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90 degree rated MCBs

jbrameld

I'm still on the quest to fix the design challenges on my theatre project and we are nearly there.

Does anyone know of a manufacturer with a 125A TPN DIN-rail MCB that has terminals rated for 90 degree operation?  The ABB breakers we're using elsewhere do have a 105 degree rating but they don't have 125A available as they re-designed it and are awaiting a new approval.  Most manufacturers don't seem to publish the terminal temperature rating but you can sometimes get it from their tech support if you can get them to respond!

The situation I have will generally have a 90-degree rated cable in trunking with grouping factors applied, and this will terminate on DIN-rail terminals (105 degree rated) at the panels/socket boxes at each end.  These terminals will then connect to the appropriate breakers/connectors via lengths of 105 degree tri-rated cable, over a reasonable length (0.5-1m-ish) inside the breaker panel where there is more fresh air generally.  The designer is however insisting that to be able to use the 90-degree basis for the cables in the trunking then all components right through to the MCB terminals need to be 90 degree rated.

Hence if anyone knows of a 125A TPN MCB with a 90 degree terminal rating then it would be of great assistance as this is the last piece of the puzzle.

Jason.

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    I'd be a bit wary of the differences between "nominal" and "actual" here. The MCB standard puts limits on the temperature rise at MCB terminals - but I have a suspicion that it's more trying to deal with dissipating heat generated by the MCB itself. In effect the cable acts as a heat sink for the MCB. As has been noted, the actual temperature of the wire connected to the MCB due to current carried through that wire is likely to be well below it's nominal conductor temperature as it approaches the MCB - so that leaves a margin where it can dissipate heat from the MCB. So while nominally the cable is simply at 70 degrees, the actuality is messier - 70 degrees in the wall, maybe naturally 35 degrees for  a single core in freer air as it approaches the MCB, but increased to 40 degrees by the heat from the MCB, with the MCB terminal itself somewhat higher again (actual numbers just made up to illustrate of course). Altering things so that the natural conductor temperature just before the MCB is just below 70 degrees might be missing some of the more subtle points of the process.

       - Andy.

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  • 0

    I'd be a bit wary of the differences between "nominal" and "actual" here. The MCB standard puts limits on the temperature rise at MCB terminals - but I have a suspicion that it's more trying to deal with dissipating heat generated by the MCB itself. In effect the cable acts as a heat sink for the MCB. As has been noted, the actual temperature of the wire connected to the MCB due to current carried through that wire is likely to be well below it's nominal conductor temperature as it approaches the MCB - so that leaves a margin where it can dissipate heat from the MCB. So while nominally the cable is simply at 70 degrees, the actuality is messier - 70 degrees in the wall, maybe naturally 35 degrees for  a single core in freer air as it approaches the MCB, but increased to 40 degrees by the heat from the MCB, with the MCB terminal itself somewhat higher again (actual numbers just made up to illustrate of course). Altering things so that the natural conductor temperature just before the MCB is just below 70 degrees might be missing some of the more subtle points of the process.

       - Andy.

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