334 HANDBOOK OF ELECTRICAL ENGINEERING
Figure 12.14 Protection devices for a high voltage feeder cables.12.6 Busbar Protection in Switchboards
Faults can occur within the busbar and riser compartments of switchboards. However, modern
switchgear is very reliable and such faults are rare. Many purchasers of switchgear specify insu-
lated and segregated busbars and risers, with all the connectors, clamps, nuts and bolts to be fully
shrouded with flame retardant material, see sub-section 7.2.4.
12.6.1 Busbar zone protection
For switchboards to be operated at voltages up to approximately 15 kV it is common practice to
avoid using differential busbar zone protection. If it is necessary to provide the maximum practical
availability of supply then busbar zone protection should be considered.
If the fault level at the high voltage busbars is high and close to the rating of the switchgear
then busbar zone protection should be given serious consideration. Busbar zone protection is usually
based on the Merz–Price circulating current scheme, with high impedance (87) relays.
Where a bus-section circuit breaker is used to divide the busbars (during abnormal operating
conditions) each set of busbars is protected as a separate zone. Each zone consists of the incomers, the
outgoing circuits and the bus-section circuit breaker(s). An accurate current transformer is connected
in each line of each circuit. All the current leaving the zone must be balanced by current from the
incomer circuits. A fault in the zone will be detected by the (87) relay. Rapid operation is required