Earth-fault loop impedance
The key idea
When a live conductor touches an earthed enclosure, the fault current is just 230 V divided by the loop impedance Zs. If Zs is too high, the current is too small to trip the breaker fast — and the enclosure stays live while someone might be touching it.
The idea
Picture the fault that this check exists for: a frayed conductor touches the metal case of a machine. The case is earthed, so a current immediately flows around a loop — out from the transformer along the line conductor, through the fault, and back along the protective conductor (CPC).
That loop is a plain series circuit, and Ohm's law runs it:
fault current = 230 V ÷ Zs
where Zs is the impedance of the whole loop — the transformer, the line conductor all the way out, and the protective conductor all the way back.
The breaker on the circuit only trips instantly if the current reaches its magnetic threshold — for a 32 A type B MCB, five times rated current, or 160 A. Below that, only the slow thermal element is left, and "slow" can mean many seconds with the enclosure live the whole time. So the whole check reduces to one comparison: is Zs small enough that 230/Zs clears the magnetic threshold?
Try it
fault current: 288 A · need ≥ 160 A
230 V ÷ 0.80 Ω = 288 A — above the magnetic threshold, so the breaker trips instantly. Lengthen the circuit (raise Zs) and watch the current fall toward the limit.
Why it matters
- This is the LV twin of the minimum-fault-current check. The HV version asks whether a relay sees a far fault; this one asks whether a breaker sees a fault at the end of a final circuit. Both fail by having too much impedance in the loop.
- It limits circuit length. Every meter of cable adds loop impedance. A circuit that passes at 20 m can fail at 60 m with the identical breaker — this check, not load current, often sets the maximum run.
- Every fix is an impedance fix. A larger protective conductor, a shorter route, a lower-impedance supply — or an RCD, which trips on milliamps of leakage and does not need the big loop current at all.
The math, if you want itOptional — the page reads completely without it
The loop is a plain series sum — the supply, the way out, and the way back:
the loop impedance
Zs = Ze + R₁ + R₂
with Ze the external impedance of the supply, R₁ the line conductor out, and R₂ the protective conductor back — the same colors as the loop diagram above. The disconnection requirement translates into a maximum Zs per device:
the pass mark
Zs,max = 230 VItrip
For the 32 A type B breaker in the widget, Itrip = 5 × 32 = 160 A, so Zs,max ≈ 1.44 Ω.
See it in Phasor
Phasor calculates Zs along the actual path through your model and compares it against the disconnection requirement of the protective device on the circuit — the same comparison this widget makes, run for every final circuit at once.