11.2 Disconnection Times (0.4 s & 5 s)

Disconnection Times (0.4 s & 5 s)

Quick Answer: For TN-system automatic disconnection taught with AS/NZS 3000 and capstone: use 0.4 s for final subcircuits ≤ 32 A, and 5 s for distribution circuits and final subcircuits > 32 A. Match that time to the protective device’s time–current curve and the corresponding maximum Zs. Mixing 0.4 s and 5 s is one of the highest-frequency exam traps in this domain.

Why Time Limits Exist

Automatic disconnection of supply is a primary protection measure against electric shock and against thermal effects of fault current. When an earth fault raises touch voltage on exposed conductive parts, the hazard persists until the protective device opens. AS/NZS 3000 therefore states maximum disconnection times — the longest acceptable clearing times for defined circuit categories under TN (MEN) conditions.

Two numbers dominate Queensland licence teaching and open-book lookup:

Always confirm the live clause/table wording in your permitted edition on the day — the 0.4 / 5 split is the durable teaching framework markers expect you to apply correctly.

0.4 s — Ordinary Final Subcircuits ≤ 32 A

Most domestic and light-commercial finals sit here: lighting circuits, socket-outlet circuits, and many fixed appliances on 10 A, 16 A, 20 A or 32 A protective devices.

Why a tight 0.4 s?

• People frequently touch Class I equipment and metal accessories on these circuits.
• Touch voltage during an earth fault can be hazardous within a fraction of a second.
• Overcurrent devices must therefore see enough fault current (If ≈ U0 / Zs) to operate on the fast part of their curve — typically magnetic trip for MCBs, or the corresponding fuse clearing band.

Design and verification implication: you must use Zs,max values associated with 0.4 s disconnection for that device. Using a more generous 5 s impedance limit on a 20 A socket circuit is wrong even if the arithmetic “looks safer for the installer.”

Practical picture

A 16 A Type C MCB on a kitchen circuit: measured Zs must be low enough that If reaches the current that clears in ≤ 0.4 s on that curve. If the run is long, the PEC undersized, or joints poor, Zs rises, If falls, and the device may take seconds in the thermal region — a fail against the 0.4 s requirement.

5 s — Distribution Circuits and Larger Finals

Distribution circuits (submains feeding distribution boards, rising mains, and similar) and final subcircuits rated above 32 A are commonly assessed against a 5 s maximum disconnection time under the TN teaching summary.

Reasons the Rules allow the longer time in those categories include the nature of the circuit (often fewer casual touch points than everyday finals, different risk balancing in the standard’s structure) — but for exam purposes you need the classification, not a philosophical debate:

• Submain from main switchboard to garage DB → think distribution → 5 s (confirm category).
• 63 A final to a large fixed machine → think final > 32 A → 5 s.
• 20 A lighting final → still 0.4 s, never “5 s because it is in a commercial building.”

Building type does not rewrite the final-subcircuit ≤32 A rule. A commercial office GPO circuit on a 20 A breaker remains a 0.4 s final.

Linking Time, If and the Device Curve

Disconnection time is not a separate magic switch. It is the horizontal axis outcome of the device’s time–current characteristic at the fault current delivered by the loop:

1. Measure or calculate Zs.
2. Compute If ≈ U0 / Zs.
3. Read the device curve (B/C/D MCB, fuse type, MCCB settings) at that If.
4. Confirm the clearing time ≤ the required maximum (0.4 s or 5 s).

Maximum Zs tables compress steps 2–4 into a single impedance limit for common devices. When tables are used, still know which time they assume. A Zs,max column for 5 s will be higher (more permissive) than the 0.4 s column for the same device — because a smaller If can still clear within five seconds on the thermal portion of many curves.

That single comparison explains why candidates who “borrow” the 5 s limit for a socket circuit invent a false pass.

Exam Traps Mixing 0.4 s and 5 s

Capstone papers love stems such as:

• “A 20 A final subcircuit feeding GPOs — maximum disconnection time?” → 0.4 s.
• “A submain feeding a distribution board — maximum disconnection time?” → 5 s (TN distribution teaching).
• “A 40 A final to a fixed cooker / machine — which time?” → 5 s because the final exceeds 32 A.
• “Commercial building, 16 A lighting — 5 s because commercial?” → No — still 0.4 s.

Other traps:

• Claiming 0.4 s applies only to RCDs — incorrect; 0.4 s is the overcurrent automatic-disconnection time category for those finals; RCDs are additional protection with their own residual-current behaviour.
• Claiming any earth fault may take 5 s on a 10 A circuit “if the RCD is present.”
• Using 0.4 s for a large distribution MCCB when the circuit is clearly a distribution circuit assessed at 5 s (and vice versa).

Relationship to RCD Additional Protection

30 mA RCDs (Chapter 8) disconnect on residual current and can clear many earth-leakage / shock scenarios extremely quickly. That is additional protection. It does not mean you may ignore earth-fault-loop impedance limits for automatic disconnection by overcurrent where that protection method is required and verified. Capstone answers that say “RCD present, therefore Zs does not matter” fail the competence.

Conversely, a healthy Zs and correct MCB do not remove RCD requirements where AS/NZS 3000 mandates additional protection on finals.

Verification Mindset on Test Day

When documenting results (Section 8 link):

• State the circuit type and device rating.
• State the applicable disconnection time (0.4 s or 5 s).
• State measured Zs and the Zs,max for that device and time.
• Fail and rectify if measured Zs exceeds the limit for the correct time — do not “reclassify” the circuit to harvest a laxer limit.

Capstone Focus Phrases to Memorise

1. Final ≤ 32 A (TN) → 0.4 s.
2. Distribution, or final > 32 A (TN) → 5 s.
3. Stricter time → stricter (lower) Zs,max.
4. Building occupancy does not convert a 20 A final into a 5 s circuit.
5. RCD ≠ substitute for correct overcurrent disconnection-time assessment.

Bridge Forward

Section 11.3 asks a different but related question: once fault current is high enough to trip in time, can the device interrupt the prospective fault current without destroying itself? That is breaking capacity versus PFC — the mechanical/thermal interruption rating, not the 0.4 / 5 clock. Keep time limits and breaking capacity as separate checklist items.