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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?
but assumes that the relationship between R1 and R2 is equal in terms of resistance. This is not the case in the UK where reduced csa is used for twin and earth cables which, when using the same calculator for prospective fault voltage, would result in higher values and, in consequence, shorter required disconnection times.
But to some extent our use of reduced c.p.c.s is mitigated by main bonding - as the shock (inside a building) is likely to be, not to true earth, but to an earthed piece of metalwork (exposed- or extraneous-conductive-part) that's connected to the MET. The touch voltage is then reduced, as the victim is only exposed to the v.d. across R2 within the installation - and is spared the corresponding v.d. along the DNO's PE or PEN. There can't be solid guarantees of course as there are many variables that are out of our control - but reasonable assumptions tend to suggest that often bonding will balance out the effects of reduced c.p.c.s.
but assumes that the relationship between R1 and R2 is equal in terms of resistance. This is not the case in the UK where reduced csa is used for twin and earth cables which, when using the same calculator for prospective fault voltage, would result in higher values and, in consequence, shorter required disconnection times.
But to some extent our use of reduced c.p.c.s is mitigated by main bonding - as the shock (inside a building) is likely to be, not to true earth, but to an earthed piece of metalwork (exposed- or extraneous-conductive-part) that's connected to the MET. The touch voltage is then reduced, as the victim is only exposed to the v.d. across R2 within the installation - and is spared the corresponding v.d. along the DNO's PE or PEN. There can't be solid guarantees of course as there are many variables that are out of our control - but reasonable assumptions tend to suggest that often bonding will balance out the effects of reduced c.p.c.s.
