- Inverse Time Relay Definition: An inverse time relay is defined as a relay where the operation time decreases as the actuating quantity increases.
- Operating Time Relationship: The relay’s operating time is inversely proportional to the magnitude of the actuating quantity, meaning higher quantities result in faster relay operation.
- Pick Up Value: The minimum actuating quantity required to initiate the relay’s operation is known as the pick-up value.
- Mechanical Accessories: Inverse time relays use mechanical accessories, such as a permanent magnet in an induction disc relay or an oil dash-pot in a solenoid relay, to achieve inverse time delay.
- Definite Time Lag Relay: This type of relay operates after a set time delay, independent of the magnitude of the actuating quantity, ensuring consistent operation time above the pick-up value.
An inverse-time relay operates faster as its actuating quantity rises above pickup. Its operating time follows a defined characteristic and time setting; it is not generally a simple inverse proportion to the input magnitude.
A typical set of characteristics of an inverse time relay is shown below.
In the graph, OA represents the pickup value. At an actuating quantity of OA, the illustrated operating time is OA’. At OB it is OB’, and at OC it is OC’.
Below pickup, the relay does not start. Just above pickup, an inverse-time characteristic can have a long operating time. The exact threshold, curve shape and operating tolerance come from the relay specification.
As the actuating quantity becomes much larger than pickup, the operating time approaches a finite minimum rather than zero. Measurement, processing, mechanism and output time prevent truly instantaneous operation.
An inverse time relay whose actuating quantity is current is an inverse-time overcurrent relay.
Electromechanical relays create their characteristic through operating torque, restraining torque and mechanical travel. Numerical relays instead calculate a selected IEC, IEEE or manufacturer curve in software.
In a historical induction-disc relay, a permanent magnet produces braking torque as the rotating disc cuts magnetic flux. A solenoid relay can use a piston and oil dashpot to produce an inverse time relay. These mechanisms illustrate older implementations, not a requirement for every inverse-time relay.
In an oil-dashpot relay, oil viscosity resists piston movement. A larger actuating current produces more solenoid force and can move the plunger faster. The resulting time-current relationship depends on the complete mechanism and its calibration, so engineers use the specified relay curve rather than assuming exact inverse proportionality.
Definite Time Lag Relay
A definite-time relay adds an intentional delay for protection coordination. After the actuating quantity crosses pickup and remains above it, the relay starts its timer and operates when the set delay expires.
Over the element’s specified operating range, the set delay does not vary with input magnitude. Actual contact operation still includes measurement and output tolerances. If the input falls below dropout before the timer expires, the relay may reset immediately or after a configured reset delay.
