Arc flash publication

Assessing and managing the risk of arc flash
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ASSESSING AND MANAGING THE RISK OF ARC FLASH
First edition
May 2018
Published by
Energy Institute, London
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Registered charity number 1097899
+44 (0)207 467 7100

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Published by the Energy Institute
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3
CONTENTS
Page
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4.1 What is an arc flash? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4.2 Arc flash causes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4.3 Arc flash consequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4.4 Arc flash risk management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 Causes and likelihood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Live working . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Arc flash causes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.1 System design and construction errors . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.2 Maintenance and asset management deficiencies . . . . . . . . . . . . . . . . . 14
2.3.3 Human and organisational factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 Potential high risk tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.1 Deliberate and inadvertent live working . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.2 Disturbance of conductors during normal operational tasks . . . . . . . . . . . 16
3 Dangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1 Factors that affect arc flash dangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.1 The proximity of personnel to the arc flash . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.2 The duration of the arc flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.3 Electrical equipment design and condition . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.4 Electrical equipment location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.5 Short circuit current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1.6 Others factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1.7 Common relationships between factors . . . . . . . . . . . . . . . . . . . . . . . . . 19
4 Managing the risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1 Risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1.1 Assessing likelihood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.2 Assessing danger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2 Risk reduction controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2.1 Prevention controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2.2 Mitigation controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.3 Reassessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
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Contents continued
Page
Annexes
Annex A References, bibliography and pertinent regulation . . . . . . . . . . . . . . . . . . . . . 35
A.1 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
A.2 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
A.3 Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
A.3.1 UK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
A.3.2 USA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Annex B Abbreviations and acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Annex C Incident energy assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
C.1 Estimating the severity of an arc flash . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
C.1.1 IEEE 1584 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
C.1.2 Lee method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
C.1.3 German Accident Insurance Organisation BGI/GUV-I 5188 E . . 41
C.1.4 IEEE 1584 simplified low voltage method . . . . . . . . . . . . . . . . . 41
C.2 Good practice when estimating the severity of an arc flash . . . . . . . . . . . 43
C.2.1 General and data gathering . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
C.2.2 Chosen method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
C.2.3 Calculation methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Annex D Site, location, equipment and task likelihood assessments . . . . . . . . . . . . . . . 47
Annex E Practical example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
E.1 Task description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
E.2 Initial risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
E.3 Additional controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
E.3.1 Prevention controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
E.3.2 Mitigation controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
E.4 Reassessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
E.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
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LIST OF FIGURES AND TABLES
Page
Figures
Figure 1 Incident energy variation with working distance (IEEE 1584 empirical model) . . . . . 17
Figure 2 Incident energy variation with trip time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 3 Example arc flash risk assessment matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 4 Likelihood assessment overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Figure 5 Risk assessment matrix with all descriptions (illustrated for context) . . . . . . . . . . . . 27
Figure C.1 Simplified low voltage protection characteristic (125 A MCB IEC 60898) . . . . . . . . . 42
Figure C.2 Incident energy estimation for typical low voltage protection devices . . . . . . . . . . . 43
Figure C.3 Switchboard illustration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Figure E.1 Example network illustration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Figure E.2 Risk assessment with existing controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Figure E.3 Risk assessment with additional controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
Tables
Table 1 Example arc flash likelihood scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Table 2 Example of unmitigated arc flash health and safety consequence scale . . . . . . . . . . 25
Table C.1 Incident energy calculation comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Table D.1 Site, location, equipment and task likelihood assessments . . . . . . . . . . . . . . . . . . . 48
Table E.1 Likelihood assessment with existing controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Table E.2 Severity assessment with existing controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Table E.3 Likelihood assessment with additional controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Table E.4 Severity assessment with additional controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
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FOREWORD
Arc flash poses a significant risk to personnel working on electrical systems, with high consequences
including severe burns and death, as well as damage to equipment. There are numerous methods
for calculating incident energy (the heat generated by an arc flash) and varying levels and standards
of personnel protective equipment (PPE) for working on or near live equipment. Preventing direct
contact live working should reduce the risk of arc flash, but a risk will always remain due to the
limited ability for equipment enclosures to limit internal arc flash energies. Furthermore, there may be
occasions when it may not be reasonably practical to work on equipment that is dead. Understanding
the risk of arc flash, and how to manage it, can seem daunting.
The objective of this publication is to provide a practical approach to the management of arc flash
risk within electrical installations. It provides general guidance on the causes of arc flash events and
the dangers to personnel, along with a risk assessment process and hierarchy of control measures
that can be used to reduce the likelihood of an arc flash and reduce its potential dangers. However,
it does not provide detailed guidance on how to calculate incident energy levels (but does provide a
brief overview of the key methods), nor detailed technical information about electrical systems.
The guidance seeks to address uncertainty within industry of when an arc flash risk may exist and
how it should be assessed and controlled, as well at what level in the organisation the risk should
be managed (i.e. by the frontline or management level controls). It also provides an overview of the
various methods for calculating incident energy.
Whilst this publication will be informative for electrical specialists and non-specialist alike, and will
give non-specialists an understanding of the causes of arc flash and how to manage the risk, it is
primarily intended as:
a) guidance to inform the organisation on arc flash risk management, and
b) a practical tool to aid electrical engineers and others with specialist electrical knowledge to
assess arc flash risks.
Therefore, it does not purport to contain all of the technical detail required to manage arc flash risk.
The user of this publication should be familiar with local regulations relating to working on electrical
equipment and have specialist electrical knowledge when applying the guidance in practice.
The information contained in this publication is provided for general information purposes only.
Whilst the Energy Institute (EI) and the contributors have applied reasonable care in developing this
publication, no representations or warranties, expressed or implied, are made by the EI or any of the
contributors concerning the applicability, suitability, accuracy or completeness of the information
contained herein and the EI and the contributors accept no responsibility whatsoever for the use of
this information. Neither the EI nor any of the contributors shall be liable in any way for any liability,
loss, cost or damage incurred as a result of the receipt or use of the information contained herein.
The EI welcomes feedback on its publications. Feedback or suggested revisions should be submitted
to:
Technical Department
Energy Institute
61 New Cavendish Street
London, W1G 7AR
e: technical@energyinst.org
+44 (0)207 467 7100

7
ACKNOWLEDGMENTS
Assessing and managing the risk of arc flash was developed by Luke Taylor (TNEI Services Ltd),
and produced by the EI Arc Flash Working Group (AFWG), with oversight by the EI Power Utility
Committee (PUC) and EI Electrical Committee (EC). During this project, AFWG members included:
Graham Beale Engie
Sue Beveridge SSE
Dibyendu Bhattacharya BP
Roger Bresden Saudi Aramco
Alan Dickson (Chair) Scottish Power
Fangtao Dai SSE
Stuart King (Secretary) Energy institute
Justin Mason BP
Nia Roderick RWE
Zaur Sadikov Shell
Lee Sinfield BPA
Jonathan Slark Valero
Graeme Smith Uniper
Konstantinos Vatopoulos Aramco Overseas
Stephen Wilkinson Phillips66
Peter Woodcock RWE
During this project, PUC members included:
Graham Beale (Chair) Engie
Steve Gilmore Uniper
Wolfgang Hahn EDF Energy
Philip Horner Centrica
Edward Jamieson RWE
Stuart King (Secretary) Energy institute
Ian Kinnaird Scottish Power
Daniel Rawdin SSE
Konstantinos Vatopoulos Aramco Overseas
During this project, EC members included:
Dibyendu Bhattacharya BP
Geoff Fulcher F.E.S. (EX) Ltd.
Neville Harris Valero
Gary Holcroft HSE
Justin Mason BP
Toni Needham (Secretary) EI
Zaur Sadikov (Chair) Shell
Lee Sinfield BPA
Jonathan Slark Valero
Chris Turney F.E.S. (EX) Ltd.
Stephen Wilkinson Phillips66
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8
Project management and technical editing were carried out by Stuart King (EI).
EI acknowledges the following individuals for their contributions during the development or review
of this project:
Francois Bathellier Statoil
Scott Davidson Maersk Oil
Paul Donnellan Shell
Mick Gaskill Shell
Martin Gillard Shell
Mark A. Metzdorf BP
Bill Moir BP
Ken Morton HSE
Doyin Owoka Centrica
Andrew Pitt HSE
Affiliations are correct at the time of contribution.
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9
1 INTRODUCTION
1.1 OBJECTIVE
The objective of this publication is to provide a practical approach to the management of the
risk of arc flash within electrical installations. The publication provides high level guidance
on the causes of arc flash events and the dangers to personnel, along with a risk assessment
process and hierarchy of control measures that can be used to reduce the likelihood of an
arc flash and reduce its potential dangers. The guidance seeks to address uncertainty within
industry of when an arc flash risk may exist and how it should be assessed and controlled.
Whilst this publication will be informative for electrical specialists and non-specialist alike,
and will give non-specialists an understanding of the causes of arc flash and how to manage
the risk, it is primarily intended as:
a) guidance to inform the organisation on arc flash risk management, and
b) a practical tool to aid electrical engineers and others with specialist electrical
knowledge to assess arc flash risks.
Therefore, it does not purport to contain all of the technical detail required to manage arc
flash risk (although it gives examples of common methods). The user of this publication
should be familiar with local regulations relating to working on electrical equipment and
have specialist electrical knowledge.
1.2 SCOPE
The guidance provided is applicable to electrical power systems consisting of single and
three-phases of alternating current (a.c.) with a frequency of 50 or 60 Hertz (Hz) and
operating phase to phase voltage of 400 volts (V) up to, and including, 52 kV. The lower
limit of 230/400 V is in line with accepted practice within IEEE 1584 and the upper voltage
limit bounded by equipment specifications to IEC 62271-200. This guidance is principally
applicable to metal-clad switchgear as opposed to open terminal installations.
The general risk assessment process described in this publication likely can be applied to
direct current (d.c.) systems; however, the electrical properties of d.c. systems differ from a.c.
systems and therefore some technical details contained within this publication may not be
applicable to d.c. systems, meaning the user should exercise caution if using the guidance in
this publication for d.c. systems.
The guidance is applicable to all electrical tasks ranging from the intrusive maintenance
of electrical equipment to the normal operation of electrical equipment, such as switches,
within their intended design capability.
The guidance is aimed at managers and operators of electrical equipment (as well as any
other roles where people are exposed to arc flash risk) and focuses on the potential arc flash
causes and mitigation under their control.
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1.3 DEFINITIONS
The following terms are used throughout, and so it is useful to define these first:
−
− 'Arc Flash' is a dangerous condition associated with the release of energy caused by
a current flowing through an electrical arc plasma, also called arc fault current and
arc current.
−
− The reference to 'incident energy' in this publication relates to the potential heat
radiated from an arc flash.
−
− In the context of this publication, the term 'danger' predominantly relates to the
potential danger to personnel of an arc flash (see 1.4.3).
−
− 'Live conductor', in the context of this publication, is a conductor that is energised or
has not been 'correctly isolated' (see below).
−
− 'Exposed live conductor' is a live conductor that does not have a mechanically fixed
barrier to prevent the contact of personnel or their tools with the conductor.
−
− 'Live working' is defined in this publication as a task performed by personnel that,
at any moment during the task, places personnel in a location nearer than a safe
distance to an exposed live conductor such that it is capable of being inadvertently
touched or approached by a person. It is applied to parts that are not suitably
guarded, isolated, or insulated.
−
− A 'correctly isolated' conductor is one which has been disconnected from all possible
sources of electricity with an adequate isolating gap. Procedures should be in place
to ensure the conductor can only be re-energised when the task being performed
on the isolated conductor is complete. Note: if the process of isolating a conductor
places personnel in a location without a mechanically fixed barrier to prevent the
contact of the personnel or their tools to an exposed live conductor the task should
be considered to be live working.
−
− 'Proven dead' is when a correctly isolated conductor has been tested with a calibrated
instrument, designed for this purpose and known to be functioning correctly, and
confirmed as dead. Note: when proving a conductor dead, personnel are likely to
be placed in a location without a mechanically fixed barrier to prevent contact,
by themselves or their tools, to a live conductor and therefore the task should be
considered to be live working.
−
− 'Live equipment' is equipment that is energised or has not been correctly isolated
and proven dead.
−
− A 'normal operational task' is a task performed by personnel on live or dead electrical
equipment, that at no point in time results in live working, and is used as part of
the equipment’s intended design capability, such as the operation of a switch. If an
operational task results in live working then it is not classified as a normal operational
task and should be subject to live working restrictions.
1.4 BACKGROUND
1.4.1 What is an arc flash?
An arc flash is an uncontrolled electrical arcing fault through air. An arc flash may occur for
the reasons described in 2.1. The possible causes of an arc flash are described in 2.3 and are
summarised in 1.4.2.
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1.4.2 Arc flash causes
The primary function of an electrical power system is to transport electrical energy and
therefore it is normally energised and commonly referred to as 'live'. Electrical equipment is
designed to remain safe whilst energised and during normal operational tasks that are within
its intended design capability; however, the following scenarios result in a risk of arc flash:
−
− Live working (defined in 1.3) – discussed in 2.2.
−
− Equipment failure – potential causes of equipment failure are discussed in 2.3.1 and
2.3.2.
−
− Management of human and organisational factors – example inadequacies that
cause a risk of arc flash from human error are provided in 2.3.3. A human error
may be an accidental action, an incorrect intended/instructed action or a deliberate
action.
Some commonly performed tasks are particularly susceptible to the scenarios listed here and
as such may carry an increased risk of arc flash. These common tasks are described in 2.4.
1.4.3 Arc flash consequences
Danger to personnel
The dangers to personnel of an arc flash include death or serious injury caused by one or
more of the following effects:
−
− burning, leading to death as a result of shock, organ failure or secondary infection;
−
− severe burning to skin and flesh;
−
− permanent or temporary visual impairment;
−
− permanent or temporary hearing impairment;
−
− physical injury from the explosion and ejection of equipment components and
molten metal, and
−
− respiratory damage from toxic gases.
Equipment damage
It is highly likely that equipment (within which an arc flash has occurred) will be damaged
beyond repair and cannot be placed back into service. The equipment is likely to suffer the
following damage:
−
− arc erosion of exposed conductors;
−
− permanent thermal damage to surrounding insulating material;
−
− deformation of conductors and enclosures, and
−
− extensive internal and external sooting.
Financial and business consequences
An arc flash has the potential to result in serious financial and business impacts through the
following possible consequences:
−
− sustained loss of process through loss of critical electrical equipment such as a
switchboard;
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−
− sustained loss of process through explosion damage to wider infrastructure;
−
− the death or serious injury of personnel;
−
− regulatory fines and civil/criminal liability, and
−
− reputational damage.
1.4.4 Arc flash risk management
Management of the arc flash risk should be performed through risk assessment and, where
necessary, implementation of mitigating controls.
The assessment of the arc flash risk can be performed using a standard risk assessment
approach which considers both likelihood and potential consequences on a risk matrix (as
also stated in NFPA 70E-2015); this publication provides an example of such a risk matrix.
Guidance on assessment of the likelihood of an arc flash is provided in 4.1.1. Guidance
on assessment of the potential consequences is limited to the dangers to personnel and is
provided in 4.1.2. The level of risk is determined by considering both aspects together.
A hierarchy of controls to reduce the level of arc flash risk is provided in 4.2. Preventing an
arc flash is the best protection from all the potential health and safety, equipment, financial
and business consequences. With this in mind, the arc flash controls provided are based on
a hierarchy which prioritises preventing the causes of arc flash. Mitigation controls to reduce
the arc flash dangers to personnel are provided but are deliberately positioned lower in the
hierarchy.
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2 CAUSES AND LIKELIHOOD
2.1 INTRODUCTION
For an arc flash to occur an arc must be developed in air between at least two electrical
conductors, which may include a live-phase and ground. Air is an insulator with a high
dielectric/insulating strength. A gap containing air is commonly used in electrical equipment
to provide insulation between electrical conductors. Clean and dry air should withstand
the electric field associated with approximately 1 000 V for each millimetre of conductor
separation (Note: this is a conservative approximation; in practice this will vary according to
the shape of the conductors and resulting electric field).
The common causes of arc flash listed in this section result in a breakdown in the insulating
properties of air. This breakdown is physically caused by either pollution of the air or by
an electric field in excess of the insulating strength of the air or, for an internal event, to
the switchgear enclosure. For an internal event the main cause of excessive electric fields
between conductors is the conductors moving closer together; this reduction in separation
can be permanent or transient. Arc flash can also take place when conductors are being
moved away from each other (as in opening a switch) or due to partial discharge in the
insulating material. Once an arc is developed, the surrounding insulating air is quickly ionised,
significantly reducing its insulating strength. When insulating air becomes ionised, the
continuation of the arc and/or the formation of other arcs is facilitated.
2.2 LIVE WORKING
It is reasonable to assume that all electrical equipment is live unless correctly isolated and
proven dead. Live working in the context of this guidance, defined in 1.3, occurs when
personnel are in close proximity of an exposed live conductor.
Live working should only be considered when it can be proven that it is unreasonable in all
circumstances to perform the task dead and reasonable in all circumstances to perform the
task live. Annex A references relevant regulations in this regard (see A.3. for a UK example).
Live working greatly increases the risk of arc flash caused by pollution of the insulating air
and/or the accidental introduction of a foreign object such as tools or debris between live
conductors, as well as working on equipment that is not well maintained.
The avoidance of live working significantly reduces the risk of arc flash and any
specific task performed on equipment that is correctly isolated and proven dead
carries a lower risk.
It should be noted incidents have also occurred caused by neighbouring live equipment,
although the equipment worked on had been isolated and proved dead. There is also likely
to be a risk of arc flash during the process of isolation and proving dead. The process of
proving dead carries a risk of live working. Other tasks that carry a risk of live working are
provided in 2.4.1.
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2.3 ARC FLASH CAUSES
The possible causes of an arc flash can be placed into three distinct groups:
−
− System design and construction errors in electrical distribution system design,
specification, manufacture, construction or installation.
−
− Equipment maintenance and asset management deficiencies – insufficient
management of electrical equipment and the physical and electrical environment in
which it is located.
−
− Human and organisational factors – inadequate management of electrical tasks,
personnel access, permissions, procedures and competencies.
2.3.1 System design and construction errors
Arc flash root causes relating to design and construction can be associated with either
the manufacture of individual items of electrical equipment or the combination of whole
electrical systems during construction.
−
− Design and specification – if the distribution system is designed with a very high
magnitude of potential short circuit current, and equipment is specified with an
insufficient short circuit rating, then an arc flash could result during short circuit
conditions. Incorrect specification of mechanical and environmental aspects such
as 'forms of separation' and 'IP rating' (i.e. internal segregation/ingress protection
marking) also increase the risk of an arc flash.
−
− Equipment manufacture – a common cause of arc flash is mechanical failure of
critical components causing arcing between energised conductors.
−
− Construction – during construction and installation of electrical equipment, errors
can occur that result in arc flash. One common example of this is foreign conductive
object entry into equipment caused by poor construction, control of equipment and
cleanliness when top covers may be open and concurrent overhead work activity is
taking place. Should equipment cleaning activity prior to energisation not remove
construction debris this may result in a switchgear internal fault and possible arc
flash incident.
2.3.2 Maintenance and asset management deficiencies
The condition of electrical equipment and/or the physical environment in which it is
contained may change over time. Incorrect management of these changes could result in
an arc flash.
−
− Maintenance – failing to appropriately maintain electrical equipment in accordance
with manufacturer’s instructions and/or a risk-based maintenance plan, or making
decisions based on inexperience or lack of knowledge, can result in an increase in
the risk of an arc flash; this includes failure to retrofit components resulting from
manufacturer’s safety notifications. It should also be noted that assembly error
during maintenance may result in arc flash upon re-energisation of the equipment
(see 2.3.3).
−
− Inspection – failure to undertake preventative inspection, maintenance and testing
of equipment as defined in the relevant benchmark standards (e.g. BS 6423,
BS 6626 and BS 6867) and as prescribed in manufacturer’s instructions. Failure
to undertake post-fault inspection and maintenance of equipment as defined in
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the relevant benchmark standards (e.g. BS 6423, BS 6626 and BS 6867) and as
prescribed in manufacturer’s instructions. Failure to undertake a periodic fit for
purpose/ageing equipment assessment or have adequate procedures for the
management of change.
−
− Frequency of operation – maintenance planning should consider the frequency of
operation to prevent an increased arc flash risk. An increased risk could result from
either over-operation leading to component fatigue or under-operation resulting in
seizure or the loss of lubrication of moving components.
−
− Overstressed equipment – an increase above equipment rating of either the
continuous normal operating load, or the potential load under short circuit fault
conditions, increases the risk of an arc flash. An overload may result under abnormal
conditions such as during testing or system reconfiguration and may not be
immediately obvious. See HSG 230, Keeping electrical switchgear safe.
−
− Equipment contamination – a common cause of arc flash is a change to the physical
environmentinwhichequipmentiscontained.Anincreaseinconcentrationofairborne
dust may occur due to a change in site manufacturing processes. Degradation of
building components, such as roofs or drainage, or changes to groundwater levels,
can also result in an arc flash through direct contamination of electrical equipment.
−
− Atmospheric conditions – high humidity or temperature changes can result in either
condensation or a decrease in the electrical insulating strength of air and increase the
risk of arc flash. Because condensation and/or high humidity may only occur during
specific weather conditions or due to failure of/changes to local building heating,
ventilation or air conditioning equipment, this issue may not be immediately obvious.
2.3.3 Human and organisational factors
Human and organisational factors refers to aspects of the task, the environment, or the people
involved that make human error more likely. Human error should be actively prevented by the
correct management of operational/task procedures, personnel competence, the operational
environment, and other human and organisational factors issues (e.g. fatigue, ergonomics,
supervision, etc.). The following management and individual failures result in many common
causes of arc flash incidents:
−
− failing to ensure that correct procedures exist, are regularly updated based on
operational experience, and are enforced to ensure tasks are performed correctly;
−
− allowing unauthorised access to hazardous electrical locations, especially to those
with no relevant training;
−
− lack of training of electrical system operators for racking in/out power circuit breakers;
−
− failing to ensure personnel who have permission to access and perform tasks on
electrical equipment are competent and adequately trained;
−
− racking in/out circuit breakers while the bus is energised;
−
− failure to consider and communicate the correct course of action when equipment
fails, and
−
− use of inadequately rated or operated metering and/or isolating equipment.
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2.4 POTENTIAL HIGH RISK TASKS
2.4.1 Deliberate and inadvertent live working
Deliberate live working is where a deliberate decision has been taken to work live (see 2.2).
For want of a better phrase, 'inadvertent' live working in this case refers to the following:
−
− Work potentially carried out live through poor management of the risk.
−
− Work potentially carried out live in spite of management of the risk, e.g. situations
where it is reasonable to believe equipment is dead but this cannot be proven
without testing. Often the tests required to determine arc flash/electrocution risk
involve a risk of inadvertent live working, i.e. even if the risk has been managed
(e.g. equipment de-energised), in theory the work to test that equipment may still
be being conducted live unintentionally if, for any reason, the de-energisation of the
equipment failed.
The following tasks carry a high risk of inadvertent live working and therefore have the
potential to cause an arc flash:
−
− proving a circuit dead;
−
− applying or removing temporary earths on phase conductors;
−
− fault finding, performing measurements with a temporary portable multi-meter or
voltage meter, and
−
− removal of a bolted or hinged door or cover that could result in exposed live conductors.
2.4.2 Disturbance of conductors during normal operational tasks
Normal operational tasks may carry an increased likelihood of arc flash if the task involves the
disturbance or movement of conductors that have not been correctly isolated and proven
dead. The reason for this is the potential development, caused by increased or reduced
conductor separation, of an electric field of sufficient magnitude to cause the breakdown of
insulating air (described in 2.1).
If moving conductors and surrounding conductors are not fully enclosed within their specified
and designed enclosures there is potentially an increased risk of arc flash. A further increase
in likelihood of arc flash occurs when the speed, direction or force with which conductors
are moved is not controlled by mechanical and stored energy systems. Examples of specific
potentially high risk normal operational tasks are listed as follows (the list is not intended to be
exhaustive):
−
− Removal or insertion of fuses into a circuit.
−
− Removal or insertion of a switch or voltage transformer (withdrawal/racking).
−
− Removal or insertion (racking) of a switch, circuit breaker, or low voltage motor
control centre into an earth connection/position.
−
− Operation of a manually operated and powered switch.
−
− The operation of switches or circuit breakers with a design capability limited to
opening or closing during normal load current flow – this carries an increased risk of
arc flash from inadvertent overload.
−
− Operation of 'dependent manually operated' switchgear and switchgear without
'anti-reflex' functionality – this carries an increased risk of arc flash and should be
avoided.
−
− Any task within the location of electrical equipment that is not installed, maintained
and managed correctly – this has a potentially high arc flash risk.
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3 DANGERS
An arc flash has the potential to cause death or serious injury as a result of shock, arc-
flash burn, thermal burn, or blast due to the uncontrolled release of a high energy gas
plasma when personnel are nearer than a safe distance to the arc flash. The potential injuries
are summarised in 1.4.3. The potential severity of the danger is location-specific. Published
guidance and methods for estimating the potential danger exist only for the danger of
burning (these methods are detailed in Annex C). Death caused by burning often results from
shock, organ failure or secondary infection and as such the level of heat exposure required
to cause death is not quantified by published guidance.
3.1 FACTORS THAT AFFECT ARC FLASH DANGERS
The factors that affect the dangers of an arc flash are described in this section, and are listed
in order of certainty and significance, with the most certain and significant listed first.
3.1.1 The proximity of personnel to the arc flash
The dangers an arc flash poses to people reduces by increasing their distance to the energised
equipment. For example, an increase in distance of 1 m can provide a very significant
reduction in the danger resulting from the heat of the arc flash; this is illustrated in Figure 1.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 2 4 6 8 10
Heat
exposure
(%
of
incident
energy
at
the
standard
working
distance)
Working distance (m)
Incident Energy Exposure Variation with Working Distance
IEEE 1584 '<1kV
Switchgear'
IEEE 1584 <15 kV 'Open
Air'
IEEE 1584 '>1kV&<15kV
Switchgear'
Figure 1: Incident energy variation with working distance (IEEE 1584 empirical
model)
3.1.2 The duration of the arc flash
The dangers of an arc flash increase with its duration. The duration is controlled by the trip
time of electrical circuit protection devices, examples of which are fuses or circuit breakers.
The trip time is the most significant electrical variable impacting the danger resulting from
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the heat of the arc flash, but it may not reduce the other dangers to personnel. The variation
of incident energy with trip time is illustrated in Figure 2. If the trip time is halved the incident
energy is halved.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0.00
0.50
1.00
1.50
2.00
Incident
energy
(%
of
energy
calculated
for
2s
trip
time)
Trip time (seconds)
Incident Energy Variation with Trip Time
(IEEE 1584)
Figure 2: Incident energy variation with trip time
3.1.3 Electrical equipment design and condition
An enclosure may provide some protection to personnel from the dangers of an arc flash;
however, it is also possible that an enclosure may amplify the dangers by concentrating energy
or introducing projectiles. The design and physical condition of any electrical equipment
enclosure will impact the dangers resulting from an arc flash. A reduction in danger can only
be assessed if the equipment is successfully type tested to one of the arc flash containment
standards as part of the original equipment manufacturer's design verification as discussed
in 4.2.2.
3.1.4 Electrical equipment location
The danger resulting from an arc flash may be influenced by the dimensions and layout
of the location. A larger and less congested location will assist with the dissipation of heat
and gases away from personnel, whereas a smaller more congested location will result in
concentration of some of the arc flash dangers.
Restrictions posed by the location (e.g. a tight space) may prevent or delay the ability of
personnel to move away from the arc flash, and therefore potentially increase the dangers.
The proximity of other potential hazards in the location to which an arc flash may be a
catalyst, such as toxic or flammable substances stored next to electrical equipment, will
increase the dangers.
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3.1.5 Short circuit current
The potential magnitude and variation in single or three-phase short circuit current magnitude
(alternatively expressed as electrical supply short circuit impedance) at a particular location
will impact the potential dangers. The relationship between short circuit current and arc
flash danger is complex and often counter-intuitive. An increase in short circuit current at a
particular location may decrease or increase the potential danger; the impact this will have on
danger is entirely dependent on the electrical protection devices installed and their operating
characteristics (the variation in fault clearance time with fault level).
3.1.6 Others factors
The following factors may also influence the dangers of an arc flash, either positively or
negatively, but are listed here because they cannot be controlled, their influence is relatively
small and/or their influence is complex and indirect via changes to the primary factors listed
previously:
−
− equipment/conductor design and operating voltage;
−
− conductor spacing;
−
− system earthing, and
−
− equipment topology/geometry.
3.1.7 Common relationships between factors
The common relationship between the following factors prevents intuitive 'rules-of-thumb'
to be derived for their influence on the arc flash danger:
−
− A decrease in arcing fault current can result in an increase in arc flash duration.
−
− As equipment rated voltage increases the conductor spacing increases and the typical
working distance between personnel and an arc energy flash increases.
−
− An increase in operating voltage can result in an increase to arcing current, towards
the magnitude of bolted fault current.
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4 MANAGING THE RISK
The first step in managing arc flash risk is, whilst considering any existing controls in place, to
assess the likelihood and consequences of an arc flash. An example risk assessment template
is provided in 4.1 with guidance on how to determine the likelihood and potential health
and safety consequences. Unless the risk is as low as reasonably practicable (ALARP) then it
is recommended that control measures are implemented to further reduce the risk; guidance
on selecting control measures is provided in 4.2. Note that certain control measures may be
required by local regulation (see A.3).
The process of assessing risk and concluding what controls are reasonably practicable is
subjective and each individual/company will have their preferred methods and approach. The
methods/approach included in this document are intended to guide the risk assessor and can
be adapted to suit their templates and methods.
4.1 RISK ASSESSMENT
To assess the risk of arc flash, the potential likelihood of an arc flash and its potential health
and safety consequences should be determined, taking into consideration any existing
controls that are already in place to reduce its likelihood or consequences. The resulting arc
flash risk will be the combined impact of both these conclusions and this can be presented
within a risk assessment matrix, an example of which is illustrated in Figure 3.
Theoretical Improbable Remote Rare Possible Likely Frequent
Low level safety
incident
Medium level
safety incident
Lost time
accident
Fatality or
serious injury
Likelihood
Health
and
safety
consequence
Catastrophe
Multiple fatalities
some off site
Tolerable risk
Action level
Intolerable risk
Figure 3: Example arc flash risk assessment matrix
The example risk assessment matrix provides three classifications of risk denoted by the colour
of the cell, either green, amber or red. The example classifications are described as follows:
−
− Green (tolerable risk): the arc flash risk is as low as practicable.
−
− Amber (action level): the arc flash risk is relatively low but additional control measures
to reduce the risk further should be considered.
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−
− Red (intolerable risk): the arc flash risk is significant and the task assessed should not
be performed until rigorous controls are in place to reduce or eliminate the risk.
The areas of the matrix that are grey are not usually considered applicable to an arc flash
incident alone (but will likely be classified as red or even dark red) but are more applicable
when considering whether the arc flash can act as an ignition source, e.g. when considering
oil-filled switchgear, local fuel storage, etc.
The classifications (either the terminology used or the numbers and placement of colours,
etc.) can be modified to align with the organisation’s existing risk assessment process if
necessary. Some organisations may also choose to add a numerical indicator of risk to each
box in the grid, often as a means to classify risk levels for internal communication.
Notethat,althoughthelikelihoodassessmentisdiscussedbeforetheconsequenceassessment,
the order in which the assessment takes place is not critical, as the risk level is a combination
of both assessments. However, this publication does recommend that prevention controls (to
reduce the likelihood of arc flash) should be implemented before mitigation measures that
simply reduce the danger to personnel in the event of an arc flash.
When conducting the risk assessment, the risk assessor may need to make use of data
gathered elsewhere, e.g. from previous risk assessments, procedures, bowtie diagrams,
accident reports, etc. all of which may contain useful information on the controls that are/
should be in place and how they may fail (or have failed in the past).
4.1.1 Assessing likelihood
It is recommended that the likelihood of an arc flash is determined on a scale, ideally the
same one used for the assessment of other risks within the organisation. A typical scale,
with a range of examples to help the risk assessor determine where on the scale to place the
likelihood, is illustrated in Table 1. The range of descriptions in each row is provided to help
the risk assessor determine the most suitable likelihood on the scale. The organisation may
wish to provide alternative or additional descriptions.
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Table
1:
Example
arc
flash
likelihood
scale
Likelihood
scale
(answer
based
upon
the
following)
Theoretical
Improbable
Remote
Rare
Possible
Likely
Frequent
Experience
A
similar
event
has
not
yet
occurred
in
our
industry
and
would
only
be
a
remote
possibility
A
similar
event
has
not
yet
occurred
in
our
industry
A
similar
event
has
occurred
somewhere
in
our
industry
A
similar
event
has
occurred
somewhere
in
the
organisation
A
similar
event
has
occurred,
or
is
likely
to
occur,
within
the
lifetime
of
10
similar
facilities
The
event
is
likely
to
occur
several
times
in
the
facility’s
lifetime
This
is
a
common
occurrence
(at
least
annually)
at
the
facility
Controls
This
is
a
non-
foreseeable
event
with
current
controls
This
is
a
credible
event
but
requires
the
failure
of
several
layers
of
protection
This
is
a
foreseeable
event
but
requires
the
failure
of
more
than
one
layer
of
protection
An
event
has
occurred
in
industry
on
similar
equipment/
plant
with
similar
controls
With
current
controls,
an
event
could
occur
during
the
remaining
lifetime
of
the
facility
With
current
controls,
an
event
has
occurred
during
the
lifetime
of
the
facility
With
current
controls,
an
event
occurs
annually
or
more
often
Equipment
condition
The
operating
conditions,
age
and
design
of
the
plant
give
no
reason
for
concern
when
considering
potential
mechanisms
of
degradation
or
failure
The
operating
conditions,
age
and
design
of
plant
indicate
that
failure
is
credible;
however,
it
is
unlikely
with
current
mitigations
Active
mechanisms
of
degradation
or
failure
are
foreseeable
but
unlikely
to
cause
failure
at
the
present
time
There
are
potentially
active
mechanisms
of
degradation
or
failure
recognised
and
the
controls
in
place
leave
a
credible
residual
risk
of
failure
There
is
some
anticipation
of
failure.
The
potential
in
known
areas
is
controlled
but
with
some
residual
risk
There
is
anticipation
of
failure
of
a
specific
system
in
the
short/
medium
term
Incidents
have
occurred
recently
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Likelihood
scale
(answer
based
upon
the
following)
Theoretical
Improbable
Remote
Rare
Possible
Likely
Frequent
Site-specific
assessment
(see
Figure
4
and
Annex
D)
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Location-
specific
assessment
(see
Figure
4
and
Annex
D)
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Equipment-
specific
assessment
(see
Figure
4
and
Annex
D)
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Task-specific
assessment
(see
Figure
4
and
Annex
D)
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Table
1:
Example
arc
flash
likelihood
scale
(continued)
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In addition, Annex D provides four assessment questionnaires that can be used to help
determine in which area of the scale the likelihood is likely to reside (these are summarised
in Figure 4). The assessments include questions about the condition of equipment, site
or location, and about the task being performed, and capture the causes of arc flash
discussed in 2.3. The person conducting the risk assessment can answer either 'yes' or
'no'. The answers to these questions are positioned on a region of the likelihood scale to
indicate what influence the question will have on the likelihood of an arc flash. Furthermore,
based upon the guidance notes in the questionnaire and other knowledge that will likely
be known to the person conducting the risk assessment, the assessor may also be able
to determine the influence that the question will have on the likelihood of an arc flash
occurrence with greater accuracy than just these broader regions. For example, question S.1
relates to safety procedures, but clearly the quality of procedures can vary and therefore
the impact upon likelihood will also vary. Annex E provides an example assessment for a
typical electrical task.
Likelihood
assessment
Site-specific
assessment
S.1 Safety
procedures
S.2 Record
keeping
S.3 Asset
management
S.4 Site
maintenance
S.5 Condition
assessment
S.6 High risk
equipment
Location-specific
assessment
L.1 Exposed live
conductors
L.2 Known
equipment
problems
L.3 Access
control
L.4 Safety
controls
L.5
Environmental
issues
L.6 House-
keeping
Equipment-
specific
assessment
E.1 Age
E.2 Safety
notices/known
problems
E.3 Enclosure
and covers
E.4 Equipment
condition
E.5 Signs of
failure
E.6 Equipment
maintenance
Task-specific
assessment
T.1 Competence
T.2 Working
conditions
T.3 Exposed
conductors
T.4 Higher risk
task
Figure 4: Likelihood assessment overview
+44 (0)207 467 7100

25
4.1.2 Assessing danger
It is recommended that the health and safety consequences (danger) of an arc flash are
determined on a typical scale, ideally one used for the assessment of other health and safety
risks within an organisation. An example of a typical scale, with a range of examples to help
the risk assessor determine where on the scale to place the health and safety consequences,
is illustrated in Table 2. The other business risks described in 1.4 can be concluded using a
similar scale modified to suit the perceived dangers for a particular business; however, these
are beyond the scope of this publication.
Table 2: Example of unmitigated arc flash health and safety consequence scale
Example health
and safety
consequence
Description
example
Casualty example Example arc flash
incident energy
magnitude*
Catastrophe Catastrophic health/
safety incident
causing widespread
fatalities within or
outside a facility
>10 fatalities
(multiple fatalities
off-site)
This severity level from
a single arc flash event
cannot be achieved
without the influence of
a supplementary hazard
such as a process
explosion initiated by
the arc flash
Multiple fatalities,
some off-site
Very major health/
safety incident
2–10 fatalities (or 1
fatality off-site)
Fatality or serious
injury
Major health/safety
incident
1–2 person fatality or
permanent disability
incident
>1,2 calories/cm2
<1,2 calories/cm2
at
typical working distance
(lost time injury possible
due to injury to arms
and hands)
Lost time accident High impact health/
safety incident
Lost time accident
Medium level
safety incident
Medium impact
health/safety
incident
Medical treatment
injury (no time off
work)
Low level safety
incident
Low impact health/
safety incident
First aid treatment
(minor cuts and
grazes)
To assist with determining the potential health and safety consequences, a typical incident
energy value is included in the far right column of Table 2. The methods that can be used
to estimate arc flash incident energy are described in Annex C alongside details of the
complexities associated with this estimation. The energy levels in Table 2 are provided as
guidance only and in practice the transitions between consequences cannot be represented
by a specific level. Furthermore, the danger is not dependent on incident energy alone.
1,2 calories/cm2
(5 joules/cm2
) is widely recognised as the level of heat above which a second
degree burn is possible without additional mitigating controls. The heat level at which a third
degree burn is possible or at which death may be a consequence is less certain. For incident
energy levels above 40 calories/cm2
NFPA 70E recommends that greater emphasis should be
placed on de-energising equipment when there is an arc flash risk, as the sound, pressure
and concussive forces associated with an arc flash incident at working boundary would mean
+44 (0)207 467 7100

26
that PPE is ineffective (note that this does not mean that de-energising equipment should
only be considered at higher incident energy levels – indeed, de-energising equipment is the
preferred method irrespective of energy level involved).
Unless it is proven otherwise, it is recommended that the assumed unmitigated consequence
of an arc flash is the fatality of the person or persons performing the task.
In the case of equipment that has been tested and is proven to offer internal arc flash
containment and tested to a recognised standard, the potential consequence of an
internal arc flash should be found in the test report. It should be noted that any internal
arc flash protection proven by equipment manufacturers will be limited to specific
locations, tasks, arcing currents and durations and may not be applicable to maintenance
tasks performed on the equipment. Any specific equipment limitations of internal arc
flash containment should be understood when assessing the potential health and safety
consequences.
For clarity, Figure 5 illustrates how the examples in Tables 1 and 2 align with the risk
assessment matrix.
+44 (0)207 467 7100

27
Experience
A
similar
event
has
not
yet
occurred
in
our
industry
and
would
only
be
a
remote
possibility
A
similar
event
has
not
yet
occurred
in
our
industry
Similar
event
has
occurred
somewhere
in
our
industry
S
imilar
event
has
occurred
somewhere
in
the
organisation
Similar
event
has
occurred,
or
is
likely
to
occur,
within
the
lifetime
of
10
similar
facilities
Event
likely
to
occur
several
times
in
the
facility
lifetime
Common
occurrence
(at
least
annually)
at
the
facility
Controls
This
is
a
non-
foreseeable
event
with
current
controls
This
is
a
credible
event
but
requires
the
failure
of
several
layers
of
protection
This
is
a
foreseeable
event
but
requires
the
failure
of
more
than
one
layer
of
protection
An
event
has
occurred
in
industry
on
similar
equipment/plant
with
similar
controls
With
current
controls,
an
event
could
occur
during
the
remaining
life
time
of
the
facility
With
current
controls,
an
event
has
occurred
during
the
lifetime
of
the
facility
With
current
controls,
an
event
occurs
annually
or
more
often
Equipment
condition
The
operating
conditions,
age
and
design
of
the
plant
give
no
reason
for
concern
when
considering
potential
mechanisms
of
degradation
or
failure.
The
operating
conditions,
age
and
design
of
plant
indicate
that
failure
is
credible,
however
it
is
unlikely
with
current
mitigations.
Active
mechanisms
of
degradation
or
failure
are
foreseeable
but
unlikely
to
cause
failure
at
the
present
time.
There
are
potentially
active
mechanisms
of
degradation
or
failure
recognised
and
controls
in
place
leave
a
credible
residual
risk
of
failure.
There
is
some
anticipation
of
failure.
The
potential
in
known
areas
is
controlled
but
with
some
residual
risk.
There
is
anticipation
of
failure
of
a
specific
system
in
the
short/medium
term.
Incidents
have
occurred
recently.
Site-specific
assessment
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Location-specific
assessment
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Equipment-specific
assessment
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Task-specific
assessment
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Positive
response
to
all
areas
assessed
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Potential
for
improvement
identified
in
one
or
more
areas
Likelihood
Description
example
Casualty
example
Example
arc
flash
incident
energy
magnitude*
Catastrophic
health/
safety
incident
causing
widespread
fatalities
within
or
outside
a
facility
>
10
fatalities
(multiple
fatalities
off-site)
This
severity
level
from
a
single
arc
flash
event
cannot
be
achieved
without
the
influence
of
a
supplementary
hazard
such
as
a
process
explosion
initiated
by
the
arc
flash
Health
and
safety
consequence
Catastrophe
Very
major
health/
safety
incident
2
–
10
fatalities
(or
1
fatal
off-site)
Multiple
fatalities,
some
off-site
Major
health/safety
incident
1–2
person
fatal
or
permanent
disability
incident
>1.2
calories/cm2
Fatality
or
serious
injury
High
impact
health/
safety
incident
Lost
time
accident
Lost
time
accident
<1.2
calories/cm2
at
typical
working
distance
(lost
time
injury
possible
due
to
injury
to
arms
and
hands)
Medium
impact
health/safety
incident
Medical
treatment
injury
(no
time
off
work)
Medium
level
safety
incident
Low
impact
health/
safety
incident
First
aid
treatment
(minor
cuts
and
grazes)
Low
level
safety
incident
Tolerable
risk
Action
level
Intolerable
risk
Theoretical
Improbable
Remote
Rare
Possible
Likely
Frequent
Figure
5:
Risk
assessment
matrix
with
all
descriptions
(illustrated
for
context)
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28
4.2 RISK REDUCTION CONTROLS
As previously noted, the risk assessment considers the current controls already in place to
manage arc flash risk. Upon determining the risk level, where the risk is deemed unacceptable
or there is a need or desire to reduce the risk further, additional controls should be considered,
and furthermore the effect that these additional controls will have on the risk level should
be considered in order to determine whether they would reduce the risk level to a level
acceptable to the organisation (see 4.3).
The controls to reduce the risk of arc flash are listed according to a hierarchy of control,
with a specific focus on passive and active technical controls to consider before use of PPE is
contemplated. Those higher in the hierarchy (ERICPD) should be considered first:
−
− Eliminate – redesign the task to remove the hazard.
−
− Replace – replace the hazard with an alternative.
−
− Isolate – use engineering controls to prevent the hazard.
−
− Control – use administrative controls to reduce the hazard.
−
− PPE.
−
− Discipline – effective communication and compliance assurance.
The controls are separated into 'prevention' and 'mitigation' controls. Generally speaking,
the prevention controls aim to reduce arc flash likelihood, and are listed in 4.2.1 – this is so
that recommendations can be made against each question in Annex D. Annex E illustrates
how these assessments can be used to identify the controls that will be most effective in
reducing the risk. Mitigation controls typically aim to reduce health and safety consequences
to personnel and are typically listed in 4.2.2. Note that this is not a perfect division, as there
is overlap between the types of risks the controls are attempting to address (for example,
the information in switchroom signage might help prevent or mitigate the effects of an arc
flash depending on what information is being provided). However, the controls are divided
this way within this publication to align with the risk assessment methodology described, to
make it easier for the reader to consider the most appropriate measures.
Note that for newly designed sites, arc flash mitigation measures could potentially be built
into the design, whereas for existing sites it may be difficult to retroactively modify the site
to reduce incident energy levels.
It should be noted that the controls provided in 4.2.1 and 4.2.2 are examples only. This
publication does not intend to provide a definitive list of controls, and the success of these
controls will likely require procedures, competence assurance, maintenance, risk assessments,
the following of management of change processes, adequate supervision of safety-critical
tasks, etc. but, for reasons of brevity, cannot be described within this publication.
4.2.1 Prevention controls
Site-specific controls
−
− Eliminate:
−
− At a site level, the philosophy for asset management should be to eliminate
arc flash risk where reasonably practicable. This could be considered during
the development of electrical projects where the opportunity may arise to
modify electrical systems to remove the arc flash hazard.
+44 (0)207 467 7100

29
−
− Replace:
−
−
At a site level, the philosophy for asset management should then be to
consider the replacement of equipment, where reasonably practicable,
assessed as having a high arc flash risk potential. This could address individual
pieces of equipment as well as significant parts of the electrical infrastructure
(e.g. entire switchboards).
−
− Sites should be built to applicable codes and standards, and quality assured
during installation of plant and equipment.
−
− If the site contains dependent manually operated switchgear then it should
be considered whether this can be replaced/retrofitted with a mechanically
operated type.
−
− Confirm electrical equipment ratings are adequate for the environment
in which they are contained. If any significant electrical or environmental
changes have occurred on the site since equipment adequacy checks were
performed then the adequacy checks should be repeated and alternative
equipment installed where necessary.
−
− Control:
−
− Electrical safety procedures should be developed, communicated and
enforced to ensure live working is prevented, unless the special circumstances
described in 2.2 can be proven.
−
− Ensure up-to-date documentation exists. Asset registers and electrical single
line diagrams should be maintained and accurately reflect actual electrical
network topology and signage.
−
− Equipment safety alerts, which inform of known equipment defects, should
be actively monitored. Ensure safety alerts are clearly communicated and any
recommended mitigating actions are undertaken. If active monitoring has
not been performed for a period of time, equipment manufacturers should
be contacted directly to check for any safety alerts.
−
− Ensure maintenance is adequate. An asset register and maintenance schedule
should be developed and maintained to ensure equipment is adequately
maintained in accordance with manufacturer’s instructions and, if necessary,
considering frequency of use and operational experience. This should also
consider isolation arrangements, access to components, location, etc.
−
− Discipline:
−
− A site-specific audit process should be established to measure compliance
against applicable electrical legislation, standards, codes of practice and
guidance.
Location-specific controls
−
− Isolate:
−
− Access to areas containing potentially hazardous electrical equipment should
be restricted to the minimum number of personnel necessary to perform the
task safely and restricted to competent persons only. Switchrooms should
be locked and access to keys restricted to predetermined personnel. Extra
provision for supervision and training should be made available to ensure
safe undertaking of essential work and cleaning.
+44 (0)207 467 7100

30
−
− Ensure movement by personnel away from equipment and equipment rooms
is unobstructed.
−
− Control:
−
− Switchgear and other equipment should be managed in a way compliant
with industry guidelines, including monitoring its condition, maintenance,
testing and fault finding.
−
− Manage harsh environments. Wet, high-humidity or dusty environments
should be prevented in the locality of electrical equipment, or it should be
maintained within the rating of the electrical equipment.
−
− There should be clear signage in locations, e.g. on switchroom doors and on
individual switchgear warning of the risk of arc flash and, where possible,
providing more information (e.g. the size of arc flash boundary, the incident
energy level, class of PPE to wear, etc.).
−
− Communications between the location control function (e.g. Operations)
should be established to ensure that there is an awareness of the presence
of staff within electrical equipment areas.
−
− Discipline:
−
− Electrical equipment and its surrounding environment should be maintained
in a clean and tidy condition.
−
− The access and authorisation process for electrical equipment areas should
be reviewed on a regular basis.
−
− A location-specific audit process should be established to measure compliance
against applicable electrical legislation, standards, codes of practice and
guidance.
Equipment-specific controls
−
− Eliminate:
−
− The philosophy for equipmentspecific asset management should be to
eliminate arc flash risk where reasonably practicable.
−
− Replace:
−
− The philosophy for equipment-specific asset management should then be to
eliminate arc flash risk where reasonably practicable through the replacement
of high risk electrical equipment.
−
− Restrict short circuit levels and/or reduce fault clearance times.
−
− Isolate:
−
− Equipment architecture and forms of separation should be considered; if the
arc flash risk results from there being a potential for exposed live conductors,
then internal barriers and forms of separation within equipment should be
provided or improved. When replacement equipment is procured, consider
improving the specification of internal barriers and other forms of separation.
−
− Control:
−
− Where there is a risk of potential interference or inadvertent operation of the
equipment, the equipment should have suitable locking mechanisms (e.g.
earth trip push buttons).
+44 (0)207 467 7100

31
−
− Discipline:
−
− Electrical equipment enclosures and covers should be maintained in their
original state and be kept in a good condition.
−
− Prior to use, the condition of equipment should be investigated to determine
whether it is showing signs of excessive heat, noise, vibration or any signs of
the ingress of dust, water or other foreign objects.
Task-specific controls
−
− Eliminate:
−
− Live working should not take place unless a specific assessment reveals that
live working is the only reasonably practicable option. Live working should
not be performed unless the special circumstances described in 2.2 can
be proven. Tasks that result in a risk of live working (see 2.4.1), should be
avoided if a suitable alternative is available.
−
− Replace:
−
− Avoid high risk tasks. The high risk tasks described in 2.4 should be avoided if
a suitable alternative is available, for example circuit isolation via a local fault
make/break rated mechanically driven circuit breaker instead of the removal
of fuses.
−
− Control:
−
− All personnel in the location of potentially hazardous electrical equipment
should be informed of the potential arc flash risks and be trained and
competent for the task which they are expected to perform, including
trained to understand the risk of arc flash and, where appropriate, in its
management.
4.2.2 Mitigation controls
Increase working distance (eliminate)
An increase in the distance between personnel and the possible location of the arc flash will
reduce all types of the arc flash dangers to personnel. This reduction is illustrated for incident
energy only in Figure 1. The working distance can be increased by doing the following:
−
− Moving any existing barriers, rerouting existing walkways.
−
− Changing working procedures; for example, operation of alternative equipment via
existing remote control functionality.
−
− Performing the task from an alternative location with a lower risk. An example of this
would be switching from an alternative location such as an upstream or downstream
switch.
−
− Remote operation of equipment, e.g. by use of remote racking equipment.
Examples of ways this can be done include: extended racking tools; use of portable racking
devices; modified doors which allow racking with the main door shut; painted areas on the
floor around switchgear where the incident energy is below tolerable level (however, caution
is advised that such markings be suitably conservative so as not to create a false sense of
security); moving equipment (e.g. circuit breakers) to safe areas to be worked on; using
remotely controlled devices/robots, etc.
+44 (0)207 467 7100

32
Reduce arc flash duration (replace)
Reducing the duration of an arc flash is the most significant electrical change impacting the
potential heat related dangers to personnel, although it may not decrease the other dangers
listed in 1.4.3. This reduction is illustrated for incident energy only in Figure 2. The duration of
a potential arc flash can be reduced by implementing one or more of the following controls:
−
− A permanent change to existing protection device settings.1
It should be borne in
mind that any changes should be assessed, approved and well documented under a
management of change process.
−
− A temporary change to existing protection device settings during the duration of a
task. Some relays have a maintenance option to enable an easy and timed transition
to alternative settings.1
However, caution is strongly advised. If the change is timed
and the work takes longer than planned, this can have consequences for either the
current work or other planned tasks. If the change is conducted manually, controls
should be put in place to prevent or catch errors and to ensure (and assure) that the
settings have been changed back to their original state.
−
− Specification of a dedicated arc flash protection device (these may need to be
upgraded if already present). Such devices may consist of optical sensors (arc flash
detection relays), and a fast acting circuit breaker or arc current diverter (eliminator
devices, terminator devices), along with an alarm warning that the device has tripped
(prompting control room operators to take appropriate action). Note that there is a
danger if protection coordination clearance times are excessive, or inappropriately
rated protection elements are used. Even when settings of protective devices provide
protection, use of longer than required coordination time intervals between adjacent
protective devices can lead to sustained fault currents and high incident energy
arcing faults. Protection studies should be independently reviewed.
−
− Specification of a fast acting unit protection scheme. The resulting protected zone
and resilience of the scheme during an arc flash should be carefully considered.
Reduce arc flash current (replace)
Reducing the arc flash current is likely to reduce the potential pressure wave (arc blast) resulting
from an arc flash, but may actually increase the heat related dangers to personnel as the fault
duration may be extended; this should be fully understood prior to pursuing this method of
control. Arc flash current can be reduced by one or more of the following methods:
−
− Prevention of parallel supplies via a robust interlock arrangement.2
−
− Permanent or temporary removal of parallel supplies such as transformers and/or
generators.2, 3
−
− Specification of replacement equipment, such as transformer, with a higher
impedance.3
−
− Install a guaranteed method of fault level reduction such as a fault-limiting reactor. 3
1
The modification of protection settings to reduce trip time may 1) increase the risk of tripping during transient
events such as motor starting or transformer inrush and 2) decrease security of supply in the event of an arc flash
or other fault conditions; these potential impacts should be fully understood by the person responsible for any
modifications.
2
The removal of parallel supplies to a location may decrease the security of supply (increased circuit outages) under
fault conditions. Preventing parallel supplies with an interlock may result in the need for a circuit outages during
maintenance or repair scenarios.
3
Increasing supply impedance has the potential to negatively impact motor starting, system losses, voltage harmonic
and/or transient voltage distortion.
+44 (0)207 467 7100

33
Arc flash containment (isolate)
Electrical equipment designed and tested to prove a level of arc flash containment can be
specified and can provide enhanced protection to personnel from the dangers of an arc flash
during routine operation. The arc flash containment will be limited to particular maximum
current levels, arc flash durations, personnel locations, equipment tasks and arc flash hazards
dependent on mechanical barriers in place. Gaseous and/or vaporous products may still be
released from equipment, necessitating immediate evacuation or further ventilation. The
following standards provide a method for testing arc flash containment: IEEE C37.20.7-2007,
IEC 62271-200 and IEC TR 61641:2014.
Personnel protective equipment
Additional PPE over and above site norms can be specified to provide the last line of protection
of personnel to the dangers of an arc flash. NFPA 70E 2015 edition provides a tabulated
recommendation of PPE based on the potential incident energy which should be determined
using the methodologies provided in IEEE 1584. This assessment should be undertaken for
all credible network configurations.
The NFPA 70E recommendations are based on PPE rated with a minimum arc thermal
performance value (ATPV) which is tested to IEC 61482-1-1. If PPE tested to VDE 0682-306-1-2
is to be specified then it is recommended that the BGI/GUV-I 5188 E method is used to
calculate the required classification of PPE.
PPE should be of the correct rating for the risk (and not just the highest level of protection)
and should be appropriate for the worker, i.e. correctly fitting. Be aware that arc flash PPE
can be fairly cumbersome and so affect worker performance. This is one reason why it should
be considered a measure of last resort; another reason is that PPE only protects against some
of the consequences of arc flash (usually heat energy).
Note that personnel should be trained in the correct use of PPE. This should include providing
an understanding of which PPE is appropriate to use for what incident energy levels, how it
should be worn, and what its limitations (in terms of the protection it provides) are.
4.3 REASSESSMENT
When all the necessary control measures have been implemented the risk assessment should
be repeated. The objective of repeating the risk assessment is to determine whether an
acceptable level of risk has been achieved with the controls in place. Company policy will
dictate what is considered an acceptable level of risk and what controls are reasonably
practicable. In the case that the residual risk is still unacceptable then the task should not
proceed until the risk can be reduced to ALARP.
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