CHAPTER 5 - System Protection.pptx

BEX 42803/ BEF 33203/ BEE 4213 – Utilisation of Electrical Energy
• Earthing System
• Protection Against Electric Shocks
• Earth Faults Protection
• Protection Against Voltage Surges
2

• Earth: The conductive mass of the Earth, whose
electric potential at any point is conventionally
taken as zero.
• An earth is defined as a connection to the
general mass of earth.
3

4
(1) Earth electrode
(2) Earthing conductor
(3) Protective conductor
(4) Electrical installation
(5) Bonding conductor
(6) Main earthing terminal
(7) Removable link

• A conductor or other metal is ‘earthed’ when it
is effectually connected to the general mass of
earth by means of a metal rod or a system of
metal water pipes or other conducting object.
• ‘Solidly earthed’ or ‘Bolted earthed’ when it is
earthed without the intervention of a fuse,
switch, circuit breaker , resistor, reactor, or
solenoid.
5

• Earthing is meant by having every item of
apparatus and every conductor shall be
prevented from giving rise to earth leakage
currents.
• It is carried out by ensuring any metal liable to
become charge should be earth and every part
of the earthing circuit should be properly
installed.
6

• Other alternatives to earthing that could be
employed are by having an all insulted
construction, double insulation and by having
an isolation.
• Earthing protects people and equipment from
potentially dangerous over-voltages and
leakages associated with electrical equipment
in homes, offices, retail outlets and industrial
plant.
7

Grounding
• Connecting equipment and points on electrical systems
to the earth or an earth substitute.
• Purpose is to limit overvoltages between the equipment
and the earth due to lightning, faults, etc.
Bonding
• Connecting equipment together and to the system
neutral point.
• Purpose is to limit voltages between equipment and to
provide a path for ground fault current.
8

9

• Reason (1) – Minimise overvoltages
10
Lightning
strike
Lightning
arrester
transformer
Service earth rod
Transformer earth rod
Utility neutral conductor
Utility phase conductor

• Reason (2) – Limit voltage potential on
equipment enclosures
11
400V motor
415V
feeder
conductors
Ungrounded
motor frame
Insulation
breakdown
240V
400V motor
Equipment
earthing
conductor
Grounded
motor frame
Insulation
breakdown
0V
Earthing surface Earthing surface

• Reason (3) – Provide a low-impedance path for
fault current
12
400V motor
415V
feeder
conductors
Ungrounded
motor frame
Insulation
breakdown
400V motor
Equipment
earthing
conductor
Grounded
motor frame
Insulation
breakdown
IF
IF
I = 0A
IF
IF
Overcurrent
devices
Overcurrent
devices

• Designing a safe earthing system means
providing the easiest and shortest path for the
fault current without exposing a person to
electric shock.
• Satisfactory earthing is the most important part
of an electrical installation because operation
of all the protective devices depend upon it.
13

• The total impedance of the conductor,
connecting the metal-work of the installation
to the earth electrode (earth continuity
conductor) measured between the earth
electrode and any other part of the installation
at supply frequency should not exceed 1.0 .
• If the resistance is higher than 1.0  (excessive
resistance), it is an indication of improper
earthing.
14

• Earth lug terminal rusty
• Loose wire connection
• Layers of paint on the electric apparatus
• Loose connection of earth wire to the plug and
socket outlet
• Loose connection between conduit and
terminal box
15

• T = Terre (French word for earth)
• I = Isolation (all live parts isolated from earth or
supply connected to earth through an impedance
for isolation.
• N = Neutral (in AC system, the earth point is
normally the neutral point).
• S = Separé (neutral and protective functions
provided by separate conductors).
• C = Combiné (neutral and protective functions
combined in a single conductor, PEN).
16

• TT system (earthed neutral)
17
One point at the supply source is connected directly to earth. All exposed- and
extraneous-conductive-parts are connected to a separate earth electrode at the
installation. This electrode may or may not be electrically independent of the source
electrode.

• TN-C system
18
The neutral conductor is also used as a protective conductor and is referred to as a
PEN (Protective Earth and Neutral) conductor.

• TN-S system
19
The protective conductor and the neutral conductor are separate. On
underground cable systems where lead-sheathed cables exist, the protective
conductor is generally the lead sheath.

• TN-C-S system
20
In the TN-C-S system, the TN-C (4 wires) system must never be used downstream of
the TN-S (5 wires) system, since any accidental interruption in the neutral on the
upstream part would lead to an interruption in the protective conductor in the
downstream part and therefore a danger.

• IT system (isolated neutral)
21
No intentional connection is made between the neutral point of the supply source
and earth. Exposed- and extraneous-conductive-parts of the installation are
connected to an earth electrode.

• IT system (impedance-earthed neutral)
22
An impedance Zs (in the order of 1,000 to 2,000 Ω) is connected permanently
between the neutral point of the transformer LV winding and earth

23

• Simplest solution to design and install. Used in
installations supplied directly by the public LV distribution
network.
• Does not require continuous monitoring during operation.
• Protection is ensured by special devices, the residual
current devices (RCD), which also prevent the risk of fire
when they are set to 500 mA.
• Each insulation fault results in an interruption in the
supply of power, however the outage is limited to the
faulty circuit by installing the RCDs in series or in parallel.
• Loads or parts of the installation which, during normal
operation, cause high leakage currents, require special
measures to avoid nuisance tripping.
24

25
TN-C system
TN-S system

• Requires the installation of earth electrodes at regular
intervals throughout the installation.
• Requires that the initial check on effective tripping for the
first insulation fault be carried out by calculations during the
design stage, followed by mandatory measurements to
confirm tripping during commissioning.
• Requires that any modification or extension be designed and
carried out by a qualified electrician.
• May result, in the case of insulation faults, in greater damage
to the windings of rotating machines.
• May, on premises with a risk of fire, represent a greater
danger due to the higher fault currents
26

27

• Solution offering the best continuity of service during
operation.
• Indication of the first insulation fault, followed by mandatory
location and clearing, ensures systematic prevention of
supply outages.
• Generally used in installations supplied by a private MV/LV or
LV/LV transformer.
• Requires maintenance personnel for monitoring and
operation.
• Requires a high level of insulation in the network (implies
breaking up the network if it is very large and the use of
circuit-separation transformers to supply loads with high
leakage currents).
28

• An electric shock is the pathophysiological effect
of an electric current through the human body.
• The degree of danger for the victim is a function of
the magnitude of the current, the parts of the
body through which the current passes, and the
duration of current flow.
• When a current greater than 30 mA passes
through a human being from one hand to feet, the
person concerned is likely to be killed, unless the
current is interrupted in a relatively short time.
29

30
Zones time/current of effects of AC current on human body when passing from left hand to feet

• A direct contact refers to a person coming into
contact with a conductor which is live in
normal circumstances.
31
Is = Touch current

• An indirect contact refers to a person coming
into contact with an exposed-conductive-part
which is not normally alive, but has become
alive accidentally (due to insulation failure or
some other cause).
32
Id = Insulation fault current
Is = Touch current

33

• Protection by the insulation of live parts
• Protection by means of barriers or enclosures
• Use of residual current operating device, which
operate at 30 mA or less, and are referred to as
RCDs of high sensitivity
34
RCD
Envelope
Insulation

• The earthing of all exposed-conductive-parts of
electrical equipment in the installation and the
constitution of an equi-potential bonding
network.
35
• Automatic disconnection of the
supply of the section of the
installation concerned, in such a
way that the touch-voltage/time
safety requirements are
respected for any level of touch
voltage (Vc).

Vc (V) 50 < Vc  120 120 < Vc  230 230 < Vc  400 Vc > 400
System
TN or IT 0.8 0.4 0.2 0.1
TT 0.3 0.2 0.07 0.04
36
• Maximum safe duration (disconnecting time) of
the assumed values of AC touch voltage (in
seconds) not exceeding 32 A:

37

• The impedance of the earth-fault loop consists mainly
in the two earth electrodes (i.e. the source and
installation electrodes) in series.
• The magnitude of the earth fault current is generally
too small to operate overcurrent relay or fuses, and
the use of a residual current operated device is
essential.
• Protection by automatic disconnection of the supply
used in TT system is by RCD of sensitivity:
38
A
n
R
I
50

 RA is the resistance of the earth electrode for the installation

39
TN-C

• In all TN systems, any insulation fault to earth
results in a phase to neutral short-circuit.
• High fault current levels allow to use overcurrent
protection but can give rise to touch voltages
exceeding 50% of the phase to neutral voltage at
the fault position during the short disconnection
time.
• The use of CB, fuses, and RCDs may be necessary
on TN-earthed systems.
40

41

• A permanent monitoring of the insulation to earth
must be provided, coupled with an alarm signal (audio
and/or flashing lights, etc.) operating in the event of a
first earth fault.
• During a phase to earth fault, the current passing
through the electrode resistance RnA is the vector sum
of the capacitive currents in the two healthy phases.
• The use of CB, fuses, and RCDs may be necessary on
IT-earthed systems
42

• Earth leakage devices are based on the principle
that the amount of current entering a device
should be exactly the same as the amount of
current leaving the device.
• Earth leakage protection devices are designed to
trip for fault currents between 10 and 100 mA and
for interrupt times between 40 and 100
milliseconds after a fault current is sensed.
• It has been gradually replaced by Residue Current
Devices (RCD) such as RCCB.
43

44

• For regular houses 40A / 63A, single phase or
depends on supply demand and main switch
capacity, ELCB's sensitivity is not exceeding
100mA.
• For heater water, it should posses separate
additional ELCB (other than no. 1 above) with
sensitivity is not exceeding 10mA is needed.
• For load which higher than 100A, 3 phases,
ELCB's sensitivity must not exceed 100mA.
45

46
3wire 3phase supply
Circuit Breaker
Trip coil
Current Transformer
To load
earth
fault

• During normal operation the current from supply L flows
through N1, to the load and then flow through N2 before
returning to N.
• Since the number of turns of N1 and N2 are the same
and the current through them is also the same the
resulting flux in the iron core is zero.
• However, if there is a leakage from the load to the
ground, a certain amount of current will flow to the
ground.
• As such current through N1 and N2 will not be the same
resulting in some magnetic flux setting up in the core.
• The fault sensing winding will trip the tripping device if
the leakage current falls within 10 to 100 milliampere.
• A test circuit consisting of a push button and a test
resistor is normally included in the ELCB as shown. Its
function is to create an unbalance current in the N1 and
N2 windings, and the current flow through the test
resistor is great enough for the fault sensing winding to
trip the circuit breaker.
47

• There are two main reasons why RCD’s are used:
– To provide additional and a higher level of protection than
that given by direct earthing, against electric shock and also
against fire risk caused by earth leakage currents. Where
fuses and miniature circuit breakers (MCB’s) are the only
means of earth fault protection, it is possible for earth fault
currents to flow undetected and cause fire risk (or touch
voltage problems).
– The use of an RCD will prevent the flow of a sustained
leakage current above the sensitivity of the RCD thus
greatly reducing shock and fire risk. All live conductors in
the protected circuits should be disconnected in the event
of earth leakage current flowing.
48

• Terms associated with RCD’s:
– RCCB:
Residual Current Circuit Breaker used in distribution boards to protect individual
or groups of circuits
– RCBO:
Residual Circuit Breaker with overcurrent protection. This is a combined
MCB/RCD and provides overload, short circuit and earth fault protection in one
unit
– SRCD:
Socket outlet with combined RCD
– PRCD:
This is a portable RCD unit with an inbuilt plug top and socket
outlet
49

• Single Phase RCD
50
LOAD
Relay
Neutral
Phase
Detection Coil
Test Button

• Three Phase RCD
51
Earthed
Metalwork
Amplifier
Magnetic Core
L1
L3
L2
Test Resistor
LOAD
Trip Relay
Neutral
Detection Coil
Test Button

Regulation D22 (Basic Earthing Requirements)
States that earth leakage protection may be provided by means of
fuses or excess current circuit breakers if the earth fault current
available to operate the protective device and so make the faulty
circuit dead exceeds:
1. 3 times the current rating of any semi enclosed fuse or any
cartridge fuse having a fusing factor exceeding 1.5, used to
protect the circuit, or
2. 2.4 times the rating of any cartridge fuse having a fusing
factor not exceeding 1.5, used to protect the circuit, or
3. 1.5 times the tripping current of any excess current circuit
breaker used to protect the circuit.
52

• A single phase 240 V, 15 kW 50Hz motor circuit
operating at 0.8 power factor lagging is protected by a
cartridge fuse having blowing current of 110 A. A fault
occurs in the circuit causes a current of 220 amperes to
flow through the earth continuity path. As a result of
poor contact due to a lock nut and bush connecting a
steel conduit to a metal box, the resistance of this
conduit connection alone is 1.35 Ω. State:
a) whether the fuse will rupture
b) the amount of heat produced at the metal box
c) the degree of risk, if any, of a fire developing
53

• Yes. The rating current:
Fusing factor:
• I2R = (220)2 x 1.35 Ohms = 65.34 kW.
• High Risk
54
A
V
kW
Ir 125
.
78
8
.
0
240
15



41
.
1
125
.
78
110


A
A
FF
Since according to regulation
D22, the protection is by a
cartridge having a fusing factor
not exceeding 1.5. Then the
maximum current in the fault is
2.4 x 78.125 A = 187.5 A.

• What is a voltage surge?
– A voltage surge is a voltage impulse or wave which
is superposed on the rated network voltage.
55

• Voltage surge is characterised by:
– The rise time (tf) measured in μs
– The gradient (S) measured in kV/μs
• A voltage surge disturbs equipment and causes
electromagnetic radiation.
• Furthermore, the duration of the voltage surge
(T) causes a surge of energy in the electrical
circuits which is likely to destroy the
equipment.
56

57

• Atmospheric voltage surges
• Operating voltage surges
• Transient overvoltage at industrial frequency
• Voltage surges caused by electrostatic
discharge
58

• Lightning risk. Between 2,000 and 5,000 storms are
constantly forming around the earth. These storms
are accompanied by lightning which constitutes a
serious risk for both people and equipment.
• Strokes of lightning hit the ground at a rate of 30 to
100 strokes per second.
• Lightning affects transformers, electricity meters,
household appliances, and all electrical and
electronic installations in the residential sector and
in industry.
59

• Lightning discharge values given by the IEC
lightning protection committee:
60
Beyond peak
probability
P%
Current peak,
I (kA)
Gradient,
S (kA/μs)
Total duration
(s)
Number of
discharges
n
95 7 9.1 0.001 1
50 33 24 0.01 2
5 85 65 1.1 6

• A sudden change in the established operating
conditions in an electrical network causes
transient phenomena to occur.
• These are generally high frequency or damped
oscillation voltage surge waves.
61

• The opening of protection devices (fuse, circuit-
breaker), and the opening or closing of control
devices (relays, contactors, etc.).
• Inductive circuits due to motors starting and
stopping, or the opening of transformers such as
MV/LV substations.
• Capacitive circuits due to the connection of
capacitor banks to the network.
• All devices that contain a coil, a capacitor or a
transformer at the power supply inlet: relays,
contactors, television sets, printers, computers,
electric ovens, filters, etc.
62

• These overvoltages have the same frequency as
the network (50, 60 or 400 Hz).
63

• Phase/frame or phase/earth insulating faults on a
network with an insulated or impedance-neutral,
or by the breakdown of the neutral conductor.
When this happens, single phase devices will be
supplied in 400 V instead of 230 V.
• A cable breakdown. For example, a medium
voltage cable which falls on a low voltage line.
• The arcing of a high or medium voltage protective
spark-gap causing a rise in earth potential during
the action of the protection devices.
64

• In a dry environment, electrical charges accumulate
and create a very strong electrostatic field.
• For example, a person walking on carpet with
insulating soles will become electrically charged to a
voltage of several kilovolts. If the person walks close
to a conductive structure, he will give off an
electrical discharge of several amperes in a very
short rise time of a few nanoseconds.
• If the structure contains sensitive electronics, a
computer for example, its components or circuit
boards maybe damaged.
65

• Primary protection devices (protection of
installations against lightning).
• Secondary protection devices (protection of
internal installations against lightning).
66

• The purpose of primary protection devices is to
protect installations against direct strokes of
lightning.
• They catch and run the lightning current into the
ground. The principle is based on a protection area
determined by a structure which is higher than the
rest.
• There are three types of primary protection:
– Lightning conductors
– Overhead earth wires
– The meshed cage or Faraday cage
67

• The lightning conductor is
a tapered rod placed on
top of the building. It is
earthed by one or more
conductors (often copper
strips).
68

• These wires are stretched over the structure to be
protected. They are used for special structures: rocket
launch pads, military applications and lightning protection
cables for overhead high voltage power lines.
69

• This principle is used for
very sensitive buildings
housing computer or
integrated circuit
production equipment.
• It consists in
symmetrically multiplying
the number of down strips
outside the building.
70

• These handle the effects of atmospheric,
operating or industrial frequency voltage
surges.
• They can be classified according to the way
they are connected in an installation: serial or
parallel protection.
71

• This is connected in series to the power supply
wires of the system to be protected.
72

• Transformers
– They reduce voltage surges by inductor effect and make
certain harmonics disappear by coupling. This
protection is not very effective.
• Filters
– Based on components such as resistors, inductance
coils and capacitors they are suitable for voltage surges
caused by industrial and operation disturbance
corresponding to a clearly defined frequency band. This
protection device is not suitable for atmospheric
disturbance.
73

• Wave absorbers
– They are essentially made up of air inductance coils
which limit the voltage surges, and surge arresters
which absorb the currents. They are extremely
suitable for protecting sensitive electronic and
computing equipment.
74

• Network conditioners and static uninterrupted
power supplies (UPS)
– These devices are essentially used to protect highly
sensitive equipment, such as computer equipment,
which requires a high quality electrical power supply.
– They can be used to regulate the voltage and frequency,
stop interference and ensure a continuous electrical
power supply even in the event of a mains power
failure (for the UPS).
– On the other hand, they are not protected against
large, atmospheric type voltage surges against which it
is still necessary to use surge arresters.
75

• The parallel protection is adapted to any
installation power level. This type of
overvoltage protection is the most commonly
used.
76

• Main characteristics
– The rated voltage of the protection device must
correspond to the network voltage at the installation
terminals.
– When there is no voltage surge, a leakage current
should not go through the protection device which is on
standby.
– When a voltage surge above the allowable voltage
threshold of the installation to be protected occurs, the
protection device abruptly conducts the voltage surge
current to the earth by limiting the voltage to the
desired protection level, Vp.
77

• When the voltage surge disappears, the protection
device stops conducting and returns to standby
without a holding current. This is the ideal U/I
characteristic curve:
– The protection device response time (tr) must be as short
as possible to protect the installation as quickly as
possible.
– The protection device must have the capacity to be able
to conduct the energy caused by the foreseeable voltage
surge on the site to be protected.
– The surge arrester protection device must be able to
withstand the rated current, In.
78

(1) Voltage limiters
• They are used in MV/LV
substations at the
transformer output, in
IT earthing scheme.
• They can run voltage
surges to the earth,
especially industrial
frequency surges.
79

(2) LV surge arresters
• Low voltage surge arresters come in the form of
modules to be installed inside LV switchboard.
• They ensure secondary protection of nearby
elements but have a small flow capacity.
• Some are even built into loads although they
cannot protect against strong voltage surges.
80

(3) Low current surge arresters
• These protect telephone or switching networks
against voltage surges from the outside
(lightning), as well as from the inside (polluting
equipment, switchgear switching, etc.).
• Low current voltage surge arresters are also
installed in distribution boxes or built into
loads.
81

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