7.3 Disconnection Times (0.4 s / 5 s) & Earth Fault Path

Automatic Disconnection of Supply (ADS)

Automatic disconnection of supply is a core fault-protection method in AS/NZS 3000: when an earth fault occurs, protective devices must disconnect the faulty circuit within a maximum time so touch voltages on exposed conductive parts do not persist long enough to cause dangerous shock.

Two classic Wiring Rules figures every NZ regulations candidate must recognise:

These are classic exam anchors, not a licence to ignore the standard. Your permitted AS/NZS 3000 edition contains application tables that assign disconnection times by system voltage, circuit type, and installation conditions. Under open-book pressure:

1. Identify whether the stem describes a final subcircuit or a distribution/submain arrangement.
2. Open the ADS / disconnection time tables for that system.
3. Apply the time to the protective device operating-current calculation or measured Zs limit.

Do not invent a third time (for example mixing RCD 40 ms figures with ADS 0.4 s figures without reading the stem). RCD trip-time criteria (300 ms / 40 ms under ESR unsafe framing) and ADS maximum times (0.4 s / 5 s) answer different questions: residual-device performance vs earth-fault clearance by the installation’s protective scheme.

Why Time Limits Exist

When an active conductor contacts earthed metal, the exposed part rises toward a touch voltage determined by fault current and earth-path impedance. The longer that voltage remains, the greater the shock risk. ADS limits duration. Equipotential bonding and low PE impedance limit voltage magnitude. Together with basic insulation, they form the fault-protection package. RCDs add residual-current disconnection for many low-level faults, but the 0.4 s / 5 s ADS framework remains central to how AS/NZS 3000 expects earth faults to be cleared when overcurrent devices provide fault protection.

Earth Fault Loop Path (MEN System)

In New Zealand’s common MEN (Multiple Earthed Neutral) arrangement, a typical earth-fault current path for a fault from active to exposed earthed metal is:

Active conductor → fault (e.g. active to case) → protective earthing conductor (PE) → main earthing / MEN link → neutral conductor → return to the source (transformer/generator star point)

Teach the loop as a series path. Every joint matters:

Missing MEN is a classic failure mode: without the main neutral–earth bond at the correct location, the intended low-impedance return for earth fault current is broken or degraded. Protective devices may not see enough fault current to operate in 0.4 s or 5 s, and exposed metal may remain hazardous under fault. High-impedance PE joints, corroded electrodes used as the only path, or open circuit protective conductors produce the same exam outcome: timely ADS fails.

The Zs × Ia Relationship

Let:

• Zs = earth fault loop impedance (ohms) at the point of fault
• U0 = nominal voltage active to earth (e.g. 230 V class systems—use the value in your tables)
• Ia = current that causes the protective device to operate within the required disconnection time

Rough teaching relationship:

Fault current If ≈ U0 / Zs

For the device to disconnect in time:

If ≥ Ia ⇒ Zs ≤ U0 / Ia

So maximum Zs tables (or calculations from device time-current characteristics) define how low the loop impedance must be. If measured or calculated Zs exceeds the maximum for that breaker/fuse and disconnection time, the installation does not achieve ADS by that overcurrent device alone—even if the breaker is the “right amp rating” for load current.

Implications candidates must verbalise:

1. Cable size and PE size affect Zs—not only active CCC for load.
2. Device type and rating set Ia (a 32 A Type C MCB needs more fault current to trip magnetically in time than a smaller/faster device—use manufacturer or standard curves/tables).
3. Measurement of earth fault loop impedance is a verification activity (later testing chapters); this section owns the principle.
4. RCDs can disconnect on residual current far below magnetic trip levels, which is why additional residual protection is so valuable on final subcircuits—but you still design and verify the earth path for ADS and for touch-voltage control.

Worked logic (qualitative example)

A socket final subcircuit requires 0.4 s disconnection under the applicable table. The protective device needs Ia amperes to operate within 0.4 s. If Zs is too high because the PE is undersized, a joint is loose, or the MEN is missing, If falls below Ia and the breaker may sit in the thermal region for far longer than 0.4 s—or never trip on a high-resistance earth fault. Touch voltage can persist. That is an ADS failure, and it is electrically unsafe in substance even before you quote a regulation number.

Distribution Circuits and the 5 s Figure

Submains and distribution circuits often receive the 5 s maximum disconnection time in the classic tables because they typically feed fixed equipment or further distribution rather than handheld portable loads, and coordination/selectivity with downstream devices is part of design. Still:

• Confirm the exact circuit class in the stem against the table.
• A submain that fails to clear within 5 s under earth fault is still a failed ADS design.
• Downstream final circuits may still need 0.4 s at their own protective devices.

RCD vs Overcurrent ADS — Keep the Stories Straight

A stem about “maximum disconnection time for a final subcircuit under ADS tables” wants 0.4 s class thinking and Zs/Ia. A stem about “RCD fails to trip within … at rated residual current” wants 300 ms / 40 ms and ESR/AS testing language. Mixing them is a common self-inflicted error.

Exam Navigation for Disconnection Questions

• Flag ADS / fault protection / max disconnection time tables in AS/NZS 3000.
• Flag earthing / MEN arrangement diagrams and PE conductor rules.
• If the stem says “socket final,” reach for 0.4 s application logic; if “submain/distribution,” reach for 5 s—then verify in the table.
• If the stem describes open PE or missing MEN, answer ADS compromised / unsafe fault path, not “RCD will always save it regardless.”
• Link forward: earth fault loop impedance testing (later chapter) is how field verification proves the Zs side of this theory.