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. 

  • I struggle to see how, if you have 3 separate fuses feeding a busbar system, they will share the same neutral conductor (this is a typical and normal setup). The same principle would apply if the fuses were replaced with MCBs surely?

    As long as the MCBs are split across 3 phases, the neutral cannot be overloaded, again the same principle of feeding any system with fuses which can all operate independently. 

  • Well if it was supplied by a 4 core SWA protected by 3 fuses we would not blink. But we would pull all 3 fuses before working on it of course.

    So what are the dangerous cases ?

    How credible is  the shared neutral risk  that someone isolates one phase and then starts dismantling things in a way that interrupts the neutral for one of the phases still live ? The other one to consider is a a live to live fault and if the neutral failed what happens to the star centre voltage.

    Unlike a multcore where the neutral is obviously shared,  in singles the risk of that sort of accident is higher . So you need to ask when does it really become 3 single phase circuits and so need 3 neutrals, one paired with each live  core  - you'd need that if you wanted 3 RCBOs for example.

    I presume as it is at some point the common neutral splits to go 3 ways with the phases - it certainly would be neater if that split was nearer the  origin and regs wise it is dodgy ground.

    Mike.

  • 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”?

    As it's lighting, there is a specific requirement to provide some means of 'disconnecting simultaneously all line conductors' - so if using fuses or SP MCBs you might need to add a TP isolator - reg 559.5.5.

       - Andy.

  • Thanks. 

    Cables would come from a DB, most likely in a multi core cable of some description. This cable would then terminate into the lighting trunking system which has a busbar setup. 

    This I think would then mimic if you like a multi-core armoured cable scenario. It would then in essence be a 3 phase circuit, protected by single pole MCBs. 

  • A main switch on the lighting DB would achieve this no? This of course isn’t an ideal scenario, so perhaps an external lockable isolator which can switch off all line conductors and neutral? 

  • To add to the above, a live to live fault would be a possibility in a system that utilises fuses as well as MCBs and the same parameters would apply.

    Would the scenario be any different for a system that uses MCBs compared to fuses, or are you meaning a live to live fault in a lost neutral scenario? 

  • Unlike a multcore where the neutral is obviously shared,  in singles the risk of that sort of accident is higher . So you need to ask when does it really become 3 single phase circuits and so need 3 neutrals, one paired with each live  core  - you'd need that if you wanted 3 RCBOs for example.

    I think that if it is important to the customer to have no single point of failure, then he or she needs to dip his hand into his pocket and pay for 3 neutrals.

    And yes, I know, the MCBs will be single pole and the 3 neutrals will all be connected together in the neutral bar.

    If there is only 1 neutral, isn't that potentially a single point of failure?

  • 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.

  • 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.