2P to Be Used as 1P on 230V Single Phase Line to Neutral Load

Switching a neutral is prohibited under British Standard 7671.

Why would one use 2-pole breakers for single phase use, when 1-pole breakers are readily available and cheaper (rhetorical question - NNTR)?

Hi PWSlack.

This is a proposal from a supplier of ours. The CB's are rated 125-175AT. I'm not sure if this is available as 1P since for Schneider Easypact, above 125A ratings already comes in 2P and not as 1P, the supplier might be specifying readily available stock. Most probably this is, but I was also wondering if there are code concerns or operational (manufacturer's) concern on not using the second pole for a 2P CB. Referring to the standard of not switching the neutral, then this type of installation would be acceptable. Would tripping operate satisfactorily on such intallation. Thanks for the response.

Put your supply feed on the input of your first pole, then feed the output of the first pole to the input of the second pole. Now take your load output from the output of your second pole.

You should never switch the neutral. Very good rule to live with.

However, NEC section 240.22 does allow a double pole circuit breaker on a phase/neutral arrangement, despite what any good electrician will tell you. If, and only if, both the intentionally grounded (i.e.neutral) and the live conductor are opened simultaneously, and neither pole can operate independently. So, in the USA, yes you could do this. However, this is not a typical USA voltage so, we must look elsewhere for guidance. We have received numerous panels where I work from continental Europe, and they use double pole breakers and even double pole fuse blocks with a dummy fuse in the neutral bay for single phase voltages, (230V with a phase and a neutral conductor). Apparently continental Europe has areas where codes also allows this if both conductors are opened simultaneously. Better check your local codes for this. I understand the first stated rule, as even an interlocked circuit breaker could fail and allow only one side to open. Very dangerous situation indeed. Best not to do this, but it may be allowable where you are.

Thanks Phys.

The supplier is aware that the design was intent on not providing a disconnect for the neutral so they proposed to use only one side of a two-pole CB as a substitute for a one pole CB. Would this be acceptable? The reason might probably be that this is their available stock. Does NEC/IEC have any standards in this light. Similarly and just for discussion's sake, would a 3P breaker be acceptable to protect a single phase L-L load, with the third pole hanging.

Don't know where this is in the code, but it is not uncommon to use the two outside poles of a three pole breaker for single phase. (2 pole) This should be the same situation.

Well, I had to go find my 1975 NEC book, and read this as it's come up now several times. Seems like it's a hold on to older power systems or custom internal industrial (but antiquated) systems.

240-22. Grounded Conductors. No overcurrent device shall be connected in series with any conductor that is intentionally grounded.

Exception No. 1: Where the overcurrent device opens all conductors of the circuit, including the grounded conductor, and is so designed that no pole can operate independently.

Exception No.2: Where required by Sections 430-36 and 430-37 for motor running (overload) protection.

430-36. Fuses -- In which Conductor. Where fuses are used for motor running protection, a fuse shall be inserted in each ungrounded conductor.

A fuse shall also be inserted in the grounded conductor if the supply system is 3-wire, 3-phase AC with one conductor grounded.

430-37. Devices Other Than Fuses - In Which Conductor. Where devices other than fuses are used for motor-running overload protection, Table 430-37 shall govern the minimum allowable number and location of overload units such as trip coils, relays, or thermal cutouts.

I'm in the US as well so this is not something we contend with here, but having worked for Siemens and other EU based companies in the past, I too have seen my share of IEC power schemes. I believe the answer will have to do with what TYPE of earthing system you have, i.e. TT, TN, IT etc. Forgive me if those are incorrect, again this is foreign to me, but I would see it in IEC systems and when I would ask, was told that this is what made the difference with regards to whether or not the N can be switched. I don't really remember what those differences are, I have to look it up every time. But if you look for "Earthing systems" on Wikipedia there is a good article explaining it and further links.

I give you a "Good answer" vote because only one heater coil energised in a 2 pole breaker would increase the thermal overload tripping amps and possibly lead to overheat of a cable or load.

67model

Good answer its what we do on the odd ocassion their is a need.

If you are leaving the common and ground lines connected to the buss bars where they normally would terminate with a single pole breaker then yes you can just use one side to a two pole breaker and leave the other unused.

Although being designed as a two pole unit either side will still trip the unit as whole when its overload limit is reached so not having one side connected does not affect is ratings or current capacities.

I hate to cause confusion by contradicting PW Slack, but BS7671 does not say you can never switch the neutral.

It says if only one pole is switched it must be the live NOT the neutral. The latest range of "MK" brand BS1363 wall sockets has double pole switches which break live and neutral.

So long as both poles are interconnected and open together, there is no problem with a two pole single phase isolator or overload breaker. Obviously, as an isolation device it is safer than single pole, since neutral could transiently get a high voltage due to a fault on another breakers' circuit.

67model

This is a common electrician error, (actually a good one to make) as US electricians will also state that code says you can never switch the neutral. It is what they are taught to keep them out of trouble. They sure look at me real funny like when I read the NEC 240.22 to them.

I have gotten those looks a number of times here as well.

It always amazes and frightens me a little with how many electricians not only do not have a current code book let alone any and then justify it with their being licensed means they don't need to have one!

I did a major wiring job for a friends new shop a number of years ago that had to be signed off by the state electrical inspector. I was not there when it was inspected but the story I got from my buddy was that the inspector was there for about 5 minutes before he commented that clearly this was not done by a contractor.

It was too neat and clean plus all the wiring and conduits were of proper size for its lengths of runs and capacities. Clearly A+ work done by someone who owned and read an up to date code book!

Sigh, tcmtech post #11 - the UK "IEE Regulations" (since decades ago), say the user should be provided with information on cable sizes, correct fuse or breaker sizes for each circuit, what it feeds etc. I have a text-book on testing to the regulations (2002) which at that necessary first move in a compliance check comments "It is at this point that most readers who work in the real world of electrical installation will be lying on the floor laughing hysterically."

67model

I should have added in post #9 that

As an example, 16th Edition, Clause 464-01-02 for functional switching has "A switching device shall not be placed solely in the neutral conductor".

67model

Dear friend

Using 2 poles in series help you for 2 below mention:

1-collection heating due of over current in series contact and better performance.

2-Increasing breaking capacities in higher voltage(v1+v2)

I'm not sure this is correct. In a single phase motor situation using a 3 pole contactor you need to series the overload heaters up for the thermal overload to work correctly. This is not the case on an MCB/MCCB.

If you put a few MCBs side by side all well loaded the heat from one will affect its neighbour and a derate must be considered. (ref back of Merlin Gerin cat)

A fuse in the neutral is a complete no no but can't see an issue on a mechanically interlocked breaker. As you can't guarentee zero PD between earth and neutral the neutral now has to be considered a 'live' conductor. Look to the UK double switched sockets that now switch both live and neutral ...

Some breakers are used in series for DC applications.