Time delay devices associated with safety circuits

Dash pots-(magnetic time lag)

The usual form of time/delay is an oil dash pot having an inverse time/current characteristic, relies for its operation upon the retarding action of a plunger immersed in a reservoir of oil, together with the magnetic force generated by a flow of current through a solenoid. The plunger is attached to an iron core which is partially enclose in the solenoid. When the solenoid is sufficiently energised the iron core will attract it, but the action is retarded due to the oil, in this way a time lag is introduced

Characteristic of Oil Dash pot with Inverse Time Delay

It is important to note that as the viscosity of the oil varies with temperature, so will the operating times of oil dash pots vary. Makers will supply dash pot oil suitable for the circuit breakers and relays. The recommended oils are selected on the basis of least variation in viscosity over the working range coupled with viscosity's which will give the time delay marked on the calibration plate. These special oils should invariably be used, the time delays are usually calibrated at 15oC, unless otherwise stated and are only correct at this temperature .
Over current devices fitted with oil dash pot time lags do not operate at the current marked on the calibration scale but at a current 25% greater with the appropriate time delay. The current marked on the scale is the value at which they would operate without time delay. Some makers supply an instruction plate indicating the exact current at which the relay will operate with a given setting.

Thermal device

Depends for its action upon the heating effect of an electric current flowing through, either a bi-metallic strip, or a heating coil placed near a strip. The thermal characteristics of the two dissimilar metals is such that when sufficient heat is generated there is a movement of the strip in one direction until the relay contacts are opened.

Induction relay

Similar to the action of the watt-hour meter, consisting of a metal disc pivoted so that it is free to rotate between two poles of two electro-magnets. The disc spindle carries a contact which is arranged to bridge two contacts when the disc has rotated through an adjustable angle. A spring returns the disc to the reset position, and as, during the deflecting period, the torque exerted by the spring increases, this is compensated for by the provision of graduated slots in the discs periphery. The necessary damping of the movement is provided for by incorporating a permanent magnet through which the disc has to rotate. The upper electro-magnet contains two windings, one, the primary normally is connected to a current transformer to a winding on the lower electro-magnet. Because of the graduated slots, the inertia of the moving system prevents the disc form rotating under normal running conditions, but when overcurrent commence to flow through the external circuit, the torque generated by the interaction of the upper and lower electro magnets is sufficient to cause the disc to rotate, until either the current falls to a safe level, or the relay is operated