Checks grading between an upstream and downstream relay at a common fault current. The downstream relay must clear first, with enough margin that the upstream one doesn't also trip and take out more of the system than necessary.
Standard method — stated scope limits apply
Fault
Upstream Relay
Downstream Relay
Enter both relays and check.
About This Calculation
Protective device coordination (selectivity) means that when a fault happens, only the breaker or fuse closest to the fault opens — not every device upstream of it. Achieving that requires plotting every device's time-current characteristic on the same log-log graph and confirming adequate time separation (coordination margin) between adjacent devices across the full range of possible fault currents, not just at one point. Poor coordination shows up in exactly the way plant operators dread: a fault on one small feeder trips the main breaker and blacks out the whole switchboard instead of just that feeder.
Frequently Asked Questions
What is a typical minimum coordination time interval (CTI) between devices?
A commonly used margin between two electromechanical or digital relay/breaker time-current curves is around 0.3–0.4 seconds at the fault current level being checked, accounting for breaker interrupting time, relay overtravel/overshoot, and a safety margin — fuse-to-fuse coordination often uses tighter margins since fuses have no moving parts to overtravel.
Why must coordination be checked across a RANGE of fault currents, not just one value?
Two time-current curves that are well-separated at low fault current can cross or converge at high fault current (or vice versa) — curve shapes differ (inverse-time vs. definite-time vs. instantaneous), so adequate margin at one current level says nothing about margin at another. A full coordination study plots the whole range the system can actually see.
What is the difference between coordination and discrimination?
They’re essentially the same concept under different regional naming conventions — IEC literature more often says "discrimination," while North American practice more often says "coordination." Both describe the same goal: only the nearest device to a fault operates.
Can two devices ever be impossible to coordinate?
Yes — if the downstream device’s maximum clearing time at some fault current exceeds the upstream device’s minimum pickup time at that same current, no time-current curve shape or setting change can create separation there; the only fixes are changing device type/rating, adding an intermediate protective step, or accepting that fault range as a coordination gap.