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Overload device negate requirement for In<Iz?

E95

Hi all,

For context this is a TP, N & E socket circuit supplying a machine. The machine supplier has recommended that the circuit be supplied from a 125A MCB, however the machining cabling is not supplied from a cable rated to 125A.

I'm being told that a cable size doesn't need to be sufficiently sized for the upstream breaker, when there is an overload unit downstream and that the cabling would withstand a short circuit disconnecting quickly

Is this true? I can't find anything explicit in the regs confirming either way, aside from Ib<In<Iz

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    The telling bit of information is how the term 'overload current' is defined.

    From part 2: an overcurrent occurring in a circuit that is electrically sound.

    In other words, by definition, if the characteristics of the circuit do not permit an overcurrent without a fault occurring then no overload protection is required at all. In this case as long as fault protection is afforded by the Zs being low enough to operate the protective device in time, and the conductors being large enough to pass the fault without damage, then the circuit is compliant. Incidentally, in this instance it's possible for the circuit to not meet I(z)>I(n)>I(b) yet still be compliant. 

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  • 0

    The telling bit of information is how the term 'overload current' is defined.

    From part 2: an overcurrent occurring in a circuit that is electrically sound.

    In other words, by definition, if the characteristics of the circuit do not permit an overcurrent without a fault occurring then no overload protection is required at all. In this case as long as fault protection is afforded by the Zs being low enough to operate the protective device in time, and the conductors being large enough to pass the fault without damage, then the circuit is compliant. Incidentally, in this instance it's possible for the circuit to not meet I(z)>I(n)>I(b) yet still be compliant. 

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