The switchgear of d.c switchboards, unlike that of enclosed a.c. equipment was mounted on what was literally a board or screen of panels. On the front the open main circuit breakers, distribution breakers, knife switches and instruments were easily accessible for inspection and maintenance. Safeguards against contact were only an insulated handrail and a rubber mat’ closed gate normally prevented entry to the passage behind the board where the copper bus-bars and rheostats were mounted.
Enclosed or dead-front boards are only required for d.c. installations if the voltage is greater than 250 volts. Most of the D.C. switchboards on British vessels just prior to the general change to ac. were for 220 volt supplies.
Main Circuit Breaker
The brush type moving contacts of the generator breaker are closed against a resisting spring by a lift-and-latch handle. The handle, when raised, slips the operating pin over the latch, where it is clipped into position. Downward movement of handle pushes
the cam plate against the hinged contact holder. The leverage and pressure applied forces the brush contacts to spread on the fixed studs. This brushing action is beneficial in that contacts are wiped and any film or deposit removed. Vibration with consequent burning or welding is prevented by the pressure, sustained by the resilience of copper strips, which are clamped In the manner of a leaf 3oring. Conductivity of the copper is slightly reduced by a small amount of alloying material added to prevent work-hardening and to give the necessary resilience.
The tripping of a main breaker due to overload or when load current was being carried would causes arcing and damage to the main contacts. Arcing contacts are therefore fitted and arranged to make first and break last to protect main contacts from burning. They are of metallized carbon, sintered silver-tungsten or other material which does not weld at high temperature; such compositions are not suitable for carrying high current over long periods.
There is a set of main and arcing contacts for each connection to be made from the generator to the switchboard bus bars. They are fixed together by bars for simultaneous movement by one handle. Thus, for a two-wire insulated d.c. system the triple pole circuit breaker connects the positive, negative and equalizing connections in one operation. The equalizing contact is given a lead so that it closes first Short-circuit’ between poles is prevented by insulation.
The breaker is held in the closed position by the latching device but is easily opened by the trip. Tripping is automatic in the event of short-circuit, overload, reverse current or low-voltage faults. For normal opening, the trip can be operated manually after reducing the load with the rheostat.
Handbook of Switchgears (Mcgraw-Hill Handbooks)
Distributed Switchgear (IEE Power & Energy Series)
1953-54 Whipp Bourne Switchgear Bethlehem Steel Builder
Handbook of Switchgears
Switchgear and Control Handbook
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