Short circuit fault current calculations

I think you've gone off track here....


The trip setting of the CB has no bearing on the PSSC - that's a function of the impedance to the fault.

Hi,


i agree that the rated current setting has no effect on the PSCC but it does have an effect on the amount of current required to instantaneously trip the CB, in this case the CB requires 8 X the rated current to instantaneously trip, therefore increasing the rated current increases the amount of fault current required to instantaneously trip.

Aha I think I understand what you need,  - if you want to know the upper limit on Zs at the far end of a circuit  for the ADS to fire, you use the setting on the breaker, so if it is set to 200A and current multiplier of  five then 1kA plus 10%,   ~0.2 ohms - even if it is a 400a breaker set to 50%.

although if you have earth fault protection set it is less of an issue, as you do  not need near instant breaking for an overload, only an LE fault.

In terms of people twiddling the setting, the ones I have seen have provision to fit seals. In a controlled situation like a factory, then only authorised people should be making changes, and then it is up to them to justify, exactly like removing and fitting a larger MCB or the wrong fuse.

I'd not leave an adjustable in a public place however.

"in this case the CB requires 8 X the rated current". Are you sure of this because it seems an added complication to the device to have both trip settings change when a quite different mechanism is involved?

I would have expected the instantaneous trip to be 8 x the maximum setting - but if it clearly says otherwise then I'm happy to be corrected.

it depends how you twiddle it - unlike a normal MCB with a thermal and a magnetic part, in the modern devices there is a current transformer and some very flexible programmable electronics, so you can make the instant threshold move, or have a variable multiplier. However, as you only use the adjustable because you really do want a funny rating, then it behoves the person making the adjustment to understand the conditions well enough to make an informed choice

(and maybe to note the settings in a label on the panel and then seal it against tampering.)

Note that really this is all about faults to earth and shock protection, the fast trip multiplier part is irrelevant for overloads and slow overload is normally what matters for fires, where a 5 second response time is just fine. (hence the preference for either a seperate earth fault relay, or a breaker that includes a programmable RCD function, as then you can separate the cases quite easily.)

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.

All of the MCCBs I have seen at that rating range have a fixed value of Im.  If the Ir setting adjusts Im then such information is bound to be present on the front panel of the device.

Lyle,

Completely agree here. The Im setting is unlikely to be adjustable as it doesn't rely on a trip signal from the electronics, but is rather a magnetically induced trip induced by the current flowing (hence Im - m for magnetic). Most importantly the short circuit rating of the circuit breaker is definitely not changed when the current settings are adjusted.

However some circuit breakers have a short time setting (probably 250-400 ms) which is not the same as the instantaneous trip, but is a fixed (short) time trip instigated by the electronics. This trip, being set by the electronics, is adjustable. However I have not seen such a trip setting on MCCBs before but rather on ACBs.

Alasdair

Some makes of programmable breaker (like the micrologic below) are very complex and manage to still be very clear with their labelling

Other models may not be, and a careful read of the data sheets example of less clear one is always good.

You may well be right that in the lower current ranges, only the smaller simpler ones exist, I had not spotted that in the OP.