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Single pole breakers 3 phase trunking and shared neutral

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. 

Parents
  • We need to be a little careful of the definition of "shared neutral" here. The lighting trunking is a single 3 phase load with the neutral supplied as usual. A "shared neutral" is a neutral taken from another unexpected circuit with no obvious connection to the load in question. Trying to break down this 3 phase load in this way is not correct, and to any electrician the arrangement is obvious. As there is maintained lighting I see no problem with a 3 pole MCB feeding this load, lighting itself will not fail, it will be the emergency stuff until any fault is fixed.

Reply
  • We need to be a little careful of the definition of "shared neutral" here. The lighting trunking is a single 3 phase load with the neutral supplied as usual. A "shared neutral" is a neutral taken from another unexpected circuit with no obvious connection to the load in question. Trying to break down this 3 phase load in this way is not correct, and to any electrician the arrangement is obvious. As there is maintained lighting I see no problem with a 3 pole MCB feeding this load, lighting itself will not fail, it will be the emergency stuff until any fault is fixed.

Children
  • I see no problem with a 3 pole MCB feeding this load

    I don't think that any of us would disagree with that, but the proposal is to have 3 SP MCBs and, therefore, 3 SP circuits.