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Introduction to Electric Shock Protection
Lutfi Al-Sharif
Ph.D., C.Eng., M.I.E.E., D.B.A.
Team Delivery Manager (L&E/E&M), Station Systems
London Underground Ltd., United Kingdom
Elevator Technology 10, Proceedings of the International Conference
on Elevator Technology (Elevcon 2000), May 2000, Berlin, Germany
(Reprinted in: Lift Report [in English & German], March/April 2001
with permission & Elevator World, July 2001 (page 110-115) with
permission).
Key Words: Earthing, electric shock, equipotential bonding, electric current, residual current
devices.
ABSTRACT
This paper presents a general overview of the principles of electric shock and the systems of
protection used to prevent it in electrical installations. Although mainly built around the
United Kingdom Regulations, its principles can be applied to any country.
The paper first discusses electric shock and the effects of electric current on the human
body. It then outlines the types of earthing systems used. This then leads to the concept of an
electric fault, how it develops and how it presents a dangerous condition to people. Protective
devices are used to disconnect the supply in case of an electric fault, and these are discussed
next. The most used system is the “Equipotential Bonding and Automatic Disconnection of
Supply” system, which is further analysed.
The paper then finishes by discussing the role of Residual Current Devices (RCD) in
further electric shock protection.
1. INTRODUCTION
“Electric shock happens when the body becomes part of an energised electrical path and
energy is transferred between parts of the body, or through the body to ground or the earth. In
order for shock to occur, a potential difference or stored electrical charge must be present to
cause the current to flow. Current flowing through the highly sensitive central nervous system
can, under certain conditions, cause serious injury or death” (Casemore Electric). It is thus
important to ensure that, in the event of a fault within an electrical system, adequate protection
is provided for the users. We first look at the effects the electric current can have on the
human body.
2. ELECTRIC CURRENT EFFECTS ON THE HUMAN BODY
In the context of discussing the protection against electric shock, it is worth highlighting the
effect of the electric current on the human body. Five factors are important in understanding
the risk of electrical shock:
The magnitude of the current.
The duration of the current.
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The path it takes in the human body.
The resistance of the body.
The frequency of the electrical supply.
Table 1 shows a summary of the effects of the different magnitude of current at different
frequencies and against gender.
Table 1: Effect of electric current on the human body at various levels (Dalziel, 1961).
Current
(mA)
Effects
DC
AC
60 Hz
AC
10 kHz
Gender
M
F
M
F
M
F
Slight sensation on hand
1
0.6
0.4
0.3
7
5
Perception threshold (median)
6.2
3.5
1.1
0.7
12
8
Shock not painful and no loss of muscular control
9
6
1.8
1.2
17
11
Painful shock and muscular control lost by ½%
62
41
9
6
55
37
Painful shock- let go threshold median
76
51
16
10.5
75
50
Painful and severe shock - breathing difficulty, muscular
control lost by 99.5%
90
60
23
15
94
63
In view of the fact that the duration of the current is an important factor in the effect on the
human body as much as the magnitude of the current, a better way of showing the areas of risk
is to plot the zones against both magnitude and duration of current. This is shown in Figure 1,
which is extracted from IEC Publication 479, 1974, Zone of effects of A.C. Currents (50 or 60
Hz) on Adult Persons.
Zone of effects of A.C. currents on Adult Persons
10
100
1000
10000
0.1 110 100 1000 10000
Body Current (mA)
Time (ms)
Zone1
Zone 2
Zone 3
Zone 4
Zone 5
Figure 1: The various zones in relation to magnitude and duration of current (IEC 479,
1974).
The area is broken down into five zones. These zones are defined as follows:
• Zone 1: Usually no reaction effect
• Zone 2: Usually no pathophysiologically dangerous effect.
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• Zone 3: Usually no danger of fibrillation.
• Zone 4: Fibrillation possible (up to 50% probability).
• Zone 5: Fibrillation danger (more than 50% probability).
This emphasises the importance of not only considering the magnitude of current during an
electric shock, but also its duration. This can be clearly seen when examining the wiring
regulations, as they specify disconnection time of the supply in the case of fault. The rest of
this paper concentrates on the main method of preventing electric shock: Earthed
equipotential bonding and automatic disconnection of the supply.
3. TYPES OF EARTHING SYSTEMS
At the heart of any system for electric shock protection is an earthing system. To provide
earthing for an installation, an earthing terminal is needed. This is achieved using one of five
different methods:
TN-C System: In which the neutral and the earth terminals are combined.
TN-S System: In which the neutral and earth terminals are completely separated.
TN-C-S System: In which the neutral and earth terminals and combined, but the
separated just outside the consumer’s installation.
TT System: In which no earth terminal is provided, but the star point of the supply is
connected to the mass of the earth, and the consumer’s installation is also connected
to the mass of the earth. The earth at the consumer’s installation is usually provided
by installing local earthing systems (e.g., rods or mats).
IT System: This is similar to the TT system, with the difference that a resistor is
inserted between the star point of the supply and the earth.
Examples of the TN-S, TN-C-S and TT systems are shown in Figure 2, Figure 3 and Figure 4
respectively.
Source of Energy
Source earth
Consumer installation
L1
L2
L3
L1 L2 L3 N E
TN-S system
Exposed conductive parts
Neutral
Earth
Figure 2: TN -S system of earthing.
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Source of Energy
Combined protective and neutral conductor
PEN
Source earth
Consumer installation
L1
L2
L3
PEN
L1
L2
L3
N
E
TN-C-S system
Exposed conductive parts
Figure 3: TN-C-S system of earthing.
Source of Energy
Neutral conductor
Source earth
Consumer installation
L1
L2
L3
L1
L2
L3
N
E
TT system
N
Exposed conductive parts
Figure 4: TT earthing system.
In a TN-C-S system, the electricity supplier will provide a combined protective and neutral
conductor (PEN), which is split into a protective conductor (i.e., earth) and a neutral at the
input of the installation, as shown in Figure 3.
In general we can see that the main two types of systems are the TN type systems, in
which there is clear electrical path back to the star point of the energy source via the electrical
cable, and the TT system, in which the mass of earth completes the path back to the star point
of the electrical installation. The main difference between the two systems is in the value of
the earth loop impedance (i.e., the impedance of the return path), which is higher on the TT
system, compared to TN system. This affects the method of protection against electrical
shock.
4. EARTHING & BONDING
Before discussing how a fault takes place and how the protection system operates, it is
important to introduce the concept of earthing and bonding. An “exposed conductive part is a
conductive part of equipment which can be touched and which is not a live part but which
may become live under fault conditions.” (P 11, 16th edition, Wiring Regulations). We say
that a fault has taken place in a system if an exposed conductive part becomes live. For this
reason, any exposed conductive part of the installation should be connected to earth. This
ensures that a path exists for the fault current to flow, and to trip the automatic disconnection
devices. However, on its own this is not enough, as another risk also exists during a fault.
An extraneous conductive part is a conductive part liable to introduce a potential,
generally earth potential, and not forming part of the electrical installation. During the fault, a
person touching an exposed conductive part at the same time as he/she is touching an
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extraneous conductive part would be exposed to a dangerous potential difference. For this
reason, any metallic work which can introduce the earth potential (extraneous conductive
parts, e.g., a piece of metallic pipework) should also be bonded to the incoming earth
terminal. This ensures that during an electrical fault, and until automatic disconnection of the
supply takes place, no dangerous difference of potential exists between the metallic piece
which is connected to earth and the casing of the electrical equipment.
The conductor connecting all the exposed conductive parts to earth is called the cpc
(circuit protective conductor). The conductor connecting the extraneous conductive parts to
earth is called the main equipotential bonding conductor. The conductor connecting the
exposed conductive parts to the extraneous conductive parts is called the supplementary
bonding conductor.
Main
switchboard
Ground
Pipework
Protective
conductor
Label
Structure
Controller
A
Controller
B
Extraneous
conductive
part
Extraneous
conductive
part
cpc
cpc
Exposed
conductive
part
Exposed
conductive
part
Supplementary
equipotential
bonding
conductor
Supplementary equipotential
bonding conductor
Label
Main
equipotential
bonding
conductor
Figure 5: Diagram illustrating the concepts of equipotential bonding.
Figure 5 shows an example installation where there are two electrical controllers (e.g.,
controlling escalators) and a pipe and a piece of trusswork (representing extraneous
conductive parts). The use of the supplementary equipotential bonding is required when the
exposed conductive part and the extraneous conductive part are accessible simultaneously.
5. FAULTS CONDITIONS
As discussed in the last section, electric shock can take place if a fault develops within the
electrical installation, and a person is touching the exposed metalwork of an electric appliance
for example. When an earth fault takes place (i.e., a short circuit between a live conductor
and the earth or the exposed conductive parts), the fault current will flow from the live
conductor in the earth terminal and back to the source earth.
In a TN-C-S (Figure 6) the fault current will flow into the PEN conductor back to the
source of energy. On the other hand, in a TT system (Figure 7), the fault current will flow
back through the general earth path back to the source earth. This impedance is higher in
value than the PEN conductor impedance.
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Source of Energy
Combined protective
Source earth
L1
L2
L3
L1 L2 L3 N E
(PEN)
Exposed conductive parts Fault
Consumer installation
Z
L1
Z
PEN
and neutral conductor
Fault current
Figure 6: Fault current path in a TN-C-S
system during an earth fault.
Source of Energy
Neutral conductor
Source earth
L1
L2
L3
L1 L2 L3 N E
N
Exposed conductive parts
(effective earth impedance)
Fault
Fault current
Consumer installation
Z
L1
Z
E
Figure 7: Fault current path in a TT
system during an earth fault.
When a phase to earth fault takes place in an TN-C-S system, the value of the fault current is
usually very large. This is because of the low earth loop impedance achieved through the
PEN (combined protective and neutral conductor) return path. The principle of protection
relies on the fact that a fault current will flow from a live conductor through the earth path,
and will be of such a magnitude to trip the protection (a fuse or a circuit breaker). This
removes the source of danger for the person touching the exposed conductive part. It is
required that the supply be disconnected in less than 0.4 of a second. As the earth loop
impedance is low in the TN system, the value of the fault current is high enough to enable the
disconnection of the overcurrent devices (e.g., a fuse or a circuit breaker), which makes 0.4
second disconnection time feasible. If this disconnection time is not possible, then an upper
limit of 5 seconds is allowed, provided that the touch voltage during the fault does not exceed
50 V (see next section for explanation of touch voltages).
The fault current path during a fault in a TT system is shown in Figure 7. In this case,
the current flows back to the source’s earth through the general mass of earth. This has a
higher impedance value than that of a TN system. For these reasons the value of the fault
current is smaller that the TN system, and the value of the fault current might be too small to
trip the overcurrent protective device. In these cases it is recommended that an RCD is used,
which can limit the value of the touch voltage to 50 V as well (this is further discussed later in
the paper).
6. TOUCH VOLTAGES
During a fault, a touch voltage will develop on the exposed conductive part, which can present
a hazard to a person touching the exposed conductive part.
The touch voltage is the voltage which develops during a fault on the exposed
conductive part. It is this voltage that presents a hazard to the users of the system. Figure 8
shows how the touch voltage on the exposed conductive parts of equipment can be calculated
during a fault for a TN-C-S system.
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Source earth Fault
Z
L1
Z
PEN
Touch voltage during fault
Live conductor impedance
Vph VT
Exposed conductive parts
PEN
PEN impedance
Figure 8: Touch voltage during a fault in a
TN-C-S system.
Neutral conductor
Source earth
(effective earth impedance)
Fault
Z
L1
Z
E
Touch voltage during fault
Live conductor impedance
Vph VT
Exposed conductive parts
Figure 9: Touch voltage during a fault in a
TT system.
The touch voltage is the effective voltage an individual will be exposed to if touching the
casing of equipment. It only persists during the fault. The touch voltage during the fault is
equal to:
V V Z
Z Z
T ph PEN
PEN L
= × +
1
Where:
VT is the touch voltage.
Vph is the value of the phase voltage (usually 220 - 240 V).
ZPEN is the effective earth impedance.
ZL1 is the live conductor impedance to source.
As an example, if we assume that ZPEN is equal to ZL1, the value of VT will 115 V (assuming a
phase voltage of 230 V). If the earth loop impedance is sufficiently low, the value of fault
current would be large to allow the automatic disconnection devices (e.g., fuses or circuit
breakers) in less than 0.4 of a second. As mentioned in the last section, if the value of the
fault current is not sufficient to trip the protection device in 0.4 seconds, then an upper limit of
5 seconds is allowed, provided the touch voltage is limited to 50 V. This in practice means
that a larger circuit protective conductor has to be used, to provide a lower earth loop
impedance.
Figure 9 shows the touch voltage calculation in a TT system under fault conditions. Again,
the touch voltage during the fault is equal to:
V V Z
Z Z
T ph E
E L
= × +
1
Where:
VT is the touch voltage.
Vph is the value of the phase voltage (usually 220 - 240 V).
ZE is the effective earth impedance.
ZL1 is the live conductor impedance to source.
As mentioned earlier, it is generally not possible to have a large fault current which can trip
the protection devices. For these reasons, a residual current device is usually used to protect a
TT installation, as discussed in the next section.
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7. USE OF AN RCD IN A TT SYSTEM
Due to the larger value of earth loop impedance in a TT system, the fault current is smaller.
This causes the problem that it is not possible to disconnect in less that 0.4 seconds. For this
reason, disconnection time of 5 seconds is allowed, provided that the touch voltage during the
fault does not exceed 50 V. This protection, can either be achieved by using an overcurrent
protective device or a residual current device (the later being the preferred option).
The IEE Wiring Regulations requirement regarding the TT system is regulation
number 413-02-20
“The following condition shall be fulfilled for each circuit:
R I V
A a
× ≤ 50
, where:
RA is the sum of the resistances of the earth electrode and the protective conductor(s)
connecting it to the exposed-conductive part.
Ia is the current causing the automatic operation of the protective device within 5 s;
When the protective device is a residual current device, Ia is the rated residual
operating current I
∆
n.”
It is important to note, that even if a fault does not exist, modern equipment does draw leakage
currents. These current should be limited in value. Otherwise, they can also raise the value of
the touch voltage. For these reasons, using an RCD device to protect a TT system, can offer
protection in the case of a fault current, as well as when the system is drawing excessive
leakage currents.
8. CONCLUSIONS
The electric current can have fatal effects on the human body. The severity of shock depends
on a number of factors: Size of electric current flowing in the body; resistance of the body;
path taken by the current; duration of the current; and the frequency of the electrical supply.
The main method of protecting from electric shock is the use of the principle of
earthed equipotential bonding and automatic disconnection of the supply. There are five types
of earthing systems: TN-S, TN-C, TN-C-S, TT and IT.
The main difference between the TN systems and the TT system, is the higher value of
the effective earth impedance as compared to the PEN conductor impedance. This has the
effect that the value of the fault current becomes smaller (as it is faced by a larger earth loop
impedance). This makes it difficult to disconnect the supply within 0.4 of a second using
automatic protection devices (e.g., fuses or circuit breakers) and a more realistic time is 5
seconds.
It was shown how touch voltages develop and how they can be calculated. When the
disconnection time of 0.4 seconds cannot be achieved, the value of the touch voltage must be
limited to 50 V.
AUTOBIOGRAPHICAL NOTES
The author graduated in Electrical Engineering in 1987, and worked for two years as an
electrical and electronic lift systems design engineer. He received his M.Sc. in Remote Lift
Monitoring in 1990, and his Ph.D. in Artificial Intelligence Applications in 1992 from UMIST
(Manchester, U.K.). He was then appointed as Senior Electrical Engineer for Lifts &
Escalator at London Underground, and is still working for London Underground, currently as
Team Delivery Manager in the Station Systems Area. He is also a Corporate Member of the
IEE and a Chartered Electrical Engineer.
REFERENCES
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Casemore Electric, “Electric Shock” http://www.casemore.com/articles3.htm
Cook, P., 1996, “Protection against overcurrent”, Wiring Matters, Summer, 1996.
Dalziel, C. F., 1961, “Deleterious Effects of Electric Shock”, Charles F. Dalziel, p. 24,
Presented at a meeting of experts on electrical accidents and related matters, sponsored
by the International Labour Office, World Health Office and International
Electrotechnical Commission, Geneva Switzerland, October 23-31, 1961
IEE, 1992, “Protection against electric shock”, Guidance Notes, Number 5 in series, 16th
Edition Wiring Regulations.