Electrical relays play a critical role in the protection and control of medium voltage (MV) and low voltage (LV) switchgear systems. Relays are used to detect abnormal conditions such as over-currents, under-currents, and earth faults, and to disconnect the faulty part of the system to prevent damage and ensure safety. In this article, we will discuss the basic settings for electrical relays used in MV and LV switchgears.

Over-Current Protection Relays

Over-current protection relays are used to detect over-currents, which are excessive currents in the electrical system that can cause damage to the equipment or even start a fire. Over-current relays are typically set to trip the switchgear if the current exceeds a specified level for a specified time. The common settings for over-current relays include:

  • Pick-Up Level: The current level at which the relay will start to operate and trip the switchgear.
  • Time Delay: The time delay before the relay trips the switchgear after the pick-up level has been exceeded. This time delay allows for the detection of short-duration over-currents, which may not be harmful, and helps to prevent false tripping of the switchgear.

Under-Current Protection Relays

Under-current protection relays are used to detect under-currents, which are low currents in the electrical system that can indicate a fault or a failure in the system. Under-current relays are typically set to trip the switchgear if the current drops below a specified level for a specified time. The common settings for under-current relays include:

  • Pick-Up Level: The current level at which the relay will start to operate and trip the switchgear.
  • Time Delay: The time delay before the relay trips the switchgear after the pick-up level has been exceeded. This time delay allows for the detection of short-duration under-currents, which may not be harmful, and helps to prevent false tripping of the switchgear.

Earth Fault Protection Relays

Earth fault protection relays are used to detect earth faults, which are faults that occur when the electrical current flows to earth instead of returning to the source. Earth faults can cause damage to the equipment, as well as pose a safety risk to personnel. Earth fault relays are typically set to trip the switchgear if the current flowing to earth exceeds a specified level for a specified time. The common settings for earth fault relays include:

  • Pick-Up Level: The current level at which the relay will start to operate and trip the switchgear.
  • Time Delay: The time delay before the relay trips the switchgear after the pick-up level has been exceeded. This time delay allows for the detection of short-duration earth faults, which may not be harmful, and helps to prevent false tripping of the switchgear.