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How Protection Relays Work?

Circuit breakers are used to control the flow of energy

Protection relays — compact, self-contained devices designed to detect faults in electrical systems and trigger circuit breakers — are critical to a circuit’s defense.

“Looking” for abnormal conditions in a circuit by constantly measuring electrical quantities, protection relays monitor voltage, current, frequency and phase angle.

Let’s take a look at how protection relays work.

Protection Relay Operating Principles

Reliant upon electromagnetism to function, protection relays operate under one of the two following principles — electromagnetic attraction or electromagnetic induction.

As the nomenclature indicates, the armature of the poles of a plunger in an attraction relay is drawn into a solenoid when the relay is triggered.

The solenoid trips the circuit breaker, which then shuts the system down. Attraction relays work in both alternating and direct current circuits.

Electromagnetic induction relays are found only in AC circuits.

The conductor in the relay behaves like a magnet when it’s fed electric current. This generates a “suction” force, which connects the movable contact and the stationary contact inside the relay.

The circuit closes and the system begins to operate. When an anomaly along the lines of one of the types described above occurs, the circuit is broken; the movable contact retracts, and the system shuts down.

What They Do?

According to Schneider Electric, one of the leading manufacturers in the electrical components industry, protection relays in a power system are the “officers” that command the “soldiers”.

The soldier in question is usually a circuit breaker that can interrupt the flow of energy when called upon by the protection relay to do so.

Protection relays take their cues from the power system, by analyzing signals they get from the rest of the configuration to determine what to do in order to maintain the health of the power system.

Typical relay applications include feeder protection, transformer protection and motor protection — just to name a few.

Why Protection Relays Are Important?

While they may differ in operating principle, the basic purpose of each type of protection relay is the same.

Simply put, they close the trip circuit of a breaker or sound an alarm to get the operator’s attention. They can also disconnect the malfunctioning component to protect the rest of the circuit.

This has the effect of preventing subsequent faults as well.

Further, protection relays can defend the device suffering the abnormality.

By initiating a shutdown, the relay can prevent the flow of current in the faulty part, potentially minimizing the damage it suffers. Protection relays also serve to buffer the rest of the system from the damage a bad component could spread through the rest of the system.

This serves to “localize” the problem, which can result in significant savings when it comes to repairs, and advances your system’s performance, reliability, stability and continuity.

While the main aim of protection relays is mitigating the effects of short circuits, generators and motors can experience other types of abnormalities protection relays can detect and minimize.

Given the protection relay’s ability to localize problems, they can also show you where to look when you’re troubleshooting issues. They can also provide an indication of the type for failure for which you should be looking.

Qualities for Which to Look

Understanding how protection relays work is fundamental to specifying the proper device for the configuration you’re designing. When you’re specifying a protection relay for any given system, your primary considerations should be:

  • A. Selectivity
  • B. Speed
  • C. Sensitivity
  • D. Reliability
  • E. Simplicity
  • F. Economy

You want the relay to have the capability of localizing the problems and shutting your system down at the breaker closest to the precise point of failure.

You’ll want this to occur as soon as possible upon the detection of an abnormality too. After all, every subsequent moment of operation can increase the amount of damage suffered by the circuit.

The degree of sensitivity is critical as well. You want the device to respond to problems, without triggering false alarms.


Naturally, you want it to work with as little maintenance as possible, for as long as possible and you want it to come in at a price that makes sense in terms of the rest of the configuration.

Article written by:

I am a writer and reporter for the clean energy sector, I cover climate change issues, new clean technologies, sustainability and green cars. Danny Ovy

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