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Single pole breakers 3 phase trunking and shared neutral

vantech

Hi, 

I have a question regarding the use of single pole MCBs on lighting trunking systems.

The idea is that the trunking system contains 8 conductors, used as follows:

L1, L2, L3, N, E (supply one - grid) 

L1, N, E (supply two - central battery for emergency lighting)

The central battery supplies lighting during an outage (AC during normal operation, DC during grid failure). This aspect is fine. 

What the customer wants is to avoid a single point of failure. Preciously, on the maintained or grid supply, L1, L2 and L3 were connected to and protected by a 3 pole MCB (across 3 phases) with a shared neutral. Moving forward, the customer has queried using single pole MCBs to protect each of the phases (split across 3 phases) and a shared neutral, thus meaning if a single MCB trips, the other 2 circuits will remain live. 

I don’t see anything wrong with this, in fact, the same result would be apparent if fuses were used. The reason I raise the question is for the purposes of isolation; when using a 3 pole MCB, a single switch can isolate an entire track however when using single pole MCBs, there are then in essence multiple points of isolation. 

Am I right in thinking that single pole MCBs can be used with a single/shared neutral, as long as the line conductors are split between 3 phases and there is appropriate labelling at all connection points for “isolation at multiple locations”? 

Thanks. 

  • 0

    therefore, 3 SP circuits.

    I'm not sure I follow the thinking there - when fuses are used we still consider a 3ph circuit to be one 3ph circuit, not three 1ph circuits - we don't even demand that all thee fuses are in a common carrier - so where's the definition different for MCBs?

       - Andy.

  • 0 in reply to AJJewsbury

    Yes, but for a 3 ph circuit, a 3 ph MCB is an improvement on 3 fuses - don't we want them all to trip together to avoid phase imbalance?

    In this instance, there appears to be an express intention to have 3 SP circuits, so each should have its own neutral.

  • 0

    Depending on how the lights are wired there could be a definite advantage to having SP protection on each phase. If the lights to each work area are supplied by a mixture of each phase then the tripping of one MCB would reduce the lighting level by one third, possibly allowing work in that area to carry on. A 3P MCB tripping would trigger the EM which might not allow this.

  • 0 in reply to OlympusMons

    This is exactly it, emergency lighting isn’t suitable for continuous works, it’s for evacuation purposes. The client wants a reduced lighting level, rather than a total outage of main lighting. 

  • 0 in reply to Chris Pearson

    If you have fuses feeding a lighting sub main, and a fuse blows, you still end up with a phase imbalance. 

    I think this is a grey area to be honest. 

    If we go down the separate neutral route, we’d have this configuration inside of the trunking system:

    L1 & N1 

    L2 & N2

    L3 & N3  

    CPC 

    EM L & EM N 

    DALI 1 & DALI 2 

    Thats a trunking system with 11 poles (and this trunking system has a built in busbar system).

    Utilising 3 SP MCBs, with a single neutral, would in essence then mean a reduction in poles by 2. The trunking system does have a maximum of 11 poles, therefore it would work with individual neutrals. The consultant just wants to reduce cost and complexity, and understandably. 

  • 0

    If the trunking system has 11 conductors, and as Chris said, loss of the shared neutral would introduce one point of failure, I can't see why you wouldn't want to use them all. it isn't more complex connecting a luminaire to L2 and N2 than it is to L2 and N, as long as the labelling is correct.

  • 0

    I am in the separate neutrals camp. Ignoring the arguments about compliance. From a resilience and ease of maintenance point of view, separate neutrals allows for isolation of circuits for maintenance when the remaining circuits are still live. 

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