Page 727 - Industrial Power Engineering and Applications Handbook

y the neutral circuit

3-g load 3-g four-wire load I,, - Unbalanced current due to load-A

This CBCT will detect the unbalance as
well as the ground leakage currents
This CBCT will detect only the
ground leakage current
I,, - Unbalanced current due to load-A
Iub - Unbalanced current due to load-8
Iga Ground fault current of load-A
-
Igh Ground fault current of load-B
-

(b) On a ground fault

Figure 21.11 Detecting the unbalance and the ground leakage currents through a CBCT

occur which could affect the sensitivity of a circuit Note
using more than one CBCT. If such a reduction in In an HT system it is recommended that each circuit he protected
sensitivity is considered detrimental to the protection separately for a ground leakage as far as possible. This is to
of the system or the equipment, it is advisable to use localize the effect of the fault and to trip only the feeder of the
a separate relay with each CBCT (Figure 21.10(b)). faulty circuit. A common protection in the incoming is not
advisable, except for cost considerations, which also will be
For a common indication, alarm or trip, the relays' nominal for such a protection scheme compared to the cost of
trip conlacls may be wired in series. the main equipment. Otherwise a ground leakage in the
5 When using only single-core cables, the same method downstream may trip the whole system. which may not be
should be adopted, as discussed above. Groups may desirable. It is also possible that at the downstream. the ground
be made of 3, 3; or 4 cores (R, Y, B or R, Y, B and leakage is very feeble due to high ground loop impedance and
N) of single-core cables, and one CBCT used in circular may not be sufficient to be detected by the common ground
or rectangular form, whichever is more convenient, leakage protection probided at the incoming. It is also possible
that the setting of the relay is not so low as to detect this. Thus
for each group. to provide a ground leakage protection for each individual circuit