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MCCB poor design

lyledunn

This T/M 400A MCCB is connected directly to DNO busbars in a factory complex that has now been divided up into smaller individual units. The MCCB is misbehaving and tripping out for no apparent reason, even with the downstream unit distribution board some 10m away, disconnected. There is no obvious issue with the inter-connecting “tails” and no auxiliary, shunt or undervoltage connections. It would seem reasonable to conclude that the device is faulty or at least should be removed for testing. However, it will mean shut-down for the entire complex as the supply from the site tx to the DNO busbars will need to disconnected. Poorly thought out design by the original contractor who is the one left with the headache. 

DNO connections at the lop look a tight squeeze!

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  • I agree that the MCCB is probably faulty, but before replacing it I suggest trying the following.

    Firstly, carefully inspect the adjustable trip settings shown at the bottom of the unit. If erroneously set to the most sensitive setting, it might trip under insignificant load or earth leakage current.

    Secondly, perform an insulation test on the output cables, just in case some hidden damage is resulting in trips.

    I doubt that either of the above will be found to blame, but it seems sensible to rule them out.

  • in reply to broadgage

    can't have earth leakage - the neutral is not going through it! On some like this these the lower that half carries the trip electronics and can be removed leaving the body and main contacts in place.

    I do not recognise the model, and it may well need to be taken out as one.

    3 fuses as the first thing on the line  have a lot to recommend them ;-)

    M

  • in reply to mapj1

    3 pole with earth adjustable earth leakage. A type A RCD, you can switch it off or set it between 30 mA - 5 amp. The one in the pic looks like telemecanique 

  • in reply to AMK

    yes, true but not the right object if as in the OP installation above there is a neutral. and it is used load side - such a 3 pole sensing will not work - avy  single phase load will be seen as an LLL imbalance. The Vigi ones with the external current transformers  can be good in that situation to make a four wire sensing three wire breaking arrangement.

    Mike.

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  • in reply to AMK

    yes, true but not the right object if as in the OP installation above there is a neutral. and it is used load side - such a 3 pole sensing will not work - avy  single phase load will be seen as an LLL imbalance. The Vigi ones with the external current transformers  can be good in that situation to make a four wire sensing three wire breaking arrangement.

    Mike.

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  • in reply to mapj1

    Closer examination of the original image, no adjustable leakage trip; only thermal and magnetic protection mechanisms are in place. Recently, I received a call to a site with new tenants just moved and setting up, where one of their TP&N sub-distribution boards was experiencing tripping issues. This particular board was protected by the Schneider 4.2 device depicted above, with the RCD activad. Given that this distribution board handles single-phase final circuits, an imbalance was promptly detected. 

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