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Disconnection Times
Maximum disconnection times for TN and TT systems from BS 7671 Table 41.1.
Maximum Disconnection Times — Table 41.1
Maximum Disconnection Times — Table 41.1
| System | ≤32A Circuits (s) | >32A / Distribution (s) |
|---|---|---|
| TN (TN-S, TN-C-S) | 0.4 | 5 |
| TT | 0.2 | 1 |
BS 7671: Table 41.1
ADS Requirements
Automatic Disconnection of Supply (ADS) must disconnect within the times above to prevent sustained electric shock.
TN Systems
Achieved by ensuring the fault current is high enough to trip the protective device. Condition: Zs × Ia ≤ U₀ where Ia is the current causing operation within the required time.
TT Systems
RCDs are normally required. Condition: RA × IΔn ≤ 50V where RA is the sum of earth electrode and protective conductor resistance, IΔn is the rated residual operating current.
BS 7671: Reg 411.3.2, Reg 411.5.3
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