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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?
Presumably distribution circuits are allowed to have long disconnect times because they don't expect anyone to actually come into live contact with them very often (apart from the occasional shortly-to-be-deceased sparky fiddling about inside a DB). All the nasty end-user paraphernalia like water heaters, power tools etc are connected further downstream with 0.2/0.4s disconnect times.
Unfortunately the situation isn't quite that good. If there's a fault on a submain, the earth bar on the DB it serves - together with all the downstream exposed-conductive-parts - will likely be held at the fault voltage (e.g. 115V for TN, 230V for TT) until disconnection occurs (e.g. for the full 5s) - so you can still get a 5s shock from metalwork on a 0.4s circuit.
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.
In theory main bonding can help a bit - but in practice the benefit isn't anything like sufficient to be guaranteed to be life-saving.
Presumably distribution circuits are allowed to have long disconnect times because they don't expect anyone to actually come into live contact with them very often (apart from the occasional shortly-to-be-deceased sparky fiddling about inside a DB). All the nasty end-user paraphernalia like water heaters, power tools etc are connected further downstream with 0.2/0.4s disconnect times.
Unfortunately the situation isn't quite that good. If there's a fault on a submain, the earth bar on the DB it serves - together with all the downstream exposed-conductive-parts - will likely be held at the fault voltage (e.g. 115V for TN, 230V for TT) until disconnection occurs (e.g. for the full 5s) - so you can still get a 5s shock from metalwork on a 0.4s circuit.
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.
In theory main bonding can help a bit - but in practice the benefit isn't anything like sufficient to be guaranteed to be life-saving.
