000 K to 20 000 K in
SF6), and this temperature is reached in less than 1 ms. This means that within the arc volume and
around it, the gas rapidly heats and expands in the enclosed space. Furthermore at such a high
temperature, the conductors in contact with the arc roots are vaporized (ablation phenomenon: direct
transformation from solid state to vapour state). The arc balances its temperature, mainly losing its
power by radiation; thus the radiation power density is very high in the vicinity of the arc, and other
materials (such as epoxy, insulated material, steel) receiving this radiation, are also vaporized. As
radiation is transported at light speed, this process follows closely the arc power curve without time
delay.
Within an enclosed compartment, these two phenomena, the gas expansion and the gas production
by solid material ablation, result in the following consequences:
– immediately at the start of the event, a pressure rise in the faulty compartment. In arc-resistant
switchgear, an opening is designed in each compartment, so to relieve this pressure in
predetermined, designated areas;
– a flow of extremely hot, and possibly toxic, gases from the faulty compartment towards an
opening. For arc resistant switchgear this opening, as well as the downstream gas evacuation
system to the outside of the switchgear or outside the switching room, is included in the
switchgear design. It should be noted that such an arrangement is effective only if all doors are
closed and breakers racked-in; otherwise gases would be evacuated through these openings;

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– the hot gas flow starts in the faulty compartment, and may involve several other compartments,
ducts and exhaust systems, as well as the switching room.
The pressure wave expands at the speed of sound, whereas the gas speed depends on flow
conditions (pressure, temperature, composition, geometry of the exhaust system). As soon as the
exhaust flow is established by design or by rupture, the pressure drops quickly to zero. In standard Air
Insulated Switchgear, the pressure stage lasts typically 1-2 cycles (20-40 ms). In large Gas Insulated
Switchgear vessels, it can last up to 10 cycles.
The gas composition at the beginning of the event is mainly the one which was in the compartment
before the arc ignition, i.e. SF6 or air. When the arc lasts, most of this original gas is expelled out of
the compartment, and is replaced by the gases resulting from the vaporization of the solid materials.
These are essentially flammable gases, and it is observed, typically after 100-200 ms, that these
gases ignite and create flames of a yellow colour, that can be observed by the human eye, whereas
hot air is transparent. This combustion process is responsible for the dark dust that is found within a
switchgear after an internal arc event.
As long as the arc lasts, it continues to vaporize solid material, and feeds this flame production
process. It is noticeable that usually, when the combustion (flame) process starts, the pressure is
already back to zero within the switchgear, meaning that the flow has reached its maximum velocity
and temperature everywhere. Therefore the combustion process does not influence the pressure field.
The combustion process depends on the stochastic proportion of oxygen, and would be changed if the
compartment remained closed.
Figure 1.1 illustrates the exhaust of a gas duct during a 1 s internal arc event, at various times after
arc ignition, respectively: 40 ms, 135 ms, 260 ms, 560 ms, 880 ms, 1,1 s. Note that the last picture is
taken after the arc extinction.

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40 ms 135 ms
260 ms 560 ms
880 ms 1,1 s
Figure 1.1 – Exhaust of a gas duct during an internal arc event
Comments to Figure 1.1:
– in the first caption, only glowing particles are visible. The flow of hot air is transparent for human
eyes;
– flames arrive at 135 ms and expand outside the duct. They are very brilliant at the beginning;
– at 560 ms, dark smoke is visible in the exhaust;
– at 880 ms and later, the image is darker. There is a larger amount of dust within the flames;
– gases continue to burn even after the arc extinction.

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1.2.2 Effects within the switchgear
1.2.2.1 The arc initiation : the sound wave
The sudden expansion of the plasma volume generates a sound wave of high magnitude, typically
130-160 dB which can injure people.
1.2.2.2 Light, radiation
As described above, the arc emits a very high level of radiated power. The visible light is of high
intensity, and this can be used to detect the occurrence of an arc by dedicated sensors. This property
of an electrical arc is used in the lighting industry.
The level of radiated power is a clear danger for humans who may be directly exposed to it. This is
the subject of the Arc Flash study [22] and regulation [9] in the USA, which specify dedicated
protections to be worn by operators, according to the level of radiation they may be subjected to.
On arc resistant switchgear, when all doors and panels stay closed, an operator cannot be subjected
to direct arc radiation, as the arc stays always within the metallic enclosure.
1.2.2.3 Pressure effects
Within the switchgear itself, the first effect of an arc fault is the rapid rise in pressure in an enclosed
compartment.
The enclosure deforms and often overreaches the plasticity limit, leading to permanent deformation.
Bolted assemblies experience high concentration of stress in bolt locations. When a bolt fails the
corresponding forces are applied to the remaining nearby bolts, which also become overloaded and
collapse, leading to a rapid opening of the complete assembly, and thus an unexpected and large gas
leakage.
Therefore non-arc resistant switchgear may not withstand the structural stresses induced by the
pressure resulting from an internal arc. Covers and doors especially are weak points in the enclosure
assembly.
The following description of AIS and GIS technologies are valid for arc-resistant switchgear.
AIS technology: the resulting rise in pressure is a function of the arc voltage, current and the volume
of the switchgear compartment. The pressure peaks within the arcing compartment in 10–15 ms. In
other compartments, the peak is lower and appears later. Pressure tends to zero when approaching
switchgear vents. Flaps used to ensure proper partitioning between compartments are usually
designed to open fully in 10 to 20 ms when subjected to the pressure rise in the compartment;
GIS technology: if the arc takes place within an “AIS compartment” of a GIS switchgear, the
behaviour is similar to the AIS one. If it takes place within the vessel of a GIS, the behaviour is
different. In the vessel itself, the pressure rises until the bursting disk pressure is reached, while it
stays at zero elsewhere, as the disk is still closed. After the disk opening, the pressure starts to
decrease in the vessel, and to rise in the other compartments, the rise of pressure depending upon
the burst disk data: size and bursting pressure, as well as disk location.
Figure 1.2 shows a switchgear arrangement comprising 3 cells, the arc power curve, and pressure
development in the case of an internal arc in the cable box (AIS case) (left side) or in the epoxy
switch (closed compartment with valve – similar to GIS case) – right side.

15
1 - (in red): the cable compartment
2 - (in blue): the epoxy switch
3 - (in orange): The busbar compartment.
4 - (in grey): middle and left cells, and gas duct (cable trench, referred to as "tunnel" in the graphic).
White points: pressure sensor locations
Figure 1.2 – Arc power curve and pressure development during an internal arc
Figure 1.3 shows an AIS assembly, after a 20 kA - 1 s internal arc test (left, rear faces), and the
plastic deformation caused by the pressure on the enclosure.
Figure 1.3 – Plastic deformation of switchgear after an internal arc test
3
2
14

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1.2.2.4 Burnthrough
The arc, pushed by electromagnetic forces, may touch the enclosure steel wall which melts, creating a
burnthrough. Such burnthroughs would subject operators to direct arc radiation, in addition to the hot
gas flow it creates.
Figure 1.4 shows a typical burnthrough in an enclosure caused by an arc.
Figure 1.4 – Typical burnthrough in an enclosure caused by an arc
1.2.2.5 Material vaporization and other thermal effects
A significant part of the bus bars and surrounding materials is vaporized by the arc. This process is
used in arc furnaces to melt metal bars.
The high temperature reached in the arc region may also initiate a fire.
Figure 1.5 shows a cable compartment of an AIS switchgear, before and after a 12,5 kA - 0,5 s arc
fault test. It can be seen that the insulated parts on the sides have partly melted and deformed.
Figure 1.5 – Cable compartment before and after an arc fault test
1.2.2.6 Switchgear pollution
The compartments of the switchgear, which are involved in the gas flow path to the exit, are polluted
by the gases themselves, and dust generated by gas combustion.

17
Figure 1.6 shows the pollution after a 20 kA - 1 s fault within an AIS switchgear. The arc burned in
the right end cubicle, whose doors can be seen in the picture, highly polluted. The adjacent cubicles
are also polluted.
Figure 1.6 – Pollution within switchgear after arc fault test
1.2.3 Within the switching room
In cases where the room is involved in the gas flow (gases relieved into the room, or accidental
leakage in the enclosure), the room is subjected to the emission of hot, and possibly toxic, gases. The
possible effects within the room are detailed in the following subclauses.
1.2.3.1 Pressure effects.
Overpressure in the room requires dedicated analysis and specification of the civil work to ensure
proper pressure withstand. Figure 1.7 shows a damaged outdoor substation after an arc event. Walls
have collapsed due to the internal pressure experienced.
Figure 1.7 – Damaged outdoor substation after an arc event
1.2.3.2 Contamination effects
Contamination of the switchgear room or other equipment with gases, metallic vapours and particles
will be caused by an internal arc. This contamination may lead to significant reduction of the
remaining lifetime of installed equipment. As an example, ozone – produced by an arc in air – is
specially oxidizing the surfaces of insulating materials, leading to higher partial discharges. Oxidation
effects on the metallic switchgear parts (especially the live parts) and subsequent change of the part’s
Source:
TÜV Rheinland /
Berlin-Brandenburg
Schutzseminar 2002

18
surfaces, may lead to a deformation of the electrical field. This also influences the dielectric
properties. In addition oxidation ageing mechanisms for contacts should be monitored as described in
the IEC/TR 60943, subclause 3.3. [5]
Auxiliary or control equipment located in the room may be damaged by such oxidation, with effects
appearing only months later.
1.2.3.3 Toxicity Effects
The possible presence of toxic components require a period of ventilation after a fault before the
switchgear room may be entered, in a similar way to any fire event. When SF6 has been involved,
some by-products due to recombination with surrounding materials may be present ; toxicity of such
by-products is documented in CIGRE Brochure 234 [23] and in IEC 622271-4 [24].
1.2.3.4 Thermal effects
Gases expelled out of the switchgear are of high temperature and may ignite a fire within the room.
The hot air which is expelled first is at high temperature (several kK) but of very low density, and with
a low Cp (thermal capacity) so it transports little energy.
However, the flames resulting from gas combustion radiate about 1 000 times more than hot air
(measured by infra-red camera). Therefore the combustion phenomenon is the main cause of ignition
of material outside the switchgear. As it typically starts after 100 ms, shortening the arc duration
below this value would probably mitigate this risk.
1.2.4 People safety: the internal arc classification approach
Internal arc classification as defined in the IEC or IEEE standard, is mainly intended to prove that in
defined conditions of service, at the instant of occurrence of an internal arc:
– the enclosure remains tight and there is no significant hot gas leakage in any direction where an
operator can be present, and that there is no burnthrough of the enclosure;
– there are no moving or projected parts that could be a serious risk for an operator in the vicinity of
the switchgear.
The tightness of the enclosure results from its structural withstand to the pressure rise, and to
dedicated design strategies ensuring gas tightness of sheet metal assemblies. However, a bolted
assembly cannot be 100 % gas tight, and therefore one objective of the type test is to prove that the
level of tightness achieved is high enough for the severity of the fault targeted.
The intention is to check that an operator near the switchgear would not be hit by a hot gas jet from
the switchgear enclosure, nor an ejected solid part. The internal arc classification focuses on these
possible effects, the others being disregarded, especially those within the switching room.
1.2.5 For the distribution system
The consequences of a fault are also on the application, on the network upstream and downstream of
the switchgear, and may be considered by:
– the number of feeders impacted (generally several when an arcing fault occurs within a
switchboard);
– the electrical power system’s sensibility to service outage, caused by an internal arc and the
maximum outage time designated or allowed.
There are applications – mostly in industrial process industry and public safety areas – where
service outage times are considered as being especially critical. But these applications may require
back-up systems to further limit any service or failure outage time;
– the service continuity as a whole.

19
2. ARC EFFECTS MITIGATION STRATEGIES
2.1 GENERAL
Mitigation may apply either on the arc itself, or on the effects. Most solutions actually mitigate both,
as the goal is always to reduce the effects. Active systems mostly act on the arc itself by reducing its
duration, whereas passive systems generally act on the effects.
In this Brochure, the reference situation is considered to be a basic switchgear with no feature clearly
dedicated to any form of arc control.
2.2 PASSIVE MITIGATION OF INTERNAL ARC AND ITS EFFECTS
2.2.1 General
These are systems that naturally or by design prevent the effects of either hot gases or pressure
waves without any mechanism or intelligent system intervening. They do not use any external power
supply or stored energy.
Most of the mitigation systems which can be classified as passive are already part of the IAC
classification of the assembly (flaps, ducts, absorbers, or combinations of them), but some could be
added to an already classified design for further mitigation of the arc effects.
Installation instructions, including room arrangements, shall be considered. Special precautions are to
be taken when using exhaust plenums for the final exhaust of these ducts (which area, which
access...). Local regulations could help as they sometimes deal with rules about exhausting smoke or
hot gases.
Depending upon the implemented solution, maintenance of the mitigation system could be required
and users should refer to the manufacturer's instructions. Some basic features of the switchboard
such as door gaskets, latches, etc. will contribute to the behaviour of the switchgear in the case of
internal arc. These should be subjected to proper periodic inspection or maintenance, as required.
2.2.2 Enclosure design to cope with overpressure (GIS design)
When faced with increased pressure in its interior the equipment must be able to withstand it,
deforming without interfering with surrounding walls or leading to individual injuries and allowing the
gases to flow out of it in a controlled manner avoiding harm to anyone. For gas-tight distribution
equipment usually filled at less than 0,05 MPa (relative), the gas pressure inside the equipment can
typically increase to 0,2 MPa (relative) within 30 ms before any opening (designed or not) occurs.
Network protection systems (with the exception of current-limiting devices) are generally not capable
of eliminating the fault in such a short time; therefore the enclosure must be able to withstand it.
Figure 2.1 – Deformation of structure after an internal arc test in the SF6-filled switch

20
There are calculation methods, such as finite element method, which help improve the design, thus
reducing the number of internal arc tests, and even allowing the verification of design modifications
by simulation.
Figure 2.2 – Simulation of the distortion of metal enclosures due to pressure
2.2.3 Bursting disc or pressure relief devices (GIS design)
These are elements that break or are released at the time the pressure increases due to an internal
arc, allowing the hot gases to escape in a controlled way. These gases are typically released and
directed away from areas that can be accessed during operation (e.g. by means of exhaust ducts).
Figure 2.3 – Bursting disc operating
2.2.4 Enclosure design to relieve overpressure
One technique to mitigate the effects of overpressure is to design the switchgear so that the exterior
covers are heavily reinforced and remain intact during an arcing fault. The overpressure is relieved by
the operation of flaps which open to vent the hot gases (see Fig 2.4). Such a design is commonly
called "arc-resistant".
Such designs are type-tested to meet relevant acceptance criteria, as expressed in standards. For the
overpressure concern, these criteria are:
– Correctly secured doors and covers do not open. Deformations are accepted, provided that no part
comes as far as the position of the cotton indicators or the walls (whichever is the closest) on
every side
– No fragmentation of the enclosure occurs.
– No ejection of fragments or of other parts of the switchgear of an individual mass of 60 g or more
occur

21
– Cotton indicators do not ignite due to the effect of hot gases or burning liquids.
Figure 2.4 – Enclosure design to relieve overpressure
Fig 2.5 illustrates the rapid reduction of overpressure during a three-phase fault in air-insulated arc-
resistant switchgear
Figure 2.5 – Oscillograms of vented Internal Arc Test (overpressure and currents)
2.2.5 Exhaust ducts (design and installation)
The arc-resistant switchgear may have a plenum or duct which evacuates the gases to controlled
areas for reducing exposure to gases for individuals in the vicinity of the switchgear. The preferred
implementation of ducts is by using them to drive gases outside the switching room, either outside
the building or at least in another volume where nobody has access during normal operation. The
duct may be constructed using an actual conduit or by placing the equipment within a room
maintaining some defined distances, for example, the space between a metal enclosed switchgear
device and the back wall.

22
Figure 2.6 – Exhaust ducts
Simulation methods such as Computational Fluid Dynamics (CFD) can be used for the calculation of
the pressure distribution and variation of hot gases discharged from MV switchgear experiencing an
internal arc. CFD is typically used in:
– assessing the actual geometry of the switchgear and installation room (simulating actual electrical
installations when they differ from the manufacturer’s requirements or from the test conditions).
– analyzing the influence of the location of pressure relief openings in rooms.
– analyzing the influence of specific flap designs, or the influence of grids and absorbers.
Figure 2.7 – Isovalues of pressure (Pa) – Case of AIS switchgear,
55 ms after arc ignition (50 kA)

23
Figure 2.8 – Example of installation of a duct
2.2.6 Cooling Systems for the hot gases (design)
These involve ducts or elements which cool the discharged gases. One example of this is provision of
a path for the gases to escape having contact with a larger surface area so that they transfer part of
their calorific energy. These systems could include metallic expanded sheets, or refractory or porous
material, so that this absorbs the excess heat as the gases exit.
Figure 2.9 – Cooling system for escaping gases using a metal labyrinth
2.2.7 Control of the electric arc (design)
The electric arc inside the equipment can be controlled in such a way that it causes the least damage
possible. The energy transmitted by the arc is proportional to the current which circulates through it
and to the term known as arc voltage. This voltage depends on the dielectric, but is almost
proportional to the length of the arc. If the equipment is designed for this arc length always to be the
same and to be small, it will be possible to control the energy generated in it and thus minimize the
damage. Hence, the equipment may have sacrificial elements inside for the case of an internal arc.
Such features are usually not documented, being part of the design know-how of the manufacturers.
They affect the performance of the switchgear under arc fault conditions and are useful to achieve
internal arc classification for instance.
2.2.8 Single phase designs
Metal-enclosed designs with independent phase compartments cannot have any three-phase fault.
Internal faults can only be phase-to-earth in such designs.

24
The phase-to-earth fault current depends on the neutral impedance value of the network. Some
distribution networks are grounded via impedance modules such as high/low impedance resistors or
reactors. In such a case, arc energy of a single earth fault is smaller than that of a direct grounding
system or a phase-to-phase short circuit fault.
According to IEC 62271-200 [1], single phase tests are applicable to single phase compartments and
solid insulation technologies – refer to table below. The standard defines a rating for single phase-to-
earth arc fault current: IAe. This rating may be lower than the three phase arc fault current: IA.
Rationale for this difference is related to the actual network earthing system and is detailed §8.104.6
in the same standard.
In most MV systems in which the neutral is connected to earth through an impedance, IAe is typically
lower than 2 kA, so an order of magnitude is saved compared to IA.
One can note that the probability for single phase designs that a second fault to earth occurs in the
network following the first one in the switchgear is not zero, leading to a double earth fault, probably
at two different locations. If such a probability has to be covered, typically when operating the
network under sustained fault conditions (isolated or tuned systems), then the IAe shall be at least
0,87 IA as shown in the table.
Table 2.1: Applicability of single phase tests according to IEC 62271-200
Test
current
Number of phases/earth
for arc initiation
Action if other phase
affected
Three phase
compartments,
other than
connection
compartments:
with bare conductors IA Three N/A
conductors with site-made solid
insulation
IA Three N/A
conductors with non site-made
solid insulation
87 % IA Two
Repeat as 3 phase test
IAe One phase and earth
Single phase
compartments:
IAe One phase and earth. Repeat as 3 phase test
Connection
compartments:
Connections uninsulated or fitted
with site-made solid insulation
IA Three N/A
Connections using outer cone
plugs (screened or unscreened)
87 % IA Two
Connections using inner cone
plugs
87 % IA Two
One result of such a single phase test shall be not to ignite any other remaining phases. The type test
thus proves that arc fault current is limited to IAe by design.
2.2.9 Passive systems implemented at the building level (installation)
The standard IEC 61936-1 " Power installations exceeding 1 kV a.c. – Part 1: Common rules" [6] in
subclauses 7.5.2 and 7.5.3, mentions possible requirements for buildings regarding pressure rise due
to an internal arc.
CIGRE Brochure 602 [4] proposes information, methodology and simulation tools for possible
calculation and simulations of the pressure inside a room in case of a switchgear internal arc, as well
as relevant design rules.
Building design is influenced by the choices made about arc withstand of the switchgear, and possible
arc effects mitigation systems implemented.

25
2.3 ACTIVE ARC MITIGATION STRATEGIES
2.3.1 General
These are systems designed to limit the arc incident energy in the legitimate hope that this will reduce
its effects. Typical active mitigation strategies involve either diverting the arc from its original location
(earthing switch) or accelerating the protection system (with use of different sensors other than
traditional CTs to trip the main breaker).
Acceleration of the protection system will result in reducing the arc duration (and potentially the arc
current magnitude) and thus its incident energy. When chosen to activate an arc quenching device
(e.g. an earthing switch) the reduction in the incident energy can be more significant if the device acts
much faster than the circuit breaker can extinguish the current.
2.3.2 Arc detection by overcurrent sensing
Switchgear’s standard current sensors provide actual current information. Normal overcurrent or bus
differential protection systems can be used to clear arc faults. The time required for the detection and
interrupting of an arcing fault using modern relaying and state-of-the-art circuit-breakers is a
minimum of 50 ms, allowing for relay trip contact-closure time, plus circuit-breaker break time
(opening time + arcing time). In many cases, the operating time may be greater than 50 ms,
depending on the type of relay and the circuit-breaker technology.
As can be seen from Figure 1.2, this is too slow to ensure arc fault clearing before the pressure peak
is reached and, if forces are sufficient to blow covers or doors off (in the case of non arc-resistant
switchgear), this will happen within this time frame. Hot gases will be emitted into the switchgear
room, possibly towards an operator, if present.
Other technologies to reduce arc detection and response time are described below.
2.3.3 Arc detection by light sensing
To decrease the arcing time, one or more of the following is required: reducing arc sensing time,
reducing time for operation of protection logic, and reducing operating time for operation of the
current interrupting device. Arc sensing time can be reduced by use of optical sensors such as spot-
sensors (also known as point-sensors) or line-sensors (also known as loop-sensors) which detect
abnormal light within the switchgear assembly. They typically monitor locations within the switchgear
assembly where an arc fault can develop and their locations are normally based on specific and
detailed guidelines from the switchgear manufacturer.
The optical sensors' signals are collected by an evaluation unit which processes a trigger signal, if the
sensor input exceeds a defined threshold level. The signal generation by the optical or electrical
method is based on simple operation of an operational amplifier. The detection signal is compared
against a threshold signal.
In order to avoid unintentional tripping caused by light sources other than an electrical arc, optical
sensors are typically combined with additional current information. The evaluation unit can send a trip
signal to the arc quenching device.

26
Figure 2.10 – Typical oscillogram of an arc quenching operation, triggered by optical method, showing the very
short detection duration
2.3.4 Arc detection by pressure sensing and mitigating effects
In tight enclosures, such as a GIS compartment, integrated pressure sensors can be used to detect a
pressure increase caused by an arc fault. The principle may also be applied within air insulated
switchgear, using the pressure wave to trip the system. The pressure rise is due to expansion of gas
inside the switchgear compartment due to thermal heating by the internal arc. Pressure sensors
detect this change of pressure and give a signal to a control unit, or directly to a switching device
through a mechanical linkage, above a defined pressure threshold.
Figure 2.11 – Arc detection by Pressure method
The green curve in Figure 2.11 represents the fault current over time. The blue curve shows the
pressure as seen by the sensor. The steep rise in pressure, when the blast reaches the sensor, is used
to trigger the pressure sensor at 555 hPa relative pressure. The pressure sensor gives a continuous
signal (red line) whenever the pressure is higher than the threshold.
The pressure sensor may be integrated in the wall of the gas tank of gas-insulated switchgear or in
the metal encapsulated wall of the air-insulated switchgear and detects a pressure increase caused by
an arc fault within the entire switchgear. In the case of an arc fault the pre-loaded short-circuiting

27
devices, installed in the incoming feeder cubicles and interconnected to the sensors, are switched on.
By activating such a device, the arc fault is transformed into a galvanic short circuit and the arc fault
is quenched.
For gas-insulated switchgear, the pressure increase may remain below the opening pressure of the
bursting discs and the gas tank will remain sealed. Thus, there are no pressure effects onto walls or
floors surrounding the switchgear.
For air-insulated compartments the pressure sensor may be installed in the roof or in the rear wall of
each cubicle, and work onto a common tripping shaft/linkage to operate an arc-quenching device for
the switchgear.
Figure 2.12 – Air-insulated and gas-insulated switchgear with arc mitigation systems using pressure methods
2.3.5 Arc detection by sensing sound signals
Products using the sound signature of an arc to detect its presence can be found in the market but as
previously mentioned they also utilize others properties of an arc in their detection logic. When using
light as the other attribute of the arc event, the intensity of both the light and the pressure wave
producing the sound must be above certain thresholds. Moreover the difference in the speed of the
light and the sound signals must be characteristic of an arc. Indeed an arc generates a unique time
delay signature that differentiates it from other sources of light and sound.
As described in Figure 2.13 below, the logic will detect the light signal above the threshold value, and
then waits for the pressurized sound signal which should be received within a design specific wait-
time (usually around one millisecond) if an arc is taking place in the switchgear. If the sound signal is
received with higher than the threshold value as well as within the design specific wait-time, a trip
signal is issued. When optimized, this system can detect the arc event in as quickly as one
millisecond.

28
Figure 2.13 – Arc detection from light and sound signals
For a higher level of reliability and redundancy multiple sensors must be located within the switchgear
(up to five depending on the switchgear design). A maximum coverage distance of 1 metre from the
possible arc incident location is recommended. Figure 2.14 provides illustration of possible locations.
Figure 2.14 – Example of sensor placement in 2-high design switchgear,
side view
To effectively detect the arc, fast and reliable light and sound sensors are required; the example
illustrated in Figure 2.15 is using sensors made of LEDs, bare fibre and a membrane.

29
Figure 2.15 – Example of light and pressure sensor technology
The light fibre picks up the flash of an arc event from the bare fibre and transmits the light signal to
the logic. On a second fibre a LED emits light. This light is transported through the fibre and is
reflected back by the diaphragm, and collected by the same fibre back to the main unit. During an arc
flash event, the diaphragm vibrates due to the pressurized sound wave creating a signature (sound
signal) which is recognized by the logic. The unique combination of the light and sound signals is used
to detect the arc and generate a trip.
The trip signal can be then used to either open the main breaker or trigger an arc quenching device
when available.
2.3.6 Arc detection by sensing mechanical deformation
The over-pressure due to an arc, even in a non-tight compartment, leads to some deformations of
dedicated parts which could be used as information to possibly trip a system. Some realisations are
using the movement to directly trip a mechanical short-circuiting device while some others use the
deformation to activate a contact used as signal by some control system.
2.3.7 Arc detection by temperature sensor
Although temperature is an effect of arcing faults, all known temperature sensors have a response
time much longer than acceptable in arc protection. Thus there is no application known to date of any
temperature sensor in arc effects mitigation systems.
2.3.8 Processing
Generally processing can be described as getting information as input to the arc mitigation system
and finally operating an arc quenching device which then provides the arc mitigation effect. Signal
processing can be realized using digital or analogic electrical or mechanical technology based on
requirements.
This Chapter relates to different arc detection principles which deliver a sensing signal. These sensing
signals can exceed defined threshold levels in order to be recognized as indication for an internal arc
occurrence. Evaluation and processing of the sensing signals can be based on:
– r.m.s. values: this measurement should cover at least two half cycles of the signal (recognized
method to calculate an r.m.s. value). For a current signal this results in a total processing time
(from arc occurrence until arc extinguishing) greater than 20 ms at 50 Hz (16 ms at 60 Hz);
– instantaneous values: this delivers the advantage of an immediate detection if a threshold level is
exceeded, but it is sensitive to signal distortions. AND-gating with others signals (e.g. light) or
filtering could avoid false tripping in such cases. Using instantaneous values of sensing signals a

30
typical signal processing time (from arc occurrence until generation of the trigger signal) of below
2 ms can be reached;
– sensing signal’s rates of rise: examples for this are dI/dt, dU/dt, dIv/dt. Depending on the chosen
time interval (dt) this deduction method could deliver the same advantages and drawbacks as
monitoring an instantaneous value (valid for short time intervals) or as calculating an r.m.s. value
(valid for long time intervals).
For a complete arc mitigation system the following components are required:
– an arc detection (sensing) device;
– an evaluation device that also creates a trigger signal and so “decides” on arc versus non arc;
– a unit creating an arc quenching device signal (depending on the type of arc quenching device);
– an acting device (the arc quenching device itself).
Separate components supply flexibility in installation and selection of these devices, perhaps from
different manufacturers. On the other hand separate units need interfaces which must be aligned and
might affect the overall system’s performance due to a potential increase of signal run time and
influence on failure rates.
Components as highly integrated devices are conceivable as:
– arc detection sensors incorporating signal evaluation and the trigger signal creation
and/or
– arc quenching devices using a defined, standardized input signal, incorporating the arc quenching
device signal treatment.
Market available complete arc mitigation systems that use light plus current sensing (these are the
most available state-of-the-art non-mechanical sensing solutions) are available as:
– a three components arrangement, combining evaluation (incl. trigger signal) and arc quenching
device signal creation in one device
or
– four components arrangements which split the evaluation (incl. trigger signal) unit and the arc
quenching device signal creation into two separate devices in order to adapt the arc quenching
device signal to the arc quenching device used.
Example of market available mechanical signal processing:
– An overpressure caused by the internal arc results in a mechanical movement of dedicated parts
within the switchgear especially designed to react on pressure. This movement is directly and
mechanically linked to the latch of a mechanical pre-loaded arc quenching device and trips it (e.g.
tripping an earthing switch, also refer to figure 2.12). These kinds of mechanical systems typically
show total processing time (from arc occurrence until arc extinguishing) of 30 ms to 100 ms. This
total processing time relates to the mechanical movement until the threshold level is reached and
the switching time for the pre-loaded arc quenching device. Details of the signal processing time
until the trigger signal is supplied are not specified/known.
While the actuation – at least for one shot devices – cannot be tested, the signal processing can be
routinely tested by applying sensing signals directly (e.g. light) or applying otherwise produced signals
(e.g. secondary CT output) to the evaluation device. The properly produced arc quenching device
signal can now be measured.

31
For the routine testing of a mechanical signal processing a mechanical movement might be applied
that leads to an unlatching of the pre-loaded arc quenching device. This routine test can only be
conducted if the later proper function of the mechanical functional chain is ensured.
2.3.9 Short-circuiting devices
2.3.9.1 General
It appears that speed of the global system is a fundamental parameter if the goal is to avoid the
pressure peak. Using very fast detection principles and devices would become useless, in that regard,
if the associated actuator needs several tens of milliseconds to operate. That means tripping a circuit-
breaker, no matter how fast, will not provide the same performance as use of a fast short-circuiting
device as an actuator. Such a device could be mechanical, and acting as an earthing switch or only as
a short-circuit between phases.
The mechanical switch in most cases is composed of a pre-compressed spring assisted by an
electromagnet as shown below.
Figure 2.16 – Mechanical switch schematic
The triggering signal received from the arc detection control unit energizes the coil and the
mechanical switch short circuits the three phases which may or may not be grounded. In some cases
where the operating time of arc short-circuiting device is not critical, an existing earthing switch inside
the switchgear can be used.
Typically the sensor then triggers the short-circuiting device associated with the incoming feeder
within milliseconds thereby transforming the arc fault into a bolted fault which is cleared by the
upstream circuit-breaker.
The association of a fast detection principle, as light detection, and a fast acting device could provide
efficient peak arc current limitation as illustrated below.

32
Figure 2.17 – Event sequence of an arc extinguisher (AE)
Figure 2.18 – Current limitation by an arc extinguisher (AE)
In Figure 2.18, the arc is initiated at T1. The arc elimination sequence is started and at T2 the short-
circuiting device closes and the arc is quenched at T3. The energy released is 40 kJ for phase one in
comparison to several MJ for usual fault duration (cleared by a circuit-breaker with a standard
protection plan).
The key advantage of this approach is the minimization of damage to the switchgear due to the rapid
extinguishing of the arc fault. A key disadvantage may be the risk of inadvertent operation of the high
speed switch.
Attention is drawn to the fact that the device must be connected to the faulty conductors. As an
example, an open incoming unit may have a fault on the cables which will not be cleared by a short-

33
circuiting device connected somewhere else in the switchboard. Various single line diagrams may
create situations in which several short-circuiting devices will be needed to cover all situations.
2.3.9.2 Self-tripped short-circuiting device
Several commercially available Ring Main Units (RMU) can be fitted, during manufacturing, with a
dedicated short-circuiting device within the gas tank, including a pre-charged spring, and an over-
pressure detector which can trip the short-circuiting device in case of fault within the tank. In such
designs the system is not resettable and any operation leads to the need to replace the RMU.
Other implementations are also available with a pre-charged earthing switch, usable as a normal
earthing switch, with making capacity, but are also fitted with a tripping system operating in case of
over-pressure in some compartment of the assembly.
2.3.9.3 Existing earthing switch or dedicated earthing device
An earthing switch with fault making capacity can be fitted with a stored energy mechanism to be
used for such a short-circuiting function. The closing order for such an earthing switch may be
provided either by some mechanical sensor, or by an electronic tripping system.
Some other short-circuiting devices are only used for arc control function. They are usually provided
with a dedicated detection and tripping system and are designed to act much faster than earthing
switches. When the making operation is faster than 5 ms, the peak value of the arc current is
reduced, compared with a longer closing time. Various technological solutions do exist, some
resettable, and others using replaceable components. According to manufacturers, the short-circuiting
device may be available as a separate product or only provided with the full system (meaning with
detection and relay).
2.3.9.4 Short-circuit between phases or to earth
The generally available solutions establish a short-circuit between the three phases and ground (like
earthing switches). However, it is also possible to extinguish the arc by only making a short-circuit
between phases, and such devices have been proven to be efficient on four wire systems. Their
applicability on any neutral management system is undocumented to date.
A short-circuit between phases, or between phases and earth, would divert the current from the
arcing channel to the newly established short-circuit which would extinguish the arc. The overall
process is basically transferring the arc from an unintended location (internal arc in the switchgear) to
a dedicated location for arc extinction (the arc chamber of the upstream circuit breaker). However to
be effective as a solution the process should be very fast (including the detection delay) in order to
avoid the pressure peak and to minimize the energy delivered by the arc
The calculation of the short current is necessary to determine the required capability of the short-
circuiting device and the impact of such a solution on the overall performance of the protection
system. Analysis can be derived from industry standards such as IEC (e.g. IEC 60909-0 "Short-circuit
currents in three-phase ac systems – Part 0: Calculations of currents" [7]) or IEEE (e.g. IEEE Std 551-
Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power
Systems [8]) or any other relevant document. It is however important to highlight that in this matter
the peak short-circuit current (referred also to as peak withstand current or close and latch current)
will be of great importance as it will relate to the duty that the short-circuiting device will face when
closing against the arc fault current, especially when arc extinction is required within a cycle (< 20 ms
at 50 Hz) after its initiation.
According to IEC 60909-0 [7] the following equation can be used for estimating the peak short-circuit
current, knowing the X/R ratio viewed from the fault point. This expression provides a conservative
rather close approximation of the peak current values for the situation where the circuit X/R ratios
viewed from the fault location are greater than three which is largely representative for most
distribution circuits. 𝐼𝐴𝐶,𝑝𝑒𝑎𝑘 is the peak value of the symmetrical (transient) short circuit current. It is
important to consider all sources contributing to the subtransient and transient short-circuit current
including the generators and large, synchronous and, induction motors. Readers should refer to any of
the relevant IEC or IEEE standards for detailed calculations procedures.

34
𝐼𝑆𝐶−𝑑𝑢𝑡𝑦 = 𝐼𝐴𝐶,𝑝𝑒𝑎𝑘 (1.02 + 0.98𝑒
−
3
(𝑋 𝑅⁄ ))
It is expected that the upstream circuit breakers will have the required capacity to withstand and
extinguish the established short-circuit current. For the short-circuiting device as long as it involves
the three phases (balanced short-circuit), it can be expected to divert and extinguish the internal arc
with no influence on the grounding scheme. However for situation where a single phase arcing is
possible special attention must be given to the system grounding. Indeed, for an ungrounded or high
impedance grounded system, a single phase arcing fault can occur inside the switchgear without
driving a significant current until a second line becomes involved (double-line fault). The risk is limited
that such an arcing current causes an explosion. but it may damage important components or
compromise personnel safety. Therefore the sensitivity of the arc detector should be planned
accordingly.
2.3.10 Acceleration of (existing) protection relays
Acceleration of existing protection can be achieved either by detection systems providing tripping
information to an existing protection relay in order to by-pass any other protection function and to
achieve instantaneous tripping, or by manual selection. The benefit is a shorter fault duration, leading
to less risk for operators and reduced damage to the equipment in case of arcing fault.
Examples of detection systems:
– flaps contacts activated when flaps open;
– light sensor information processed by the protection relay, or by a dedicated one;
– overpressure contact on GIS
These solutions may be used to trip the incoming breaker, but the efficiency could be questionable
due to a risk of ignition of fault on incoming connection, or to trip some upstream breaker, if such a
control scheme is possible.
Example of application of manual selection:
– using a special set of protection parameters, with shorter or no time delays, for when people
access the switching room (to be implemented in the digital relays and switched "normal/special"
before access);

35
3. BENEFITS AND DRAWBACKS DUE TO ARC EFFECT
MITIGATION SYSTEMS
3.1 GENERAL
The use of switchgear equipped with an internal arc effect mitigation system is mainly a choice of the
customer. In certain cases, it can be required based on internal building pressure limits and/or
avoidance of pollution within the switching room caused by an arc fault.
Another case is the need for upgrading an existing switchgear installation regarding its internal arc
behaviour, for reduction of the arc effects.
It should be noted that IAC (internal arc classification) testing appeared in the first edition of IEC
standard 62271-200 [1], effective November 2003. Before the year 2003 switchgear’s requirements
with regard to the behaviour during internal arc events were under agreement between manufacturer
and user only. IAC classification actually does not cover any active system.
A workplace risk assessment, required by national standards, might consider an “active” system for a
workplace risk improvement. As an example, the U.S. standard NFPA 70E "Standard for Electrical
Safety in the Workplace" [9] addresses employee workplace electrical safety requirements and
considers that, in case of an arcing fault, the protection plan (breakers, relays, etc.) works normally.
Such an arc effect mitigation system will be chosen if the circumstances favour its use. The factors
taken into account in determining its use are:
– the type of available switchgear room;
– stand alone (e.g. prefabricated substation) or integrated in an industrial, commercial or
infrastructure building, including vessels, offshore platforms and similar;
– the pressure resistance of the walls, the possible pressure relief opening of the room, the
room's accessibility conditions.
– the need for maintenance work, which requires opening of switchgear covers and/or doors when
other parts of the switchgear assembly are still energized. In this case, particular maintenance
procedures are needed;
– the electrical power system’s sensibility to service outage, caused by an internal arc and the
maximum outage time designated or allowed.
There are applications – mostly in industrial process industry and public safety areas – where
service outage times are considered as being especially critical. However these applications may
require back-up systems to further limit any service or failure outage time;
– the criticality of the whole installation and other exposed devices and its sensitivity to the impact of
an arc, such as thermal effects, pressure and conductive or oxidizing gases.
Generally arc mitigation systems, active as well as passive, cannot be routine or acceptance tested as
a complete system (initiation by an arc and assessment of the mitigation effect). Nevertheless parts of
the system can be tested separately for proper function as well as complete system type tests can be
performed. However, the current situation is that there is a lack of reference document for such
performance, and any type tests have to be specified and agreed "between manufacturer and user".
3.2 ACTIVE ARC MITIGATION SYSTEMS
3.2.1 Benefits
The benefits which can be expected from such systems are:
– limiting or avoiding the release of gases and particles into the switchgear room, caused by the
internal arc in order to:
– minimize the emission of toxic gases to persons near the switchgear under fault;

36
– reduce contamination of the switchgear room or other equipment with gases, metallic vapours
and particles, caused by the internal arc. This contamination may lead to significant reduction
of the remaining lifetime of installed equipment.;
– reduce the required ventilation time before entering the switchgear room after fault;
– reduce the structural strength requirements for the switchgear room, especially for pressure
withstand, and reduce the size of exhaust openings and the switchgear room size itself.
This may allow the customer to use an existing room for the switchgear installation or a room
where it is not possible to obtain the required exhaust openings for gases or routing gases and
particles outside. Reducing room’s size is especially beneficial where available space is limited.
In all cases an appropriate pressure calculation is necessary.
In certain cases, the reduction of gases generated by the internal arc in a functional unit under
fault, may no longer require a “first level” large pressure relief volume (buffer volume) – mostly
located below the switchgear – before further releasing the gases;
– reducing the thermal energy generated by the arc, which limits the risk of reaching high
temperatures leading to a fire on components inside or outside the switchgear;
– enabling arc effect mitigation under maintenance condition which requires opening of switchgear
covers and/or doors when other parts of the switchgear assembly are still energized;
– reducing the internal arc duration below the rated value, in cases where the implemented
overcurrent protection scheme does not provide the fault clearing time to remain within the IAC
rated duration of the switchgear;
– limiting the damage within the switchgear due to the arc energy reduction.
The arc fault damage can be confined to the functional unit or compartment under fault.
The other functional units of the switchgear that were not exposed to the internal arc could be re-
energized after isolation of the faulty functional unit or compartment.
Depending on the limitation, the faulty functional unit or compartment might be re-used after repair
in accordance with manufacturer instructions, cleaning and inspection;
– possibly reducing the thermal and mechanical stress for incoming feeder cables and connections
caused by the fault current in the case of an internal arc. The internal arc detection signal can be
used to by-pass the selected protection scheme and thus accelerate the tripping of the upstream
breaker, even if a short-circuiting device ensures the arc extinction.
3.2.2 Limitations and drawbacks
Some active arc mitigation devices are transforming an arc fault into a three-phase, ungrounded
bolted short-circuit. Under these conditions the involved circuit-breaker, which is expected to clear the
fault, has to handle a transient recovery voltage which might exceed its type tested capabilities
because breaker type testing uses a grounded test circuit (IEC 62271-100 "High-voltage switchgear
and controlgear – Part 100: Alternating-current circuit-breakers" [10], figure 13).
If short-circuiting devices are installed close to generators the current flowing through the short-
circuiting device can lead to a longer period of “delayed current zero crossing” condition compared to
the case with arc faults. In such a case special attention should be paid to the application of generator
circuit-breakers (see details in IEC/IEEE 62271-37-013 "High-voltage switchgear and controlgear –
Part 37-013: Alternating-current generator circuit-breakers" [11]).
The components of an active arc mitigation system have to be considered in the calculations of MTBF,
functional safety and expected service lifetime of the installation. The lifetime can be reduced by the
adjunction of auxiliary equipment and an auxiliary power supply (if required by the system) in non
optimal conditions (e.g. electrical field and high temperature, inside MV compartments).
Three phase short-circuiting devices (with and without grounding) if initiated will establish a three-
phase short-circuit current,(even though the arc fault occurred as a single-phase to ground failure
with lower current). This increases the thermal and mechanical stress of the upstream equipment,
although normally still within the specified performances. In switchgear designs which would keep the
fault single-phase to earth, the three phase short-circuit would normally be avoided if no short-
circuiting mitigation system operates.

37
The various detection systems show specific limitations:
– Light / sound detection:
– erroneous initiating of the arc mitigation system is possible caused by other emitting sources;
– location and type of sensors will have to be adapted to a specific switchgear design;
– mechanical limitations have to be taken into account, e.g. a minimum bending radius for optical
fibres.
– Current detection:
– a phase-to-earth arc failure could provide a situation in which the fault current is too low to
activate the arc effect mitigation system, while the arc stays one phase-to-earth. This depends
on the electrical system’s neutral point treatment and switchgear design.
– Pressure detection:
– variations in the sound frequency range may be interpreted as crossing a threshold while they
should be ignored. Measurement should implement proper filters or sensitivity to get a valid
signal especially if low thresholds are considered;
– Annex B of CIGRE Brochure 602 [4] provides detailed information on these issues.
There are minimum threshold levels required to initiate the active arc mitigation system:
– low setting values of the initiation criteria have the advantage of being sensitive to low current arc
faults. However such a sensitive system gives the risk of unwanted activation;
– high setting values of the initiation criteria help to focus the system on severe faults, but create
limitations in sensitivity.
This sensitivity trade-off might result in a deviation from the expected result. Combining more than
one threshold signal overcomes this sensitivity challenge, and is commonly implemented for light
detection with a cross-check with another parameter (e.g. current).
3.3 PASSIVE ARC EFFECT MITIGATION SYSTEMS
3.3.1 Benefits
The benefits which can be expected from such systems are to:
– limit or avoid the release of gases and particles caused by the internal arc into the switchgear
room:
– minimizing the emission of toxic gases to persons near the switchgear under fault;
– reducing contamination of the switchgear room or other equipment with gases, metallic
vapours and particles caused by the internal arc. This contamination may lead to significant
reduction of the remaining lifetime of installed equipment;
– reducing the required ventilation time before entering the switchgear room after fault. This is
mainly applicable when using an exhaust duct.
– reduce the structural strength requirements for the switchgear room, especially for pressure
withstand, the size of exhaust openings and the size of the switchgear room size itself.
In all case an appropriate pressure calculation is necessary.
This is applicable using an exhaust duct, an absorber system or a combination of it, for rooms
equipped with an opening;
– use an existing room for the switchgear installation or a room where it is not possible to obtain the
required exhaust openings for gases or routing gases and particles outside. This is applicable using
an absorber system;
– reduce the thermal energy – released from the switchgear assembly – by absorbers or routing the
thermal energy outside the switchgear room by an exhaust duct; both limit the risk of reaching
high temperatures inside the switchgear room leading to a fire of components.

38
3.3.2 Limitations and drawbacks
Some limitations, or drawbacks, are identified for these systems, as:
– upgrading an existing internal arc classified switchgear, for reducing the arc effects by passive
systems, may lead to invalidity of switchgear’s type tests (if originally tested without these
systems), thus requiring new testing for validation of the classification;
– before adding a passive system – in any initial switchgear installation or later upgrading – the
available space in the switchgear room versus required additional space has to be considered as
well as limitations caused by the building structure;
– some passive systems (e.g. exhaust ducts) cannot be factory assembled and require on-site
installation expertise and additional work. On the other hand on-site upgrading of already installed
switchgear by a passive system may not be possible if it is not foreseen by design (e.g. flaps);
– passive systems are usually only beneficial under normal service conditions, meaning with all
covers and doors closed, as for the internal arc classification.

39
4. POSITION ACCORDING TO AVAILABLE
STANDARDS AND CUSTOMER'S SPECIFICATIONS
4.1 GENERAL
Arc resistant medium voltage switchgear designs were developed in Europe over forty years ago to
cater to the possibility of an arcing fault on the bare copper or aluminium buses commonly used in
Europe. Annex AA to IEC 60298, “A.C. Metal-Enclosed Switchgear and Controlgear for Rated Voltages
above 1 kV and up to and Including 52 kV” [12], was initiated in the mid-1970’s, based on German
experience, and was approved in 1981. The performance evolved to become an optional rating –
Internal Arc Classification IAC – in the publication of the IEC 62271-200 [1], in 2003. The similar
IEEE guide C37.20.7, “Guide for Testing Metal-Enclosed Switchgear Rated up to 38 kV for Internal
Arcing Faults” [2], was first issued in 2001 and revised in 2007.
While there is some disparity in requirements between the IEEE and IEC, arc resistant switchgear
standards and guides, there are also some similarities and common requirements:
The criteria for success in testing are similar and are summarized as follows:
– Criterion 1: Properly secured covers and doors do not open.
– Criterion 2: The enclosure remains essentially intact.
– Criterion 3: No openings in the enclosure wall are created by the arcing in the areas being
evaluated.
– Criterion 4: No indicators ignite due to escaping gas.
– Criterion 5: The grounding connections are maintained.
A key difference between the IEC Standard and the IEEE Guide is that the IEEE document includes an
option to confirm the absence of arc effects inside the instrument/relay compartment of the
switchgear.
4.2 IEC STANDARDS
In the IEC publications, several either directly address arcing faults, or may be used when working on
a risk assessment.
The IEC 62271-200 [1] "Metal-enclosed switchgear and controlgear assemblies" provides several hints
which are opening possibilities for arc limitation or arc effects mitigation systems:
– 4.7.101 Rated Duration of Short Circuit Current: "In principle, the rated duration of short
circuit for a main circuit cannot exceed the corresponding rated value of the weakest of its series
connected components. However, for each circuit or high-voltage compartment, advantage may be
taken of apparatus limiting the duration of the short-circuit current, such as current-limiting fuses."
Short circuit current duration could be reduced for part of an assembly by the installation of these
limiting devices, however external short-circuits are usually not going to trigger a device dedicated
to arc effect mitigation and the possible application of a reduced rating for some circuits within the
assembly shall be closely investigated.
– 5 Design and construction: "For main circuits with current-limiting fuses, the manufacturer of
the switchgear and controlgear may assign the maximum peak and Joule integral of the let-
through current of the fuses to the main circuit downstream of the fuse."
This could be a precedent, and a hint to rate more sophisticated devices.
– 6.106.1 Internal Arc test; General: "Not intended to cover the presence of gases with potential
toxic characteristics, or the hazard of fire propagation to combustible materials or equipment
placed in the proximity of the metal-enclosed switchgear and controlgear;"
From a standard perspective, the secondary effects of arcs (fire, toxic gasses, equipment damage,
etc…) are pretty much universally ignored, due to the impossibility to define proper measurements
and acceptance criteria.

40
However, the Standard provides limited information about how to qualify (for Internal Arc
Classification) a switchgear fitted with an internal arc effects mitigation system, but that does not
address the demonstration of performance of the system itself.
– 6.106.2 Internal Arc test; test conditions: "Any device (for example, protection relay) that may
automatically trip the circuit before the end of the prospective duration of the test shall be made
inoperative during the test. If compartments or functional units are equipped with devices intended
to limit the duration of the arc itself by other means (for example, by transferring the current to a
metallic short circuit), they shall be made inoperative during the test. If these devices are integral
part of the design of the compartment or assembly which prevents to make them inoperative
without modification of the construction, the relevant compartment of the switchgear and
controlgear may be tested with the device operative; but this compartment shall be qualified
according to the actual duration of the arc. The test current shall be maintained for the rated
short-circuit duration of the main circuit.
NOTE2 Because in general arc limiting devices are out of the scope of this standard and if the
switchgear and controlgear has previously been tested with the limiting device made inoperative,
an additional test may be performed to demonstrate the behaviour of this arc limiting device."
This states that such devices cannot be used to demonstrate the ratings for internal arc classification.
But the ratings may be defined considering that such a system will be implemented. For instance, a
switchgear assembly may be classified with a very short fault duration, such a duration being realistic
only with a dedicated arc extinction system.
Also the Standard does not address any maintenance situation (classification is demonstrated with all
doors and panels closed and locked). That further means that "arc flash" concerns are not covered.
(as a reminder, "arc flash" means direct exposure to arc thermal radiation, either by working on bare
live conductors, or following a fault within a non-internal-arc-rated piece of switchgear).
Some guidelines are provided in Clause 8 of the standard "Guide to the selection of switchgear and
controlgear" and especially in subclause 8.104 about internal arc risk and classification.
The IEC 62271-202 "High-voltage/low-voltage prefabricated substations" [13] extends the concept of
Internal Arc Classification from switchgear assemblies to prefabricated substations, with a similar level
of specification, basically the same assessment criteria, and no more detailed information about arc
effects mitigation.
The IEC/TR 61641 "Enclosed low-voltage switchgear and controlgear assemblies – Guide for testing
under conditions of arcing due to internal fault" [14] proposes classification criteria based on either
constructive provisions – solid insulation – or test results. Test procedures are not fully defined, and
some points remain open for discussion according to the assembly considered. Many different tests
could be performed on the basis of this document.
Some differences related to MV equipment appear, beyond the fact that no type test is defined:
– the Report does not consider that solid insulation may fail; the proposed "arc ignition protected
zones" are expected to be fault free because of solid insulation;
– the Report also addresses service continuity, through proposed assessment criterion 7;
after clearing of the fault or after isolation or disassembly of the affected functional units in the
defined area, emergency operation of the remaining assembly is possible. This is verified by a
dielectric test according to IEC 61439-2 "Low-voltage switchgear and controlgear assemblies – Part 2:
Power switchgear and controlgear assemblies" [15], subclause 10.9.2, but with a test voltage of
1,5 times the rated operational voltage for 1 min. Bending or bowing of doors and covers of the unit
under test and adjacent units is acceptable providing it can be readily restored to a minimum level
of protection in accordance with IPXXB of IEC 60529 "Degrees of protection provided by enclosures
(IP Code)" [16]. With the exception of the tested zone as declared by the manufacturer, all other
units should remain fully operable both mechanically and electrically and are essentially in the same
condition as before the test.";
– the Report considers that any arc mitigation system may be implemented and operational for the
tests.

41
The IEC 60909 series " Short-circuit currents in three-phase ac systems" provides rules for short-
circuit current calculations according to network conditions. It may be useful when considering the
actual risk level on a given installation. The parts are:
– Part 0:2001, Calculation of currents
– Part 1:2002, Factors for the calculation of short-circuit currents according to IEC 60909-0
– Part 3:2006, Currents during two separate simultaneous line-to-earth short circuits and partial
short-circuit currents flowing through earth
– Part 4:2000, Examples for the calculation of short-circuit currents
The IEC 60255 series "Measuring relays and protection equipment"; could be used to specify and
demonstrate some performance aspects of some detection systems.
The principle in the IEC standards, and especially in the IEC 62271-200 [1], is that the rated
performances shall be demonstrated (principle of verifiability stated by the ISO/IEC Directives, Part2,
5.5 [21]), and that these performances define the boundary for interactions with the larger system.
Typically, a short-circuit withstand duration shall be linked with the protection plan and its possible
back-up stages and the rated duration is the limit for the protection scheme to clear the fault
(actually, the possible reclosing operations have to be considered also on that performance). With a
similar idea, it is possible (see Clause 5 of IEC 62271-200 [1]) to define the maximum peak current
and maximum Joule integral as a boundary, if upstream devices or systems, e.g. current-limiting
fuses, are considered reliable enough to ensure such limits will not be exceeded. And it is once more
the same idea which opens the possibility to declare any duration, even rather short, for IAC
classification (see subclause 5.101 of IEC 62271-200 [1]); such short values are expected to cover the
need when dedicated arc protection system is implemented and the overall behaviour relies on the
proper function of the system as a whole, but each contributing part needs to be characterised in such
a way that it can be validated.
When coming to test procedures, this arc protection system is not considered as a whole (same
situation as any protection scheme) and the test parameters are chosen for demonstration of the
boundary values characterising the switchgear itself, thus the provision expressed at the end of
subclause 6.106.2 of the IEC 62271-200 [1]: "Because in general arc limiting devices are out of the
scope of this standard and if the switchgear and controlgear has previously been tested with the
limiting device made inoperative, an additional test may be performed to demonstrate the behaviour
of this arc limiting device.".
4.3 OTHER STANDARDS AND REGULATIONS
4.3.1 Standards
Several other publications issued by standardisation bodies address some way the concern of arcing
fault, not always limited to medium voltage switchgear. Some are listed below, and some others have
been cited by respondents to the survey (see A.4.7).
UL 2748 - 2015 Outline of Investigation for
Arcing Fault Quenching
Equipment
Covers equipment rated up to 38 kV ac
maximum.
Does not include the requirements for
sensors intended to detect arcing fault,
devices intended to trigger the
functioning of the arc quenching
equipment, or devices that are
intended to interrupt arcing fault
currents.
Does not include all the requirements
for integration and testing of arc
quenching equipment within
equipment it is intended to protect.

42
IEEE C37.20.7 - 2007 Guide for Testing Metal-
Enclosed Switchgear Rated Up
to 38 kV for Internal Arcing
Faults
Introduces Suffix C
"C" for indicators in LV instrument
compartments
IEEE 1584 - 2002 Guide for performing arc flash
hazards calculation
Provides guidance for calculation of
arc exposure; in such calculations, the
fault duration is a parameter, and any
device which shortens the duration
could be beneficial..
4.3.2 Regulations
There is no identified regulation addressing directly internal arc events. However, several regulatory
texts dealing with safety issues could be applied with the help of mitigation techniques; some are
listed below.
Document reference Title Comments
NFPA 70 E
U.S. regulation (partial)
Standard for Electrical Safety in
the Workplace
NFPA 70E arc flash specifications (to be
addressed for situations when operator
can face an open arc) in North America
has the status of regulation in almost all
states.
European Union
EN 50110
" Operation of electrical
installations"
Personnel shall wear clothing suitable
for the locations and conditions where
they are working. This could include the
use of close-fitting clothing or additional
PPE (personal protective equipment).
EU ATEX Directive
(2014/34/EU)
Requirement for no hot gases release in
the event of fault
CSA Z462-15 Workplace electrical safety This regulation calls the IEEE 1584 for
arc exposure calculation
4.4 MANUFACTURERS' DOCUMENTATION QUOTES (CLAIMED PERFORMANCES,
DEMONSTRATION...)
4.4.1 Passive mitigation
"Passive system enclosures offer arc flash containment and redirect the fault energy up and away
from the installed equipment and personnel."
"All the energy, gases and other materials produced as result of the arc incident are directed to the
upper part of the vertical section through the vent flaps and plenum, preventing front, bottom and
side exhaust for gear and personnel protection."
"The exhaust duct collects the gases produced in case of internal arc fault and leads them out of the
switchgear room."
"Two and three phase short-circuits between the primary conductors are excluded by the single pole
primary enclosure."

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