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The maximum permissible disconnection time is 0.4 s in TN system. Why and from where this value (0.4 s) is obtained?
Former Community Member
The maximum permissible disconnection time in in the event of a short circuit between a phase conductor and a body or protective conductor or a protective-neutral conductor is 0.4 s in TN system. Does anybody know this value (0.4 s) obtains from where? How this value is obtained?
I suspect the allowance for 5s disconnection is more to do with the practicalities of achieving discrimination (possibly through several stages of sub-mains) and being able to use a range of overcurrent devices (e.g. fuses) within practical loop impedances, together with the lower likelihood of faults on submains etc - rather than any comprehenive plan for shock protection.
For example, a distribution circuit protected by an 80 A BS 88-3 fuse requires a maximum Zs of 0.27 Ω. That might be difficult to achieve if Ze is anywhere near that or higher. A 5 sec disconnection time allows a Zs of up to 0.51 Ω.
I suspect the allowance for 5s disconnection is more to do with the practicalities of achieving discrimination (possibly through several stages of sub-mains) and being able to use a range of overcurrent devices (e.g. fuses) within practical loop impedances, together with the lower likelihood of faults on submains etc - rather than any comprehenive plan for shock protection.
For example, a distribution circuit protected by an 80 A BS 88-3 fuse requires a maximum Zs of 0.27 Ω. That might be difficult to achieve if Ze is anywhere near that or higher. A 5 sec disconnection time allows a Zs of up to 0.51 Ω.
