Choosing the Right Protection Relay

Protection relays enable the safe distribution of electricity from the grid. Their function is to detect anomalies in the grid that could lead to dangerous situations and, if necessary, interrupt the electrical circuit for as long as necessary.

They provide differential, overcurrent, and arc flash protection. This protection is ensured by the relay’s ability to respond rapidly.

A relay can consist of several modules, each with its own function.

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  • What are the different 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 known simply as “digital relays”, are made up of an electronic system with microprocessors. These microprocessors make it possible to analyze various grid parameters, such as voltage, intensity, or current to detect faults in an industrial control system.

    Thanks to microprocessors, these relays can record and transmit monitoring data in real-time.

    Electromechanical relays

    Electromechanical protection relays are older but are still widely used in today’s systems. As their name suggests, they use electromagnetic and mechanical forces to change state. These relays can perform only one or two functions.

  • What are the different functions of protection relays?

    Protection relays have different functions. They are often made up of several modules, each of which performs a specific function. In this guide, we will distinguish three types of functions: protection against abnormal variations in the main supply, protection against leakage currents, and protection against overloads.

    FANOX ELECTRONIC overcurrent protection relay

    FANOX ELECTRONIC overcurrent protection relay

    Protection against abnormal grid variations

    Abnormal voltage, current, and frequency variations can lead to problematic phenomena. These include overcurrents (overload or short-circuit currents), overvoltages, and voltage dips or faults. Voltage protection relays, current protection relays, and frequency protection relays make it possible to detect a fault in the circuit, switch off the section where the fault is located, and eliminate it without consequences for the rest of the installation.

    Protection against earth leakage currents

    The earth leakage current protection relay, also known as a differential protection relay, is mainly used in transformer stations, transformers, generators, and power lines.

    It transmits information about the status of the grid and can disconnect it if it detects a problem. An advantage of this type of relay is its low cost.

    Overload protection

    Eaton thermal relay

    Eaton thermal relay

    The overload protection relay, also known as a thermal relay, is used to protect electric motors when a defect occurs. It detects overloads and can open the circuit to prevent the motor from overheating. Some models can also detect a phase failure in a three-phase motor. This type of relay can be installed on the circuit itself or remotely in an electrical panel.

    Main Protection Relays:

    • Voltage protection relays
    • Current protection relays
    • Frequency protection relays
    • Earth leakage protection relays
    • Overload protection relays
  • Which relay should you choose for your application?

    To protect an electric motor

    Opt for a thermal relay, specially designed to cut off the motor’s power supply when it draws too much current over a long period.

    To protect an electrical line

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

    To protect against electric shock through indirect contact

    Choose a differential relay, which limits earth leakage currents and protects the electrical installation.

    Examples of applications: street lighting, water treatment, telecommunications, and solar energy production.

  • What other factors should you consider when choosing a relay?

    Before you buy a protective relay, there are a few more things to bear in mind:

    What is the current used in your grid?

    It is essential to know what type of current is used in the grid for which the relay is intended: direct current, alternating current, single-phase, or three-phase.

    What incidents do you want to prevent?

    You can configure the relay to trigger when it detects certain faults or malfunctions that you have previously defined. The relay actuates when the trip current is reached.

    Other possible relay components

    There are an increasing number of industrial relays with monitoring systems and communication interfaces for data transmission. It is therefore important to choose a relay that is compatible with existing systems and can communicate with them.

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