Page 724 - Industrial Power Engineering and Applications Handbook

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balance of the system. Similarly, on a single phasing,
although the balance is upset, there is no current through
the ground circuit and this scheme would not detect a
single phasing.

21.3 Ground fault protection in
hazardous areas
r
G' These are highly sensitive areas and a little higher level
of a ground fault current can be catastrophic. It is therefore
mandatory at such locations to keep their ground leakage
current (rather than the ground fault currents) low by
maintaining a certain level of ground loop impedance
and then be able to detect and isolate these currents
promptly.
The leakage current at hazardous locations such as
refineries, petrochemical plants and mines should not
exceed 15% of the rated current of the circuit or 5 A,
whichever is greater. Table 2 1.3 indicates the maximum
permissible ground leakage currents for such areas at
15% of the rated current and the recommended maximum
ground loop impedances.
The use of core-balanced CTs is quite common for
such applications. They are specially designed to detect
the ground leakage current of a circuit. This ground leakage
is then used to trip the faulty circuit through a ground

Table 21.3 Maximum impedances of ground loop for protection
by ground leakage relays in hazardous areas
Figure 21.6 An HT to LT distribution system showing Current rating Maximum permissible Recommended maximum
grounding circuits of circuit ground leakage ground loop impedance
A>k 5 z, =- IS xc
on a 240 V phase to
currenP 'C'
Amp
ground circuitb
Amp
240
~
5
32
10
5
32
15
IY
2
20
5
32
/E
5
30 5 32
32
40 6 26.7
60 9 17.8
80 12 13.3
IR = Iy = /B 100 15 10.7
_..
:. IR + /y + Is = 0 125 18.7 8.6
I50 22.5 7.1
Figure 21.7 Phasor sum of a balanced three-phase system is 175 26.2 6.1
zero 200 30 5.3
300 45 3.5
400 60 2.7
does not necessarily mean that each current phasor R, Y
or B individually is equal in magnitude and phase. Even Notes
if it has unbalanced currents in the three phases (which aHighly sensitive ground leakage relays can sense a current as low
is a likely situation in a three-phase system, see also as 0.1 A and less and at a much higher ground loop impedance.
Section 12.2(v), it will not cause a current to flow through bCalculated to allow at least 150% of the maximum permissible
the ground circuit, as illustrated in Figure 20.19(a). The current to be on the safe side. For example, for 15A maximum
current through the ground circuit will flow only when ground leakage current, maximum impedance at 240 V
any one or more of the phases has a ground fault and
forms a complete circuit through the ground loop con- --= 240 10.7 !2
-
ductor, as illustrated in Figure 20.19(b), and disturb the 1.5 x 15