Protective device coordination
Will the downstream breaker clear before the upstream one? Enter two devices and the fault current, and compare their IEC 60255 curves: trip times, the margin between them, and the TMS the upstream device needs to be selective.
| Device | Curve | Pickup (A) | TMS | Time at fault (s) |
|---|---|---|---|---|
| Upstream | Standard inverse | 250 | 0.30 | 0.737 |
| Downstream | Standard inverse | 100 | 0.10 | 0.183 |
Selective: the upstream device waits 4.03× the downstream time at 4000 A.
t = TMS · 0.14 / ((I/Is)^0.02 − 1) · Standard inverse; VI, EI and LTI use different constants.
m = t_upstream / t_downstream · Typically 1.3 or more at the maximum fault level.
TMS_up = TMS_up,now · m_required / m_actual · What the upstream device needs to reach the margin.
A clear path from inputs to results.
Coordination is the question a cable check cannot answer: at this fault current, does the downstream device clear before the upstream one? Two IEC 60255 curves, one fault level, and the margin between them — plus the TMS the upstream device would need to be selective.
How to use
- Set the downstream curve, pickup and TMS.
- Set the upstream device the same way.
- Enter the fault current and the margin you require, then read the verdict.
The formula
Reference: IEC 60255-151 standard inverse curve; margin convention from common protection practice.
Worked example
100 A downstream, 250 A upstream, 4 kA fault
- Both standard inverse; downstream TMS 0.10, upstream TMS 0.30.
- 1
At 4 kA the downstream sees 40× pickup and trips in 0.13 s; the upstream sees 16× and trips in 0.44 s.
Margin 3.4×, comfortably selective against a 1.3× requirement.
Interpreting the result
A margin below the requirement means a fault in the downstream zone can trip the upstream device, taking out more of the installation than necessary.
Assumptions & limitations
- Two devices at one fault level.
- IEC inverse curves; definite-time relays are handled separately.
- No CT saturation, no arc impedance.
- Only the maximum fault level is checked here.
Frequently asked questions
Why 1.3×?
It covers breaker clearing time, relay tolerance and the CT error; some utilities ask for 1.5×.
What does the suggested TMS do?
It is the upstream TMS that would produce the required margin at this fault current.
Should I check the minimum fault too?
Yes — selectivity at the maximum fault does not guarantee it at the minimum, where the curves are closer.
Can I coordinate a fuse and a relay?
Not in this form; fuse curves are manufacturer-specific and need the published time-current data.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.