Page 719 - Industrial Power Engineering and Applications Handbook

Grounding theory and ground fault protection schemes 21/679
21.1 Protection of a domestic or an body can endure, without damage has a relationship with
industrial single-phase system the leakage current through the body and its duration,
i.e.

21.1.1 Effects of current passing through a
human body and the body’s tolerable (21.1)
limits
where
An electric current, rather than voltage, through a human sb = shock energy in watt-seconds (Ws)
body may cause shock and can damage vital organs of Ib = r.m.s value of the leakage current through the body
the body as follows:
in Amperes (A)
tb = duration of leakage current in seconds (s)
It may cause muscular contraction, unconsciousness,
fibrillation of the heart, respiratory nerve blockage The energy, S,, for a 50 kg body is regarded safe at
and burning. These are all functions of body weight. 0.0135 Ws and for a 70 kg body at 0.0246 Ws
The muscular contraction makes it difficult to release
an energized object if held by the hand and can also :. I, (50 kg) = /? - - (21.2)
0.0135 - 0.116
make the breathing difficult. The heart, being the most K
vulnerable organ of a human body, is damaged most,
mainly by ventricular fibrillation, which may result OE
=
-
in an immediate arrest of the blood circulation (electro- and I, (70 kg) = /y - (21.3)
cution). In Table 21.1 we provide likely intensities of
body currents when lasting for more than a heart beat
(nearly 60-300 ms or 3-15 cycles for a 50 Hz system). Figure 21.1 as in IEC 60479-1 illustrates the various
zones of ground leakage current versus duration of fault
It is generally seen that a human body can sustain a and its effect on a human body.
much higher current at a lightning or switching To enable the body to sustain a high fault current it is
frequency (5 kHz or above) due to the extremely short essential that the fault interrupting device (relay or release)
duration of such surges (30 ps or less).
is quick responding. For domestic applications, it is
recommended to be less than a heart beat.
The current can pass through the heart, when the current
passes through hand to hand, or through one hand and a
foot. Current flowing between one foot to the other may Note
Prevention from ground leakage as such is important, as it causes
not be considered dangerous, but may cause muscular corrosion through electrolysis and may damage the insulation of
contraction and pain. The subsequent body fall, if it occurs, wires due to ageing.
may, however, be fatal as now the current can also flow
through the hand involving the heart. Ground fault 21.1.2 Use of ground leakage circuit breakers
protection emphasizes keeping the fault current below (GL CBs) *
the fibrillation threshold and for a period of less than a
heart beat, in the range of 60-300 ms. It has been It is likely that smaller installations, such as a domestic
established that the electric shock energy which a human light or power distribution network, and single-phase
industrial or light loads may not always meet the
requirements, as discussed in Section 21.2, Table 21.2,
Table 21.1 Likely intensities of body currents when due to the circuit’s own high impedance (other than the
lasting for more than a heart beat (= 60-300 ms) impedance of the ground conductors). It is therefore
possible that the fuses, if provided for the short-circuit
Sr. Body At frequency Intensity protection of the system, may be too large to detect a
no. current ground leakage. The ground leakage circuit breakers
(d) (HZ) (GLCBs) provide an effective solution to this problem
and are easily available and extensively used. They are
1 100 50-60 Lethal extremely sensitive to very feeble ground leakages, of
2 100 3- 10 kHz Tolerable (because of the order of 10 mA or more, and trip the faulty circuit
extremely short duration)
3 60-100 50-60 Fatal within a safe period on the smallest leakage current and
4 16-60 50-60 Breathing may become provide effective protection to the human body. Figure
difficult due to muscular 21.2 illustrates the tripping scheme for such a breaker
contraction and the principle of operation is discussed below. Normally
5 10-16 50-60 It is the let-go current and two types of GLCBs are in use, i.e. 30 mA and 300 mA.
makes it hard to release an
energized object if held by
hand *More commonly known as earth leakage circuit breakers (ELCBs)
6 5-9 50-60 No dangerous effects or residual current circuit breakers (RCCBs). They operate on the
7 14 50-60 Threshold of sensation
principle of residual current.