Page 720 - Industrial Power Engineering and Applications Handbook

21/680 Industrial Power Engineering and Applications Handbook

10000 Effects of A C currents (50 Hz) on a person
5000 at least 50 kg in weight
2000 Zone-1 - Usually no reactlon
Zone-2 - Usually no dangerous physiological
1000 effect
500 Zone-3 - Usually no danger of heart failure
(ventricular fibrillation)
1 ::: Zone-4 - Probability of heart failure up to
50%
than 50%
3 50 Zone-5 - Probability of heart failure more
s
20
E 10
K
5
2
1
0 1 02 05 1 2 3 5 10 20 30 50 100 200 500 1000 2000 5000
Current passing through the human body in mA (rms) +
Figure 21.1 EffeCcts of current leakage through a human body

-
-
-
-~- - - - - - - - - - R and electrocution, while the latter protects the circuit
-
-
I - 7 I
- a - I I Y from fire risk. Normally the 300 mA GLCB is used as
I I the incomer and 30 mA as the outgoing for the individual
- 4 1 B feeding circuits.
I I
I I
Refer to Figure 2 1.2 showing a single-phase circuit breaker
having a thermal overcurrent and a magnetic short-circuit
element, OCR. A core-balanced transformer is arranged
in the circuit as shown. When the system is healthy, the
current through the phase, I,, and the neutral, I,, will be
equal and the magnetic effect of the load current on the
primary windings, P, and P2, will be almost zero. In the
event of a ground leakage, I(, this balance is disturbed
and some of the leakage current will also flow through
one of the primary windings of this core-balanced CT, as
shown in the circuit. This will cause a magnetic flux in
the core, inducing a voltage in the secondary winding S.
In the circuit shown, it feeds a rectifier circuit provided
on the secondary side of the circuit. This unit facilitates
a direct current flow through the leakage trip coil, which
trips the breaker. These breakers are extremely fast opera-
ting on fault (operating time may be le2s than 5 ms), and
have a low let-through energy. Refer to I- - t characteristics
(Figure 21.1). The ground fault current will also be
comprised of a current through the appliance, I, (Figure
Z, -Ground fault loop impedance 21.2). But since the basic purpose of this device is to
IA - Leakage current through the appliance But for simplicity the protect the human body from an electric shock or
whole current is considered through the body only electrocution, we have considered the leakage current,
if -Leakage current through the human body I(, through the body alone to be safer. To ensure total
* -Test button to check periodically the operation of the ELCB safety, the device should be able to detect this leakage,
assuming there was nu ground current through the
appliance. When the device will be able to do so, the
Figure 21.2(a) Schematic drawing of an ELCB circuit in any case will be protected.