What Is a Protection Relay?

Paul Scott
Paul Scott
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A protection relay is an electrical device used to protect circuits, equipment, and operators from a range of undesirable electrical conditions by cutting the control circuit power when a fault is detected. These fault conditions include over-current and voltage, frequency deviations, and reversed power flows. The protection relay differs from standard switching relays in that it will only activate when subjected to very closely defined input parameters. Protection relays are generally designed to respond to one specific anomaly so most installations utilizing the relays will typically feature banks of the devices, each dedicated to a particular fault category. There are several mechanisms used in protection relays including armature, induction disc, and moving coil mechanisms.

Due to the ever present potential for accidents, electrical installations are generally well-protected with instruments and devices, which either warn of impending problems or prevent them from escalating to a point where equipment losses or injuries occur. This is particularly true of large industrial high-voltage and current installations where electrical faults often cause catastrophic damage to equipment or loss of life. The protection relay is one such device commonly encountered in substations and switching facilities protecting the equipment against faults such as current and voltage surges, frequency deviations, and reversed power flow conditions.

This protection is achieved by routing the installations control circuit power through the contacts of the protection relay. During normal operation, the contacts are closed, allowing the control circuit to remain active. If the relay senses a fault condition, it will activate, opening the contacts and breaking the control circuit. This causes the installation to shut down, protecting the equipment and personnel from damage or injury in the process.

The protection relay is, in some respects, similar to a conventional switching relay, with the main difference between the two being the sensitivity of the relay-operating mechanism. Protection relays operate subject to very specific parameters, whereas general switching relays will usually operate through a relatively wide range of voltages and currents. In many cases, the operating parameters of the protection relay can be user set to suit ambient conditions. This allows technicians to define the specific fault condition characteristics to which the relay will respond.

Most protection relays are also designed to sense one fault category, only necessitating the installation of several units to cover all potential faults. There are several types of mechanisms used in the different protection relay types, including induction disc, moving coil, and armature mechanisms, each suitable for specific conditions. Due to the critical role protection relays play in any electrical installation, they should be subject to a strict maintenance and testing regimen to ensure ongoing accuracy and integrity.

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