How to Test an Overcurrent Relay: Pickup and Trip Time
Published · 2 min read
An overcurrent relay has two things to prove: it picks up at the right current and it trips in the right time. Both are checked by secondary injection with a relay test set.
Before you start
Read the relay's settings: the pickup current (Is), the curve type and the time multiplier (TMS). Isolate the relay with its test switch or by lifting the trip and current-transformer connections, and make sure the CT secondary stays short-circuited. Never open a live CT secondary.
Pickup test
Pickup is the lowest current at which the relay starts to operate. Ramp the injected current slowly from well below the setting and note the value where the relay's pickup output changes. A slow ramp matters: a fast one overshoots the true value.
- Connect the tester's current output to the relay's phase input and its trip contact to the tester's binary input.
- Set the ramp to start at about 80% of the pickup setting and rise in small steps.
- Record the current at which the relay picks up and compare it with the setting.
- Repeat for each phase and for the earth-fault element.
Trip time test
Time is checked on the relay's curve. For the standard inverse curve of IEC 60255 (BS 142) the operating time is t = TMS × 0.14 / ((I/Is)^0.02 − 1). Check the curve and its constants in the relay manual. Other curves use different constants: very inverse 13.5 and exponent 1, extremely inverse 80 and exponent 2.
For example, with TMS 0.5 and a current of five times the pickup, the expected time is 0.5 × 0.14 / (5^0.02 − 1) = about 2.14 seconds. Inject a fixed current at several multiples, for example 2, 5 and 10 times pickup, and record the trip time of each.
- Apply the first test current suddenly (a step, not a ramp) and start the timer.
- Stop the timer when the trip contact closes and record the time.
- Repeat at the other multiples and for each phase.
- Compare each time with the calculated one and with the tolerance in the relay manual.
Report the result
Record the settings used, each injected value, the measured pickup and time and the pass or fail verdict, with the date and the tester's serial number. Test software does this automatically and keeps the same layout from year to year.