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sizing of cables for adjustable short-circuit threshold

Huw Pullen
Hi 

i have been searching the regs and concluded that there is no reg covering this.


I have a 100A MCCB but adjusted overload to 40A. I been asked to size the cable to the adjusted overload setting (40A) and not the max MCCB size.

Its not something i want to do as I've always sized to the max fault current. is there any guidance or reg that I can throw back at them to stop this sizing to the adjusted setting and potentially under sizing the cable and under certain conditions, a fault current may exist on the upstream LV panel for a longer period of time than expected resulting in damage to equipment and cables

Thanks for your input

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    I don't think the regs have any objection to relying on the adjusted rating - just as long as the setting can't be changed accidentally. It's not really all that different from being able to replace a fuse with a higher rating one or the wrong sized fusewire. If you couldn't take advantage of the adjusted lower rating, there wouldn't be much point in having adjustable circuit breakers in the first place.

       - Andy.
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  • 0
    I don't think the regs have any objection to relying on the adjusted rating - just as long as the setting can't be changed accidentally. It's not really all that different from being able to replace a fuse with a higher rating one or the wrong sized fusewire. If you couldn't take advantage of the adjusted lower rating, there wouldn't be much point in having adjustable circuit breakers in the first place.

       - Andy.
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