Distribution board overload conditions

Unless clearly stated otherwise, I would expect that a distribution board rated at 400 amps, should be suitable for continual use at 400 amps, and should also withstand a SHORT TERM overload to more than 400 amps, for whatever time it takes for a standard type of 400 amp OCPD to operate.

If it says 400 amps on the rating plate, then in my view it should withstand whatever combination of time and current that can be passed by a 400 amp fuse or MCCB.

I would consider it better practice to avoid long term 100% loading, but that is my view of good practice and not a regulatory requirement.

rather depends what constitutes a failure - bus bars rising to a few degrees above the intended is not an immediate show stopper but may affect the thermal environment of breakers mounted above, and shorten life or shift the tripping levels a bit. I'd not expect insulation to suddenly melt, but things may age a bit faster then if they were cooler. I also expect some brands to be more forgiving than others as we all know some makers are more generous with the metal sizes and the space for cooling than others.

It is is very similar to the question of how hot does the inside of the box get with a given external ambient temperature at various load levels, you'd like a 400 A board to handle 400A all day and not break a sweat, and I expect the better ones to do so. I have also seen a few that look a bit weedy.

Do derate if it is in use in a hot location.

Makers data will only get you so far.

Mike.

Mike, 

Firstly thanks for your contribution, I think you given input everyone of my posts so far, it is greatly appreciated and you and other frequent contributors are helping young/developing engineers like my myself

Are you suggesting that the distribution board busbars will be rated to continuously carry anything below the tripping point of the main switch? It seems very possible that 400A boards could indefinitely carry 500-600A (when there seems to be a lot of uncertainty on industrial diversification) without the overload component of the MCCB starting to operate?

Kind regards 

I would not expect that a "400 amp" distribution board would be able to carry to  carry say 500 amps indefinitely, but I would expect it to carry 500 amps SHORT TERM for whatever time it takes for a 400 amp fuse to operate, or for a typical 400 amp MCCB to trip.

That's the point, a 400A fuse won't trip at all until about 600A right? 

A 400 amp fuse should EVENTUALY operate at not much in excess of 400 amps. At 600 amps I would expect operation with an hour or two.

Indeed, and a 400A board protected by a 400A fuse or 3 400A fuses if 3 phase, would be a set up that requires no further protection against overload unless there is something non-standard  going on that changes things so it needs some de-rating, like it is to be used in a desert or a the hot part of a steelworks !

Mike,

That's the point, a 400A fuse won't trip at all until about 600A right?

It should blow at a lot less than 600A .... given enough time. Exact numbers vary with the exact type of fuse, but around a 45% overload should open a fuse within 'conventional time' (usually 1hr for smaller fuses, 2hr for larger ones) - much smaller overloads should still open the fuse - but it'll take longer maybe hours or even days. Don't be misled by graphs stopping at a certain time ... that's just down limited paper size and the fact that we're usually only interested in calculating actual opening times for relatively short durations and high currents (usually dead short faults and <5s).

  - Andy.

500A instead of 400 A is a 25% overload, and about  1.25² =1.56 times the temperature rise of the fuse wire that occurs at nominal  full load. This is usually enough to separate eventual failure from never without approaching failure when there is no fault but just ageing but 20-25% is about the limit.  Note that unusually cold or unusually hot ambient conditions can also alter things, but at least for copper wire elements, as the melting temperature is about 1000C,  if the starting temperature is 0C or 50C makes not a huge difference to the temperature rise needed to get there. However, things like the fuse being lagged or in a cold draft do have a noticeable effect.

Mike.