Fault current values
The key idea
A fault does not have one current — it has a current that changes over its first moments. Four standard values capture the moments that matter: I″k at the start, ip at the first peak, Ib when the breaker opens, and Ik once everything settles.
The idea
When a fault lands near generators, the current starts at its largest and decays: the machines' internal fields react over a few cycles and their contribution drops. On top of that, the first cycle carries a DC offset that makes the very first peak the biggest single instant of all.
Because equipment fails in different ways, different moments of this story get their own number:
- I″k — initial symmetrical current (RMS, at the instant of the fault). The reference value. Thermal withstand checks and protection grading start here.
- ip — peak current (instantaneous, first half-cycle). What the busbar bracing and breaker making-duty must survive mechanically. Set by I″k and the X/R ratio through the factor κ.
- Ib — breaking current (RMS, at the breaker's contact separation). What the breaker must actually interrupt. Near generators, the decay means Ib is smaller than I″k — a breaker that opens later has less to break.
- Ik — steady-state current (RMS, after the transients die). The long-duration value, for anything that must carry the fault for seconds.
Try it
Move the breaker's opening time and watch Ib slide down the decay.
I″k — initial symmetrical
10.0 kA
thermal checks, grading
ip — peak (instantaneous)
25 kA
mechanical forces
Ib — breaking at 80 ms
6.4 kA
breaker rating
Ik — steady state
4.0 kA
long-duration effects
Near a generator, the fault current is largest in the first cycles and decays as the machine's field reacts. A breaker that opens later has less current to break — that is why Ib depends on the breaker's own operating time.
Why it matters
- Each rating gets checked against its own value. Comparing a breaker's breaking capacity against ip, or busbar bracing against Ib, is the classic wrong-number-right-units mistake.
- The decay is a generator phenomenon. Far from machines, on a grid-fed network, the values converge: Ib ≈ I″k ≈ Ik. Near generation they spread apart, and the distinctions earn their keep.
- Motors join in. Induction motors feed the fault for the first cycles and then quit — raising I″k and ip more than the later values. Leaving them out can pass a switchgear check that should fail.
See it in Phasor
Phasor reports all four values per fault location and fault type, plus the κ and X/R behind ip — and compares each against the matching equipment rating, with the margin.