Static Relay

Relays are extensively used in the protection of system from fault and abnormal conditions. In today’s world, the static combined protection control & monitoring systems are used for the protection of systems.

What is a Static relay?

The static relay is the combination of both the static and the electromagnetic relay. In this relay, there is no armature and moving contacts and response is developed by the components without mechanical motion. The solid state components used are transistors, diodes, resistors, 0and capacitor and so on. In the static relay, the measurement is performed by electronic, magnetic, optical or another component without mechanical motion.

Static relay consists of the following functional circuits:-

  • Input stage with CT’s and PT’s:-

The input to the relay is obtained from the CT/PT. This output of CT/PT is given to the auxiliary CT/PT. The input stage has CT’s and PT’s, summation units, auxiliary CT’s and PT’s, filter.

  • Rectifiers and smoothing circuits:-

In these circuits, the input given from auxiliary CT/PT is given to the single bridge rectifier to rectify the fluctuating quantities. This rectifier output is then smoothened to avoid any ripples and is provided it to a level detector which levels the quantities.

  • Comparator:-

The output which is smoothened is given to the comparator to compare it with a fixed input. Thus the output is then fed to the measuring unit. The comparators used are of amplitude, hybrid and phase comparators.

  • Measuring unit:-

If the input of the comparator is above the threshold value, then the level detector gives an output otherwise it doesn’t produce output if it’s below the threshold value.

There are amplifiers, logic circuits as well as level detector circuits to complete the job.

  • Output stage:-

The output of the level detector is then fed to the output stage of the relay in which it has a PMMC (Permanent magnet moving coil) relay, a Thyristor in series with the trip coil and auxiliary switch.

Whenever there is a fault the relay operates, and it gives gate signal to the Thyristor which then energizes the trip coil so that the circuit breaker contacts gets open and the circuit is tripped. These relays do not have any arcing problems as all the entire relay system is static.

Advantages of Static Relay:-

  • Consumption of power is low.
  • The selectivity of the system can be improved by using special circuits.
  • There are no moving parts thus arcing and contact bounce is not present which saves a lot of money.
  • Vibrations and effects of gravity is not present in operation of static relays.
  • These relays are compact.
  • These relays have high speed.
  • The relays can be programmed according to the needs.
  • The settings of the relay can also be changed if the line carries charge.
  • Only one relay is sufficient for several functions.

Limitations of Static Relay:-

  • Some components are sensitive to electrostatic discharges. Even small charges can damage the components, and therefore precautions are necessary for the manufacturing of static relays to avoid components failures due to electrostatic discharges.
  • Static relays are sensitive to voltage spikes or voltage transients. Special measures are taken to avoid such problems.
  • The reliability of the system depends on a large number of small components and their electrical components.
  • The static relay has low short-time overload capacity as compared to electromagnetic relays.
  • Static relays are costlier, for simple and single function than their equivalent electromechanical counterparts. But for multi-functional protection, static relay proves economical.
  • Highly trained personnel are required for their servicing.
  • Static relays are not very robust in construction and easily affected by surrounding interference.


Thus, use of static relay is simple and the calculations regarding it are also easy.


  • Book by Sunil S. Rao
  • – image credits

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