Relay 1 (Upstream)
Relay 2 (Downstream)
Two relays at one fault level is a spot check. A protection study grades every relay in the chain across the full range of fault levels. Protection studies →
Enter a valid email address.
Opens the print dialogue. Choose “Save as PDF” to keep a copy.
Grading Curves
Frequently Asked Questions
How does this tool calculate the grading margin?
It works out each relay's trip time at the fault level you enter, from its curve, Is and TMS using the IEC 60255-151 formula t = TMS x k / ((I/Is)^alpha - 1). If a definite time stage is enabled and the fault current is at or above its pickup, the definite time delay is used instead. The margin is the upstream trip time minus the downstream trip time.
What margin counts as a pass?
The margin shows green at 0.3 s or more, amber from 0.2 s to 0.3 s and red below 0.2 s. A negative margin means the upstream relay trips first, and shows red. Confirm the margin your scheme needs against the actual relay and circuit breaker data.
Which curves can be graded?
Standard Inverse, Very Inverse and Extremely Inverse, which are curves A, B and C of IEC 60255-151 Annex A, plus Long Time Inverse. Each relay can also have a definite time stage with its own pickup and delay.
What happens above 20 times the setting?
The inverse curve is held flat above 20 times Is. IEC 60255-151 clause 4.4.1.3 sets 20 times the setting as the minimum end of the dependent time range, and relays behave differently beyond it, so check the manufacturer data for very high fault levels.