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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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  • 0

    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.

Reply
  • 0

    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.

Children
  • 0 in reply to AJJewsbury

    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.

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