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Short circuit fault current calculations

Hi,

When using an adjustable circuit breaker e.g. current rating adjustable between 15 and 36Amps. What value do you use for rated current in the prospective short circuit fault current calculation, I.e. do you use the maximum current setting (36A) or the actual current setting used in operation, (e.g. 20A).

In the example given above this can make a large difference to the maximum length of cabling that can be used and still ensure instantaneous tripping of the CB.

Also if the actual current setting is used in the calculation, where does responsibility lie if someone adjusts the setting to the maximum, which could mean that the prospective short circuit fault current is no longer enough to guarantee instantaneous tripping, which could in turn lead to a fire risk? Is that foreseeable misuse?

any help or suggestions appreciated.

thanks.

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0 AJJewsbury over 4 years ago

I'd say use the adjusted setting (e.g. 20A rather than 36A).

I've not found a specific regulation for overcurrent protective devices used for ADS, but for RCDs used for shock protection and circuit breakers used for overcurrent protection there is a general requirement that if the device is to be operated by ordinary persons that adjustment can only be altered using a key or tool and there should be a visible indication of the setting (531.3.4.201 and 533.1.3). My thinking is that such precautions wouldn't be needed if you had to assume the worst case of the adjustment being set to the highest possible setting.

Yes there is a risk of someone meddling, but in general we have that risk anyway - cartridge fuses may be replaced by one of a higher rating (not all values are physically different sizes), an incorrect size of fuse wire might be selected for a rewireable fuse - even an MCB might be replaced by one of a different rating or type or even manufacturer.

Ultimately there would be little point in manufacturing adjustable devices if lower settings couldn't be relied upon - you might as well just install a fixed device of the maximum rating.

- Andy.
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0 AJJewsbury over 4 years ago

I'd say use the adjusted setting (e.g. 20A rather than 36A).

I've not found a specific regulation for overcurrent protective devices used for ADS, but for RCDs used for shock protection and circuit breakers used for overcurrent protection there is a general requirement that if the device is to be operated by ordinary persons that adjustment can only be altered using a key or tool and there should be a visible indication of the setting (531.3.4.201 and 533.1.3). My thinking is that such precautions wouldn't be needed if you had to assume the worst case of the adjustment being set to the highest possible setting.

Yes there is a risk of someone meddling, but in general we have that risk anyway - cartridge fuses may be replaced by one of a higher rating (not all values are physically different sizes), an incorrect size of fuse wire might be selected for a rewireable fuse - even an MCB might be replaced by one of a different rating or type or even manufacturer.

Ultimately there would be little point in manufacturing adjustable devices if lower settings couldn't be relied upon - you might as well just install a fixed device of the maximum rating.

- Andy.
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Vote Up 0 Vote Down

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Short circuit fault current calculations

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