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Disconnection times outside Table 41.1: 5 s and 1 sWhy are 5 seconds allowed?
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After Table 41.1 come two regulations, one for each earthing system. They cover distribution circuits and any circuit not covered by 411.3.2.2:
A disconnection time not exceeding 5 s is permitted.
A disconnection time not exceeding 1 s is permitted.
BS 7671 · Reg 411.3.2.3 TN: up to 5 s · Reg 411.3.2.4 TT: up to 1 s · p.72
It doesn't have to. That's the wrong way to look at it. 5 s is the maximum allowed, not how long it will take. As a designer, you need to know the required time so that you can design the circuit to disconnect within it. For the yellow distribution circuit, you know you're allowed up to 5 s, so that's the limit you work to in your calculations.
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