Checks that a chain of two or more overcurrent relays in series (from the load end up to the source) trips in the correct order with adequate time separation across the ENTIRE fault-current range they could see — not just at one arbitrarily chosen current — using inverse-time (IDMT) curves from IEC 60255-151 or IEEE C37.112. Add relays to check a full radial grading chain, not just a single pair.
Standards followed
IEC 60255-151 (measuring relays and protection equipment — functional requirements for over/undercurrent protection)IEEE C37.112 (IEEE standard for inverse-time characteristics for overcurrent relays)
What it can do
- Computes trip time for every relay in the chain at any single fault current, given curve family/type, pickup, TMS/time-dial and CT ratio.
- Sweeps the full fault-current range (not just one point) for every successive relay pair, reporting the worst-case (minimum) grading margin anywhere in that range for each step.
- Plots a log-log Time-Current Characteristic (TCC) graph with every relay's curve overlaid.
- Pass/fail verdict per step and for the overall chain, against a configurable required grading margin (CTI), defaulting to the commonly used 0.4s for electromechanical relays.
Example problem
A downstream relay (pickup 100A, TMS 0.1, IEC Standard Inverse) must grade against an upstream relay (pickup 150A, TMS 0.25, same curve) at a 2000A fault.
- Compute each relay's multiple of pickup: downstream = 2000/100 = 20×, upstream = 2000/150 = 13.3×.
- Apply the IEC Standard Inverse equation t = TMS × (0.14 / (M^0.02 − 1)) to each relay using its own multiple and TMS.
- Compare the two trip times — the difference is the grading margin at this specific current.
- Repeat across the full current sweep range to find the worst-case (minimum) margin anywhere, not just at this one point — and repeat again for every additional relay pair if more than two relays are in the chain.
The full-range sweep is what determines pass/fail against the required CTI — a comfortable margin at one current doesn't guarantee compliance everywhere, which is why this calculator checks the whole range, for every adjacent pair, rather than a single point on a single pair.
Want the full step-by-step version?
Curve outputs were checked against the published IEC 60255-151 SI/VI/EI reference table at TMS=1.0 for multiple current multiples. This calculator previously shipped as two separate tools (a 2-relay full-range checker, and a separate Multi-Bus Grading tool limited to a single fault current) — they've been merged here so any chain length gets the more rigorous full-range sweep.