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Stone's Latching Relay

The Stone's latching relay is used commonly in the lighting circuits of Mk1 RBRs. Two of these devices are used - one to control the lighting and UV steriliser within the kitchen, and one for the saloon and corridor lights. The latching contactor assembly is referred to as "No 5 Contactor" on the BR schematic SC/ES/7023

Stone's latching relay

The principle of operation is fairly straightforward. Please refer to the schematic for the terms used. Assume the lights are off. When the lighting control switch "ON" button is pressed, +24V appears at the ON terminal. This pulls in the ON coil which in turn supplies power to the OPERATE coil of the main circuit breaker which then makes and supplies +24V to the lights. At this time, 24V is also applied to the HOLD coil via the normally closed contacts of the OFF control relay. This latches the main contactor on after the lighting panel ON button is released.

When the OFF button on the lighting control panel is pressed, +24V arrives at the OFF terminal and the OFF relay is actuated. The contacts open and the supply to the HOLD coil is thus interrupted. The main contacter then falls out and the lights are switched off.

The purpose of the two rectifier diodes accross the control relay coils is to inhibit sparking when the coils are switched off, theus increasing the life of the contacts.

If you find the above schematic a bit indistinct, the full size version can be viewed by clicking here.