- Overcurrent Relay Definition: An overcurrent relay is a protective device that operates solely based on current without the need for a voltage coil.
- Working Principle: When the current in an overcurrent relay exceeds a critical level, the magnetic effect of the coil activates the moving element, altering the relay’s contact position.
- Types of Overcurrent Relays: There are various overcurrent relays, including instantaneous, definite time, and inverse time relays, each designed for specific response times.
- IDMT Relay Characteristics: The IDMT relay combines inverse time characteristics with a plateau of minimum operating time when the current reaches high levels due to CT saturation.
- Application and Importance: Overcurrent relays are crucial for electrical safety, ensuring quick and appropriate responses to current surges in electrical systems.
A non-directional over current relay or o/c relay uses measured current as its operating quantity. It does not need a polarising voltage input to decide direction. A directional overcurrent function is different because it combines the overcurrent element with direction logic and a polarising quantity. Modern numerical equipment may contain many elements in one protective relay.
Working Principle of Over Current Relay
An electromechanical overcurrent relay uses current in a coil to produce magnetic force or torque. Below pickup, the restraining spring holds the moving element in its normal position. Above pickup, operating force exceeds restraint and the element changes its contact state. A numerical relay instead samples CT current, calculates the selected current quantity and compares it with a pickup setting. In both cases, pickup starts the element logic. An intentional or curve-based delay may still need to expire before a trip output is issued, and the circuit breaker then has its own opening time.
Types of Over Current Relay
By operating-time characteristic, the main types of Over Current relays are:
- Instantaneous over current relay.
- Definite time over current relay.
- Inverse time over current relay.
An inverse time over current relay, also called an inverse OC relay, operates on a selected curve. Common families include inverse definite minimum time (IDMT), very inverse time and extremely inverse time over current relay curves. An OC relay may also provide standard inverse, long-time inverse, short-time inverse, definite-time and manufacturer-specific curves. The exact names and equations depend on the relay standard and model.
Instantaneous Over Current Relay
An instantaneous over current relay has no intentional time delay.
In a simple attracted-armature unit, current in the coil magnetises a core. A hinge and restraining spring hold the iron armature away from the core below pickup, leaving the normally open contact open. When current exceeds the pickup setting, magnetic attraction moves the armature and changes the contact state. Numerical instantaneous elements perform the equivalent threshold comparison in software.
Pickup is the threshold at which the element starts or asserts. The term instantaneous over current relay does not mean zero operating time. Measurement, filtering, element logic, output contacts and the breaker all add time. Use the manufacturer’s published pickup time, overreach, accuracy and output time for coordination.
Definite Time Over Current Relay
A definite time overcurrent relay starts a fixed intentional delay after measured current exceeds pickup. It operates only if current remains above the element’s pickup or reset condition for the required interval. Pickup and delay are separately adjustable on many relays. Actual operation includes the set delay plus applicable measurement and output time, within the published tolerance.
Inverse Time Over Current Relay
An inverse-time element operates faster as the multiple of pickup rises. Its time follows a selected IEC, IEEE or manufacturer curve rather than a universal reciprocal of current. Induction-disc relays produce the characteristic mechanically, while numerical relays calculate or store it. Curve family, pickup, time multiplier, reset behaviour, minimum time and any instantaneous stage must all match the protection-coordination study.
Inverse Definite Minimum Time Over Current Relay or IDMT O/C Relay
An IDMT characteristic combines an inverse operating-time region with a defined lower time limit or asymptotic minimum set by its curve and relay implementation. The current transformer supplies the measured current but does not create that characteristic. If a CT saturates, its secondary waveform and magnitude no longer reproduce primary current accurately. Saturation can delay pickup, increase operating time or prevent an intended high-set stage from seeing the true fault level. The IDMT relay curve, time multiplier and any explicit minimum-time setting must therefore be applied separately from the CT saturation check.
