3 pole contractor fed from 3 separate MCBs

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. 

  • No problem at all. Multi pole contactors are regularly used to switch multiple circuits together.

  • 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.

  • 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. 

  • Probably OK unless any part of it is in a situation or installation which requires all live conductors - line(s) and neutral - to be disconnected for isolation, because Reg 531.1 states (1st para) that devices for ADS shall be suitable for isolation.

    Probably OK for most TN and TT systems, provided in TT at least there is a common means of isolation disconnecting all poles (including Neutral), but check special locations and other types of supply if applicable.

  • 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.

  • I seem to be posting responses in the wrong area. Sorry. See below further question. 

    If for example the contactors coil is fed from a different circuit, perhaps a control circuit, and this trips or is switched off, this will obviously extinguish the lighting (assuming it’s a normally open contactor). 

    This will obviously not bring on the emergency lighting (as the supplies to the lights will still be on). I presume this is part and parcel of controlling lighting and something that would need to form part of a risk assessment. I have done DALI relays before, which operate the same way on power failure to the DALI circuit (if the control panel is switched off, lighting switches off).

    The emergency lighting will still always work on power failure or if the lighting circuit itself trips. 

    Thanks. 

  • Well that's always a risk, but if in a work environment, it should be under someones control, so the control circuit should not be turned off by  anyone but a person in control.

    To mitigate a tripping problem of the control circuit (though i think that is a very low risk), run the circuit from a CB. not a RCD, and keep the contactor close to the DB. It wouldnt trip through overload, and should not be tripped through a fault if the cable is protected properly, and CB sized correctly.

  • Probably OK unless any part of it is in a situation or installation which requires all live conductors - line(s) and neutral - to be disconnected for isolation, because Reg 531.1 states (1st para) that devices for ADS shall be suitable for isolation.

    But only isolation of the line conductor is required - if the last line of 411.3.2.1 is to be believed...

      - Andy.

  • 411.3.2.1 only applies to TN systems. In TT systems, it is also permitted to disconnect only live conductors for the purposes of ADS, but for safe isolation (for safe working) all live conductors ought to be isolated.

    However, ADS is used in other circumstances which require all conductors to be disconnected, such as faults in IT systems, as there is no "earthed live conductor".

    Whilst I agree it's arguable there is no "Neutral" in IT systems (there could be a star-point conductor, but distribution of this is not recommended in IT systems - however, the OP does not describe whether the three single-phase circuits are connected to the same phase or different phases - we could have this situation in three circuits from an IT system or "floating generator" etc.