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Distribution board overload conditions

E95

Hi all,

I have a question regarding how overloads are protected against in distribution boards

For example, I have a 400A distribution board fed from a 400A MCCB, I assume that the distribution is rated for the full 400A continuously? Interestingly OMS made a comment on how distribution boards were constructed with a 0.6 diversity included so that a 400A board is actually only rated for 240A continuous, someone disputed this but there was no other response unfortunately.

Not to digress too far, assuming the thermal overload component doesn't start to operate until 600A what is protecting the board from an overload condition? Is this supposed to be by diversity calcs only?

I assume if this was the case there would be lots of incidents involving burnt switchboards, or the more obvious answer that the 400A board can take 600A for longer than it would take a typical protection device to operate?

Thanks for any responses Thumbsup

Parents
  • 0

    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.

Reply
  • 0

    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.

Children
  • 0 in reply to mapj1

    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 

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