The gas-operated relay is designed for this particular duty and depends for its operation on the fact that most internal faults within the transformer generate gases. The service record over many years shows clearly that the relay is extremely sensitive in operation and that it is possible to detect faults in their incipient stages, thus minimising damage and saving valuable time in effecting the necessary repairs. The gas-operated relay can only be fitted to transformers having conservator vessels, and is installed in the pipeline between the transformer and its conservator tank. The relay comprises an oil-tight container fitted with two internal elements which operate mercury switches connected to external alarm and tripping circuits. Normally, the device is full of oil and the elements, due to their buoyancy, rotate on their supports until they engage their respective stops. An incipient fault within the transformer generates small bubbles of gas which, in passing upwards towards the conservator, become trapped in the housing of the relay, thereby causing the oil level to fall. The upper element rotates as the oil level within the relay falls, and when sufficient oil has been displaced the mercury switch contacts close, thus completing the external alarm circuit.
In the event of a serious fault within the transformer, the gas generation is more violent and the oil displaced by the gas bubbles flows through the connecting pipe to the conservator. This abnormal flow of oil causes the lower element to be deflected, thus actuating the contacts of the second mercury switch and completing the tripping circuit of the transformer circuit breaker, so disconnecting the transformer from the supply. Gas within the device can be collected from a small valve at the top of the relay for analysis and from the results obtained an approximate diagnosis of the trouble may be formed. Some of the faults against which the relay will give protection are:
1. core-bolt insulation failure;
2. short-circuited core laminations;
3. bad electrical contacts;
4. local overheating;
5. loss of oil due to leakage;
6. ingress of air into the oil system.