What is the typical setting current for relay protection

The normal current for relay protection is typically set equal to the secondary rated current of the current transformer (CT) connected to the relay, often 1A or 5A, with adjustable settings depending on the type of relay.

Relay Pick-Up Current

The pick-up current of a relay is the minimum current at which the relay will operate. For overcurrent relays, the pick-up current is generally set between 50% to 200% of the CT secondary rated current, in steps of 25% . For earth fault relays, the pick-up current is usually set between 10% to 70% of the CT secondary current, in steps of 10% . For example, if a CT has a ratio of 200/1 A and the relay is set at 150% pick-up, a primary fault current of 1000 A results in a secondary current of 5 A in the relay coil, which is above the pick-up threshold, causing the relay to operate .

Current Transformer Considerations

Relays are always powered by the CT secondary, which reduces the high line current to a manageable level for the relay, typically 1A or 5A . The CT ratio is chosen slightly higher than the normal line current to ensure proper relay operation without saturation. Multiple relays can share the same CT, but the total burden must not exceed the CT's rated capacity .

Practical Settings

• Instantaneous overcurrent relays (ANSI 50): Operate almost immediately when the current exceeds the pick-up value, typically used for short-circuit protection .
• Inverse time overcurrent relays (ANSI 51): Operate with a time delay inversely proportional to the magnitude of the current, allowing coordination with downstream devices .
• Earth fault relays (50N/51N): Detect residual or zero-sequence currents and are set at a lower percentage of the CT secondary current to detect ground faults .

Summary

The normal current for relay protection is not a fixed value but is determined by the CT secondary rating and the relay type. Typical secondary currents are 1A or 5A, with pick-up settings adjusted according to the protection scheme: 50–200% for overcurrent relays and 10–70% for earth fault relays. Proper selection ensures selectivity, stability, and reliable fault detection in the power system .