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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

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    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.

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    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.

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