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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?
What about the 5s disconnection time where did that come from?
The legend I heard was that it dated from (smoke filled rooms) when the regs committee first started discussing disconnection times and one member did a quick back of an envelope calculation based on what he knew about the circuits where he worked in his normal day job - and reckoned that they'd have to permit up to 5s if his system was going to continue to comply.
There's probably a co-incidenal relationship between most fuse characteristics and an acceptable voltage drop - which usually means that if the voltage drop is acceptable the loop impedance will be such that disconnection time will probably happen within about 5s too.
What about the 5s disconnection time where did that come from?
The legend I heard was that it dated from (smoke filled rooms) when the regs committee first started discussing disconnection times and one member did a quick back of an envelope calculation based on what he knew about the circuits where he worked in his normal day job - and reckoned that they'd have to permit up to 5s if his system was going to continue to comply.
There's probably a co-incidenal relationship between most fuse characteristics and an acceptable voltage drop - which usually means that if the voltage drop is acceptable the loop impedance will be such that disconnection time will probably happen within about 5s too.
