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3 pole contractor fed from 3 separate MCBs

vantech

Hi,

Any issue having 3x single pole (single phase) MCBs, switched by a 3 pole contactor, feeding 3 different runs of lights? The 3 circuits will be on different phases. There will be a separate neutral for each circuit. 

Each pole of the contactor is rated for the connected load and the number of lights it’s switching. 

Generally contactors are fed from an interlinked 3 pole MCB, however for this setup we need to use single pole MCBs to prevent 1 circuit tripping the other 2 runs of lights if there was a fault of the need to isolate one of the circuits. 

Don’t foresee it as an issue as long as the contactor is labelled as having multiple points of isolation. To me this is no different than a big grid switch, or a contactor fed from fuses. 

Anyone think otherwise?

Thanks. 

Parents
  • 0

    I agree - no problem. Often you'd be in the same situation if the protective devices happened to be fuses.

    It's very common to have a contactor fed from several circuits - often the coil is fed from yet another (lower rated) circuit.

       - Andy.

  • 0 in reply to AJJewsbury

    Thanks. 

    So each pole is then rated at 25A. If there is say 10A of lighting per circuit/pole, this means the total accumulative load on the contractor is 30A (if it’s a 3 pole device).

    Does the same logic apply if each pole is on the same phase, or if there are for example 2 poles on L2 and 1 pole on L3? 

    Thanks. 

Reply
  • 0 in reply to AJJewsbury

    Thanks. 

    So each pole is then rated at 25A. If there is say 10A of lighting per circuit/pole, this means the total accumulative load on the contractor is 30A (if it’s a 3 pole device).

    Does the same logic apply if each pole is on the same phase, or if there are for example 2 poles on L2 and 1 pole on L3? 

    Thanks. 

Children
  • 0 in reply to vantech

    A rating of say 25 amps is 25 amps PER POLE so a total loading of 75 amps is fine, provided that no pole is loaded to more than 25 amps.

    This applies regardless of the loads switched by each pole of the contactor being on the same or differing phases.

    The most common use of a 3 pole contactor is probably to control a 3 phase motor, in which case a "25 amp" contactor is suited for a motor with a full load current of up 25 amps per phase.

    Similar arguments apply to most 3 phase equipment, A three pole 63 amp switch is for 63 amps per pole.

    For long term use I would consider it to be better practice to limit the load to about 90% of the contactor rating, that however is my personal view and not a requirement.

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