sizing of cables for adjustable short-circuit threshold

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.

I think you are worrying about the wrong thing. If it is a concern, then an anti-tamper sticker over the setting switch/knob or what ever it is and a sticky label 'Must  Be Set to 40 amps' would be the sort of thing.

Some designs have places you can wire seal the covers in place after setting, then to open it and twiddle requires tools and is then in the same league as opening the box and bypassing it with  jumper cables and we do not try to design for that. As noted above the whole point is that you can make it into a customised breaker with characteristics that match your wiring, and there is some sense in letting a large one loaf along and run cool if you can afford the space and cost.

Perhaps take a look at 433.1.1 noting the words "rated current or current setting" and the sentence regarding the current setting selected is the effective In of the device.


However, that's for overloads


typically under short circuit, reducing the effective In also reduces the threshold at which the short circuit current operates the device and in what time (ie the inverse characteristic) - so you still need to design for the SC condition at the actual fault level using the effective disconnection time. It's then up to you if you decide that upstream devices don't provide any cut off characteristic or are likely to maloperate to an extended disconnection time (and further upstream doesn't act.


As an example, just to maintain temperature rise characteristics sensibly within the switchboard, I might well have a 3200A incoming ACB and put a 1600A ACB device dialed down to 1250A as an outgoing device and then use say 4 x 185mm2 conductors to feed the downstream switchboard - and then use say 630A devices dialled down to 400A in that downstream switchboard with the outgoing circuit assessed accordingly. Under short circuit, I might ignore the cut off characteristics of the 1600A/1250A device and assume the full prospective current is applied to the 630A/400A device. But I would probably ascertain the cut off of the 3200A device and use that for the 1250A outgoer.


Essentially, you only need to design for what you consider credible rather than what might be possible - how far you go in each direction is up to your appetite for risk - BS 7671 is only minimum compliance and would allow what you describe without any "whatiffery" involved


Regards


OMS