Free Tool · IEC 60947-2 §8.3 / §8.4 · Protective Device Coordination · Electrical

Relay Coordination Check

Time-current selectivity and back-up coordination for upstream/downstream circuit-breaker pairs per IEC 60947-2 §8.3 (selectivity) and §8.4 (cascading). Enter device trip-curve parameters, specify the fault current, get a selective/cascading/partial verdict with grading-margin detail and recommended settings adjustments.

Upstream Device

Device label

I_r — rated current (A)

I_sd — short-delay pick-up (A)

t_sd — short-delay time (s)

I_i — instantaneous threshold (A, 0 = disabled)

I_g — ground-fault pick-up (A, 0 = disabled)

t_g — ground-fault time (s)

IDMT curve type

TMS — time multiplier setting

Downstream Device

Device label

I_r — rated current (A)

I_sd — short-delay pick-up (A)

t_sd — short-delay time (s)

I_i — instantaneous threshold (A, 0 = disabled)

I_g — ground-fault pick-up (A, 0 = disabled)

t_g — ground-fault time (s)

IDMT curve type

TMS — time multiplier setting

Fault & Coordination

I_k — fault current at downstream bus (kA)

Required coordination type

I_sd should be ≥ 1.5 × downstream I_i to ensure the upstream short-delay stage does not overlap with downstream instantaneous.

Minimum grading margin per IEC 60947-2 §8.3: 100 ms between consecutive time steps.

Coordination Results

Time margin at I_k

Upstream trip time (ms)—

Downstream trip time (ms)—

Upstream I²t (A²s)—

Downstream I²t (A²s)—

Selective up to (kA)—

Cascading limit (kA)—

Ground-fault selectivity—

Export coordination report

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Worked Example — 1600A ACB / 400A MCCB @ 25 kA

FAIL Not coordinated — time margin 0 ms < required 100 ms

Upstream trip time (ms)20 ms

Downstream trip time (ms)20 ms

Time margin at I_k0 ms

Selective up to (kA)20 kA

Clause referencesIEC 60947-2 §8.3 (selectivity tables), IEC 60255-151 §9.1

FAQ

What is the difference between IEC 60947-2 and IEC 60898?

IEC 60947-2 covers industrial circuit-breakers (ACBs, MCCBs rated for switchgear panels) with adjustable trip units including I_r, I_sd/t_sd, I_i, I_g/t_g settings. IEC 60898 covers smaller residential/commercial miniature circuit-breakers (MCBs, typically up to 125 A) with fixed B/C/D trip curves and no adjustable short-delay stage. Relay coordination with adjustable settings is only possible under IEC 60947-2 devices.

When is cascading coordination acceptable under IEC 60947-2?

Cascading (back-up) protection per §8.4 is permissible when the upstream device limits energy let-through I²t at the fault point to a level the downstream device can withstand — verified by the manufacturer's type-tested cascading table. Both devices must be tested as a combination. It is not sufficient to check individual device ratings. Cascading is acceptable where total selectivity is impractical (e.g. high short-circuit levels exceeding downstream Ics), but it means the upstream device may also trip for faults beyond the downstream device's breaking capacity.

How do I read a manufacturer trip curve for coordination?

Trip curves plot trip time (y-axis, log scale, typically 0.01 s to 1000 s) versus fault current as a multiple of rated current I/I_r (x-axis, log scale). The lower band is the fastest possible trip; the upper band is the slowest. To verify selectivity, the fastest upstream curve must be above the slowest downstream curve at every fault current from I_r upward. A grading margin of at least 100 ms is required at each operating zone boundary (overload, short-delay, instantaneous). Read the curves at the prospective short-circuit current I_k at the downstream busbar.

What does "total selectivity" mean?

Total selectivity (IEC 60947-2 §8.3.1) means only the downstream protective device operates for every fault within the downstream circuit, at every fault current up to and including I_k (the maximum prospective fault current). The upstream device never trips — it acts purely as a back-stop. This requires the upstream time-delay stage to be set such that its trip time always exceeds the downstream clearing time by the grading margin (100 ms minimum). Where the fault current exceeds the downstream device's breaking capacity (I_cs), total selectivity is no longer achievable and cascading or bus coupler schemes must be used instead.