Power System Protection: 5 Fundamental Concepts of Electromagnetic Relays you Should Learn Today

Relays play a key role in power system protection. Today you’ll learn the 5 fundamental concepts on electrical relays.


All types of relays essentially involve a switch and an electromagnet. The electromagnet detects the quantity of current and signals the switch.


Let’s consider the circuit below. Under ordinary conditions, the black colored electromagnet acts like a simple piece of metal. The switch remains opened.

opened electromagnetic relay

The large flow of current through circuit activates the electromagnet. The switch is attracted towards the left and right circuit is activated.

closed electromagnetic relay

NO vs NC (Normally open vs Normally closed)

In the previous section, the switch was initially opened. Such relays are technically termed as Normally open relays. There is another class of relays where the switch is initially closed and an electromagnet repels the switch. Such relays are known as Normally closed relays.

Illustration and Working of Physical device

The video below illustrates the detailed working and applications.

Instrumentation transformers

All sort of measurement equipment suffer some sort of limitations, and relays are of no exception. A relay can measure voltage and current up to certain. It is impractical to design a relay which can measure 1000 or 50,000 volts or a relay which can measure a current of 500 Amps.

However practical power systems often work on HV class of voltages (500 kV or so on). So how can we use a relay in such condition?

An instrumentation transformer bridges this gap by stepping down the voltages and current to a safe and acceptable level.

Relays vs Contactor

The contractor is a specially designed large sized relay which is used with electrical motors for switching purposes. They are often found in control panels, trolleys, substations and motor control station.


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