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What is In current in relay protection

Fiber and communications infrastructure knowledge brief Published 06 Nov 2025

Prepared for preliminary research by the ROGER FIBRE knowledge desk. Confirm optical interfaces, port or core capacity, electrical input and output, enclosure protection, thermal design and installation limits against current standards and product documentation.

The current used for relay protection is primarily defined by the relay's pickup current, which is the minimum current at which the relay begins to operate, typically set as a percentage of the current transformer's rated secondary current.

Pickup Current and Current Setting

The pickup current is the threshold at which a protective relay starts to operate, overcoming the controlling force that holds its contacts in the normal position. In normal conditions, the deflecting force generated by the relay coil is weaker than the controlling force, so the relay does not operate. When the current through the relay coil exceeds the pickup current, the deflecting force surpasses the controlling force, causing the relay to actuate and send a trip signal to the circuit breaker ( ).

The current setting of a relay allows adjustment of the pickup current to match system requirements. This is done by changing the number of active turns in the relay coil using a plug bridge, effectively modifying the current required to actuate the relay. For example, an overcurrent relay may be set to operate at 125% of the rated current of the system.

Current settings are typically expressed as a percentage of the current transformer's secondary rated current, ranging from 50% to 200% for overcurrent relays and 10% to 70% for earth fault relays ( ). Plug Setting Multiplier (PSM) and Time SettingThe Plug Setting Multiplier (PSM) is the ratio of the fault current to the relay's pickup current. It determines how quickly the relay will operate under fault conditions.

The Time Setting Multiplier (TSM) adjusts the relay's operating time, allowing coordination with other protective devices to ensure selective tripping ( ).

Types of Relays and Current Application

Common overcurrent relays include: 50 (Instantaneous Overcurrent Relay): Trips immediately when the current exceeds the pickup value, typically used for short-circuit protection ( ). 51 (Inverse Time Overcurrent Relay): Operates with a time delay inversely proportional to the magnitude of the overcurrent, providing coordination with downstream devices ( ). 50N/51N (Earth Fault Relays): Detect ground faults using residual or zero-sequence currents ( ).

Relays are powered by the secondary current of current transformers (CTs), which step down high primary currents to safe levels for relay operation. Direct connection to high primary currents is not feasible due to the large magnitude of the current ( ).

Practical Considerations

The pickup current must be carefully set to avoid nuisance tripping during normal load conditions while ensuring rapid response to faults. Coordination with other relays and breakers is essential to maintain system reliability and selectivity. Modern microprocessor-based relays allow precise digital settings for pickup current, time delays, and multiple protection functions in a single device ( ).

In summary, the current used for relay protection is defined by the relay's pickup current, adjusted through current settings and plug settings, and coordinated with system CTs and other protective devices to ensure safe and selective operation of the electrical system.

What is In current in relay protection - ROGER FIBRE ATELIER [PDF]

What is In current in relay protection

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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