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Current Relay

Last updated: September 3, 2019

What Does Current Relay Mean?

A current relay is a type of electromagnetic switch used as part of the control systems for electrical equipment.

Current relays are commonly used as sensors (current sensor/monitoring relays) to monitor current flow within industrial and other safety-sensitive equipment. This equipment uses relays to prevent malfunctions due to excessive or insufficient electrical current (“overcurrent” and “undercurrent,” respectively).

Safeopedia Explains Current Relay

A current relay is an important component of electrical safety. Electrical system failures can cause machinery or other powered equipment to malfunction, which can cause it to behave in an unsafe manner, potentially resulting in an injury.

Moreover, some hazard controls, such as ventilation systems, are powered by electrical systems. If those systems fail, the controls cease working, leaving workers exposed to additional risks. A current relay can reduce the likelihood of such a malfunction, thereby keeping workers safe by ensuring uninterrupted protection from certain hazards.

Repairing a machine that has been damaged by undercurrent or overcurrent can also expose workers to additional hazards.

Monitoring Overcurrent and Undercurrent

Current relays monitor alternating and direct currents to detect any instance where the current exceeds or falls short of a certain threshold value. If too much current flows through, the current relay will activate, switching off or reducing the amount of energy that can flow through the circuit.

In the case of an overcurrent relay, when normal current flows through the current coil, the magnetic effect generated by the coil is insufficient to move the moving element of the relay. This is because the restraining force is greater than the deflecting force. When the current increases, however, the magnetic effect also increases. When the current crosses a certain level, the deflecting force generated by the magnetic effect of the coil overcomes the restraining force. This causes movement in the moving element and changes its contact position in the relay.

If, instead, the system experiences an undercurrent, a small electromagnetic field will cause the circuit to switch off, deactivating the part of the equipment affected by the undercurrent.

Current Relay and Equipment Protection

Because relays work by reacting to the electromagnetic field generated by the system they monitor, they don’t actually need to be integrated into the specific circuit they're monitoring. For example, overcurrent relays are often used as a type of protective relay, and they function by tripping a circuit breaker once they are activated by the presence of current flows that are too high.

The quality standards that this type of protective relay must meet are prescribed by a number of different consensus standards, such as ANSI 50 and 51. While choosing a relay, the following features must be considered:

  • Physical size and pin arrangement
  • Coil voltage suitable for the circuit powering the relay coil
  • Coil resistance
  • Switch ratings
  • Switch contact arrangement (most relays are single pole double throw [SPDT] or double pole double throw [DPDT])

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