Principle of Operation and Testing


Principal of Operation

The operating mechanism consists of a solid non-metallic cylinder containing the mercury switch, counterbalanced by a smaller solid metal cylinder. Both cylinders are jointed and free to rotate about the same axis, the amount of rotation being controlled by stops.

When the relay is empty of oil, the weight of the switch cylinder predominates and the switch system rests against the bottom stop, the mercury switch being in the closed circuit position.

When the relay is full of oil, both cylinders appear to lose weight. Due to the different densities, the switch cylinder appears to lose enough weight to enable the weight of the counterbalance cylinder to predominate and rotate the whole system until it reaches the top stop, with the mercury switch in the open position.

Mounting Position

The relay should be mounted in the connecting pipe between the transformer and the conservator tank. This pipe should be as long and as straight as possible, and must be arranged to slope upwards, towards the conservator, at an angle within the limits of 3 to 7 degrees to the horizontal.

There should be a straight run on the transformer side of the relay of at least five times the internal diameter of the pipe, and at least three times this diameter on the conservator side.

A machined surface is provided on the relay body for the purpose of testing the mounting of the relay, both in the inclined direction and at right angles to the pipe where it should be horizontal.

“Alarm” Operation

When a slight or incipient fault occurs within the transformer, the gas generated will collect in the top of the relay housing. As gas collects, the oil level will fall and increasing amounts of the alarm switch will appear above the oil level. This results in gradual restoration of the apparent lost weight, until the weight of the switch cylinder predominates. The element rotates as the oil level continues to fall and eventually the alarm switch operates.

Trip Operation

When a serious fault occurs, the generation of the gas is so rapid that an oil surge is set up through the relay. This oil flow will impinge upon the flap fitted to the trip element causing it to rotate about its axis and so bring the mercury switch to the closed position, which in turn operates the tripping devices.

In the event of serious oil loss from the transformer, both alarm and trip elements operate in turn, in the manner previously described for gas collection. The oil level in the double element relay can be monitored against a graduated scale on the windows both sides.


The terminal boxes on double element relays are normally drilled and tapped M20 x 1.5mm for bottom entry by conduit or cable gland. Side entries and alternative thread sizes can be supplied for most types upon request. Alarm and tripping circuit connections are made to OBA (M6) terminal stems, in the terminal box, and secured by OBA (M6) nuts and washers. The maximum recommended Torque valve should not be exceeded when making connections.

Note: If the relay is repainted on site, care should be taken that the vent/drain hole in the terminal box is not obstructed.

Testing on Site

Double element relays are provided with a separate ballvalve to enable the injection of compressed air to be used for testing on site.

To test the operation of the alarm element, air from the Dry Air Pump or an air bottle should be admitted slowly so that the alarm element falls gradually until the switch operates.

To test the trip element, the valve controlling the Dry Air Pump or bottle is opened quickly so that the air rushes in, impinges on the flap and depresses it, operating the switch. The pressure required is dependent upon the equipment used. To facilitate on-site testing a portable foot pump providing air which has been passed through a desiccant is available.

Note: It should be appreciated that this test is no substitute for a proper works calibration of the relay.

Routine Testing

Relays are individually calibrated, in accordance with BEBS T2 (1966) and BS EN 50216-2. Values are recorded for loss of oil/gas collection to operate the alarm switch and steady oil flow to operate the trip switch. The unit is also observed to ensure the trip switch operates due to a complete loss of oil.

Assembled relays are pressure tested with transformer oil at 1.4 bar for 6 hours. Electrical circuits are flash tested at 2000 volts r.m.s. and the insulation resistance measured at 500 Volts is not less than 10 Megohoms in air.

Although specifically designed to function with transformer oil according to BS148, successful trials have also been conducted utilising Silicone coolant.

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