How to choose a protection relay

The protection relays ensure that the distribution of electricity from the electricity grid takes place in safe conditions. More precisely, their function is to detect any anomalies in the network itself that could lead to dangerous situations and to quickly interrupt. If necessary, the electrical circuit for the duration of the anomaly. Depending on the model, these devices provide differential, overcurrent, or electric arc protection.

The relays can also be composed of several modules, each with its own function.

What are the various types of protection relays?

Protection relays can be divided into two main categories: digital relays and electromechanical relays.

Digital protection relays

Digital protection relays, also simply called digital relays, are composed of an electronic system equipped with microprocessors. These microprocessors allow for analyzing various parameters of the network, such as voltage, intensity, and current, and to detect any anomalies in an industrial control system.

Furthermore, thanks to the microprocessors they are equipped with, the digital relays are able to record and transmit monitoring data in real-time.

Electromechanical protection relays

Although they are the first protection relays to appear on the market, electromechanical protection relays, or electromechanical relays, continue to be widely used, even in modern installations. As their name suggests, they use electromagnetic and mechanical forces to change state. However, electromechanical relays can only perform one or two functions.

What are the various functions of the protection relays?

Protection relays often consist of several modules, each of which performs a specific function. In this guide, we will distinguish three types of protection. Protection against fluctuations in the main supply, leakage currents (differential currents), and overloads.

Protection against mains power fluctuations

Abnormal variations in voltage, current, and frequency can lead to over currents, short circuits, overvoltages, or even simply to voltage drops. Voltage protection relays, current protection relays, and frequency protection relays allow for to detection of a fault in the circuit, disconnecting the device from the circuit with the fault, and eliminating this fault without affecting the rest of the plant.

Differential protection

The earth leakage current protection relay, is more commonly known as a residual current relay. Mainly used in transformer substations, transformers, generators, and power lines.

RCDs transmit information about the status of the network and are able to shut off the power if they detect a problem. Another advantage of these relays is their low purchase cost.

Overload protection

Overload protection relays, called thermal protection relays, thermal relays, or thermal overload relays, protect electric motors in the event of an anomaly.

More precisely, and as indicated by their name, they detect overloads, and, in order to avoid motor overheating, open the circuit. Some models can also detect a phase fault in a three-phase motor. This type of relay can be installed on the circuit itself or remotely, in an electrical panel.

Which relay to choose depending on the application?

To protect an electric motor

Choose a thermal relay, specially designed to interrupt the supply current if the motor draws too much current for too long a period of time.

To protect a power line

Opt for an overcurrent relay, or overcurrent protection relay, which offers sufficient protection for simple, low-cost equipment.

To protect against electric shock by indirect contact

Opt for a differential relay, which limits the leakage currents to earth and protects the electrical system.

Application examples: street lighting, water treatment, telecommunications, and solar energy installations.

What other factors do we need to consider when choosing a relay?

To choose a protection relay that is right for us we will have to take into consideration the factors we have already examined. i.e. the type of anomaly to which the relay will have to respond, but also other factors, such as the type of current and the presence of any accessory components.

Type of current

It is essential to know the type of current of the electrical network for which the relay is intended.

Faults to which the relay must respond

The relays can be configured to intervene when certain malfunctions or faults occur. More precisely, the relay will trip when a trip threshold is reached, also called trip current.

Accessory components of the relay

More and more industrial relays are equipped with monitoring systems and communication interfaces for data transmission. It will therefore be important to choose a relay that is compatible with existing systems and able to communicate with them.

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