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Will standard mcbs work on 12v AC?

hertzal123
The customer is using a 300va 12v tx to supply 5 no 60w takeaway heated food bags

and is having problems with the 5 no 10A ceramic fuse holders overheating on their spade terminals.

I wondered if B10 mcb,s would solve the problem,or would they be too slow to protect the tx on a short

cct on one output?

                         Regards,

                                    Hz
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  • 0
    Yes standard types of MCB will work on 12 volts AC or other much reduced voltages.

    The relatively low short circuit current from a 300 VA transformer may be insufficient to operate the magnetic trip which means waiting for the thermal trip.

    An estimate of the short circuit circuit current from the transformer would be about 10 times full load, or say 250 amps. Even a short length of intervening cable will substantially reduce the fault current.


    I would be inclined to use 6 amp MCBs rather than 10 amps to give closer protection. 6 amps is still 20% in excess of the calculated load current.


    Alternatively, consider a "like for like replacement" rather than altering the design. Fit new fuse holders, perhaps with soldered connections or decent screw terminal connections rather than crimped push on types.

    If the heaters are PTC types rather than a simple resistance, then altering the design might be best avoided.
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  • 0
    Yes standard types of MCB will work on 12 volts AC or other much reduced voltages.

    The relatively low short circuit current from a 300 VA transformer may be insufficient to operate the magnetic trip which means waiting for the thermal trip.

    An estimate of the short circuit circuit current from the transformer would be about 10 times full load, or say 250 amps. Even a short length of intervening cable will substantially reduce the fault current.


    I would be inclined to use 6 amp MCBs rather than 10 amps to give closer protection. 6 amps is still 20% in excess of the calculated load current.


    Alternatively, consider a "like for like replacement" rather than altering the design. Fit new fuse holders, perhaps with soldered connections or decent screw terminal connections rather than crimped push on types.

    If the heaters are PTC types rather than a simple resistance, then altering the design might be best avoided.
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