What is inverse timing relay

Difference between instantaneous, specific time and inverse time compared to current protection relays

When electromechanical relays were still in use, inverse timing relays, certain timing relays, and momentary relays were separate relays.

Modern protection relays combine inverse time, specific time and instantaneous properties in one device. So you can have all three types in one device.

The time-current characteristic differs for inverse time, specific time and instantaneous relay.

You can use combinations of curve types to meet your design needs. I usually use inverse time, definitive time, and instantaneous elements, all on the same relay.

  • Inverse time overcurrent relay: Slow tripping at low currents. Faster tripping with high fault currents. Used to coordinate the overload protection, which can have a high starting current. Generally most sensitive (pickups with the lowest amp) and slowest to operate.

  • Certain timing relays: will used to coordinate over another specified time or immediate protection. Generally less sensitive (higher pickup) to prevent operation at switch-on load. Generally faster uptime.

  • Momentary relay: Used when coordination is not required. Usually the least sensitive of all relays, as the relay must not be operated for any inrush current or before a downstream relay.

For more information, including application details, see:

Protection of power systems is a deep and interesting field. Good luck!