Single phase breakers on 3-phase circuit

[mfarrow]

Hi

This may provoke some debate...

Lighting system on 3-phase TT supply with RCCB incomers on 4 distribution boards, each supplying two lighting circuits, feeding 4-core cables to local rotary isolators. Rotary isolators then feed strings of luminaires spreading load across 3 phases.

Is it permissible for the circuits within the distribution boards to comprise of 3 1-phase breakers rather than a single 3-phase breaker?

I guess it's your interpretation of final circuit, but I'd like everyone's opinion if I can.

Thanks
Mike

 

[sparkiemike]

what did they do before they had circuit breakers i.e. fuses

 

[flameport]

feeding 4-core cables to local rotary isolators

The neutral is shared between the 3 phases?

If so, this arrangement is not permitted, as isolating one MCB will leave the circuit live due to the shared neutral.
It also means that if the neutral failed or was disconnected for whatever reason, the lighting circuits would get 400V across them.

 

[holmslaw]

..

 

[JohnW2]

If the lighting is across 3 phases with a shared neutral then it is a TP&N circuit and must be controlled by a 3 pole breaker. But if each phase has a separate neutral from the dist board then it is 3 separate 1 phase circuits and should be controlled by individual 1 pole breakers. Flameport has stated the reasons why.

I think that I am maybe not completely understanding what is being described. I thought it was being said that the lighting loads 'spread across 3 phases' were connected between each of the phases and (a common) neutral. If that were the case, then there surely would not be very much functional difference between the two scenarios described above, the only real difference being the matter of exactly where the neutrals of the three lighting loads combined and became one?

Kind Regards, John.

 

[holmslaw]

..

 

[sparkiemike]

Have a look at 559.6.2.3.

EDIT, I think the device could simply be a linked switch rather than a 3-pole breaker?

 

[JohnW2]

Thats why I have very clearly described the only two possible situations. It saves waffling and asking pointless questions.

It is true that I can sometimes be dim. Could you perhaps explain to me what you were very clearly describing when you wrote "If the lighting is across 3 phases with a shared neutral". Many thanks.

Kind Regards, John.

 

[JohnW2]

Have a look at 559.6.2.3.
EDIT, I think the device could simply be a linked switch rather than a 3-pole breaker?

Thanks. That certainly seems to explitly answer the question posed. As you say, it appears that 559.6.2.3 would be satisfied by a TP&N switch/isolator, without the need for a 3- or 4-pole breaker.

Kind Regards, John.

 

[Paul_C]

It also means that if the neutral failed or was disconnected for whatever reason, the lighting circuits would get 400V across them.

The same thing happens with a TP&N feeder to a sub-panel from which completely independent 2-wire single-phase circuits are run if the feeder neutral goes open circuit.

 

[sparkiemike]

559.6.2.3 Groups of luminaires divided between the three line conductors of a three-phase system with only one common neutral conductor shall be provided with at least one device that simultaneously disconnects all line conductors.

It is quite a common arrangement to have a common neutral on a large lighting system. L1, L2 and L3 are connected to the single pole MCB, the 1st luminaire you connect L1 and neutral and earth, the second luminaire L2 and neutral and earth and the 3rd luminaire L3 and neutral and earth and keep going unitl all are done.

I am not aware of a requirement for a 3-pole MCB - only a 3-pole switch for isolation, as said before would you view it differently if it was a 3ph&N switch fuse?

 

[JohnW2]

It also means that if the neutral failed or was disconnected for whatever reason, the lighting circuits would get 400V across them.

The same thing happens with a TP&N feeder to a sub-panel from which completely independent 2-wire single-phase circuits are run if the feeder neutral goes open circuit.

... and presumably also in an installation such as mine. I have a 3-phase supply feeding multiple single-phase CUs, the neutrals of all circuits coming together at the post-meter Henley block. If the (then common) neutral went open-circuit upstream of that Henley, the L-N pd feeding any of the CUs (hence all connected final circuits) could presumably become anything from 0V to 400V ('nominal'!), depending upon the respective loads connected to the various phases at the time.

I've never thought about this before and, if I'm thinking right, I suppose it's a bit unsettling. Currents would presumably be low, since they would be flowing through loads in series, but the voltage could well be a problem.

I suppose exactly the same thing could happen with adjacent properties on different (single) phases with a neutral common to the properties, if the neutral supply went O/C upstream of that neutral common to the two properties.

Have I got this right?

Kind Regards, John.

 

[Spark123]

It makes a mess if you have an open circuit neutral! Have seen it on a housing estate where they had battle of the phases, the phase with higher loading's houses got off pretty unscathed, the other two house's phases had a lot of appliances go bang! TVs smoking, lamps blowing etc etc.

 

[Paul_C]

You've got it John. I have exactly that sort of arrangement here. From the pole at the road carrying the normal 240/415V 4-wire system, there are two phases plus neutral which run about 100 ft. to a pole in my next door neighbor's front yard. He and I are fed from one phase, with the house on the opposite side of him on the other phase. Any break on that 100 ft. run of neutral would result in the 415V being divided between my/my neighbor's house and that other house in proportion to whatever loads were connected at the time.

The same sort of thing would happen with a break in the neutral feeder to the pole by the road, which runs about 200 ft. from the transformer. In that case it would be affected by the multiple earth connections on the neutral, which would tend to keep the voltages from varying as much as they would otherwise do with unbalanced loads.

 

[westie101]

Firstly taking the original lighting circuit. In theory if the load on all of the three phases was the same there would be no neutral current so, again in theory, there is actually no need for a neutral! There are plenty of 3wire 3c phase motors about!

Loss of neutral where loads are unbalanced as they are on most of the LV distribution is a nightmare with possible lethal effects