Handbook of Electrical Installation Practice - BeKnowledge

72 Handbook of Electrical Installation Practice
Fig. 3.6 Duplicate feeders and duplicate transformers.
(a rare but not unknown occurrence), then that substation (or section of busbars if
provided with a bus-section switch) must be repaired before its supply can be
restored. The location of faults is greatly facilitated by the provision of earth fault
indicators at each cable terminating box around the system. These make unnecessary
the practice of repeatedly closing onto a fault in order to locate its whereabouts,
or the time consuming technique of isolating each section in turn in order to conduct
insulation tests.A visual inspection of the earth fault indicators will determine which
section of the ring main is faulty. Earth fault indicators with remote monitoring are
also available at increased cost which must be balanced against the cost of delay in
locating the fault and carrying out the necessary switching to restore supplies.
Where continuity of supply is crucial and even short unplanned outages due to
cable faults cannot be tolerated, it is necessary to provide circuit-breakers throughout
the ring main and to operate the system as a closed ring in normal circumstances.
In this mode of operation conventional current and/or time graded protection will
not provide proper discrimination since the direction of power flow around the ring
main is likely to vary under different load conditions. In order to provide continuity
of supply to all substations in the event of a cable fault, it is therefore necessary
to employ a differential protection scheme for each leg of the ring main or to install
a suitably graded scheme utilising directional overcurrent and earth fault relays.
ON-SITE GENERATION
Generators may be installed on site for one or more of the following reasons: