Fault Loop Impedance on a Long Rural Final Subcircuit
A 2.5 mm² copper final subcircuit supplies a 20 A socket-outlet circuit in a rural shed 85 m cable-route length from the main switchboard. The supply is a 230 V single-phase MEN system with a 63 A service fuse and a 20 A circuit breaker (Type C) protecting the subcircuit. The measured earth fault loop impedance at the furthest socket is 1.9 Ω. The installer has also measured the impedance at the main switchboard as 0.35 Ω. The shed has a concrete floor and earthed metal fixtures. The client reports that a portable tool recently caused the breaker to trip only after a noticeable delay, and sometimes the RCD trips instead.
1. Evaluate whether the measured 1.9 Ω earth fault loop impedance is acceptable for this circuit, showing the reasoning.
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The acceptability of 1.9 Ω depends on the protective device and the disconnection time required. For a 20 A Type C circuit breaker, instantaneous magnetic tripping occurs at 5–10 times the rated current, so the current required for instantaneous operation is at least 100 A (5 × 20 A). At 230 V, the maximum loop impedance for that current is approximately 230 / 100 = 2.3 Ω, so 1.9 Ω would allow the breaker to trip magnetically in theory.
However, the relevant requirement is the disconnection time for the circuit. For a socket-outlet final subcircuit in a TN system, the required disconnection time is generally 0.4 s, and the breaker must operate within that time at the fault current available. At 1.9 Ω the prospective fault current is about 230 / 1.9 = 121 A, which is only just above the magnetic threshold. Manufacturing tolerance means the actual magnetic trip point may be at the top of the 5–10× band, i.e. 200 A, and at 121 A the breaker would rely on thermal operation, which is far too slow for a 0.4 s requirement.
The measured 1.9 Ω is therefore marginal and should not be accepted on calculation alone. The installer must verify against the device's published trip curve and the required disconnection time, and must also confirm that the protective earthing conductor and circuit length are consistent with the measured value. The symptom of delayed tripping supports the conclusion that the circuit is operating in the thermal region rather than the magnetic region.
2. What practical remedies are available, and how should the installer verify the remedy is effective?
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The most direct remedy is to reduce the loop impedance by increasing conductor size on the subcircuit, particularly the active and protective earthing conductors. Increasing the 2.5 mm² cable to 6 mm² or larger over the 85 m run will substantially reduce both the phase and earth path resistance. The protective earthing conductor must be sized in accordance with AS/NZS 3000 for the increased active conductor. An alternative is to relocate or add a distribution board closer to the shed, reducing the length of the high-impedance section.
Changing the protective device is another option: a Type B breaker has a lower magnetic threshold (3–5×), which would operate at a lower fault current, but the device must still achieve the required disconnection time and the circuit must remain adequately protected against overload and short circuit. A combined RCD/MCB may provide additional protection, but an RCD does not cure a high loop impedance for the purposes of fault protection; it provides additional protection and does not replace the requirement for adequate fault current disconnection.
Verification after the remedy must include a fresh earth fault loop impedance measurement at the furthest point, confirmation that the prospective fault current exceeds the magnetic trip threshold with margin, and a check against the device's trip curve for the required disconnection time. The results should be recorded on the verification schedule and compared with the design calculation.