Protection Relays: and why they are important

• 2023-07-27

Be it in our houses, factories or business depots, we’re surrounded by electrical equipment and appliances, to the point where a day without electricity might be unimaginable. Thus, learning about and investing more in how to effectively safeguard your electrical equipment would be a good idea. Having sufficient knowledge about these concepts helps ensure you get the most bang for your buck.

What are protection relays?

A relay is an automatic protection device that breaks its contacts upon sensing any abnormality in the system. This in turn triggers the circuit breaker which then trips, disconnecting the faulty portion of the electrical circuit from the rest of the healthy circuit. Invented about 150 years ago, the earliest electrical relays can be traced back to the 1830s. Back then, electromagnetic thermal expansions were used for making electrical relays. While these are in use even today, solid state relays are preferred as they have more accuracy, are cost efficient and are space-saving.

Types of protection relays

Whether you are an electrician, business owner or a home-maker, here are some types of protection relays you should know about:

For single-phase supply:

• single phase preventer relays
• single phase voltage protection relays
• single phase current protection relays

For three-phase supply:

1. phase sequence relays
2. overload, underload relays
3. power factor correction relays

Phase preventer relays

What are they?

A single-phase preventer relay is a control device that detects any failure in three phase supply and energizes its auxiliary relay contacts, which is then given to the MCB/MCCB (Miniature Circuit Breaker/Molded Case Circuit Breaker) that will turn off the supply to the equipment. When one of the three phases is missing from the three-phase supply, it is known as single phasing. 

How do they work?

A single-phase preventer relay has 8 terminals - 3 terminals for three-phase supply, 2 terminals for single-phase auxiliary supply powering the device, and 3 terminals as relay contacts. The interior circuit monitors the phase sequence for any abnormality or imbalance continuously. On the occurrence of single-phasing, it immediately shuts off the power supply to the equipment.

Types of phase preventer relays

There are mainly two types of phase preventer relays: 

1. Current-sensing
2. Voltage-sensing

Voltage protection relays

What are they?

A voltage protection relay is a device that continuously monitors the incoming voltage to the device and shuts down when the input values are outside a preset range. They are primarily used to detect changes in voltage due to generator failure, low voltages caused by power interruptions or short circuits, etc. The device shuts down the machine to protect it from such fluctuations in voltage.

How do they work?

These protection relays operate when the voltage across the phases crosses the specific preset value. The purpose of these relays is to protect loads against voltage drops or spikes. When a fault is detected, the relay contacts of the circuit trip the circuit breaker, cutting off the connected circuit and shutting down any connected devices, thus ensuring its safety. 

Types of voltage protection relays

There are mainly three types of voltage protection relays:

1. Under voltage protection relay
2. Over voltage protection relay
3. Under voltage and over voltage protection relays

Multispan has a range of products for each of these types, to help face any challenge in the workplace.

Current protection relays

What are they?

A current protection relay or overcurrent relay is a device that continuously keeps track of the current flowing through the circuit and reacts when it crosses a predetermined threshold. Overcurrent relays are used in many different applications and systems in order to help protect against electrical faults like short circuits or overloading. Overloading of current in the circuit may lead to damage to the equipment or electrical fires, so the relays help prevent them.

How do they work?

These relays provide protection by disconnecting from the circuit on detecting a deviation from the value set previously. This only happens when the value of the current is greater than the relay setting time. An overcurrent relay typically consists of two main components - a sensing device and a control device. The sensing device is used to ‘sense’ the current flowing through the circuit, while the control element decides the tripping current, usually based on user settings.

Types of current protection relays

Depending on the time of operation the overcurrent relay can be categorized into the following types:

1. Instantaneous overcurrent relays
2. Inverse time overcurrent relays with definite minimum time (IDMT)
3. Definite time overcurrent relays

Frequently asked questions

What is the importance of electrical protection?

Despite all the progress technology has made, no device can work without electricity. Electrical dropouts and blackouts are extremely disturbing. They can cause real costs, work or progress loss and even physical damage to the equipment hardware. Therefore, adequate protection against such situations is important for every industrial system.

What is meant by protection in the electrical system?

Protection in the electrical system is provided by automatic circuit-breaking devices like protective relays, fuses and circuit breakers. Upon encountering abnormal conditions, they isolate the faulty section of the circuit from the rest of the healthy circuit. They are preventive measures against damage to expensive pieces of equipment.

What is the need for protection?

However much a company invests in the improvement of its electric grid, it is not 100% reliable. The electric grid is not stable; the likelihood of the electrical supply companies supplying voltage above or below the nominal value is always high. Thus, a company can’t depend on them, at the risk of permanent damage to expensive units or worse, electrical fires. Protection is always advisable.

What are the protection systems for electrical systems?

There are several protection schemes in the current industry scene and new ones keep being invented as protection engineers face new challenges every day. The fundamental concept behind them stays the same. The four most basic protection schemes are:

1. Overcurrent protection scheme
2. Differential protection scheme
3. Distance protection scheme
4. Direction protection scheme

What are the most common protection devices?

The elemental components of a power protection system are:

1. Protection relays
2. Fuses
3. Circuit breakers
4. Instrument transformers
5. Surge protection devices
6. Electrostatic Disturbance protection (ESD) devices

Still facing an issue with picking the components for a power protection system for your company? Contact us here and we’ll be sure to help!