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Single-Phase Breaking on a Two-Pole Breaker?

12/26/2017 10:36 PM

So in general, the panel shop I use says most commonly for a single phase component most firms will use a 2 pole breaker. In other words the break the neutral also not just the phase

What are the pros and cons of this?

I know its generally more expensive. The only pro I can see is the 2 pole breaker gives you a terminal to land a neutral wire in a panel without a separate terminal.

Any thoughts?

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#1

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 12:15 AM

Where do you live?

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#2

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 2:33 AM

Most 2-pole breaker setups I've seen connect to two live buses. Wired (rather than plug-in) breakers can be connected to a neutral, however.

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#12
In reply to #2

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 5:46 PM
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#3

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 5:15 AM

For an electrical system where the Neutral is firmly connected to Earth such as in the TN or TT networks there is no real need for the Neutral to be switched as it is held at earth potential. In these situations a single pole Circuit Breaker (MCB) is sufficient, but a Residual Current Device (RCD) adds extra personal protection, especially in the case of the TT system where fault loop impedance is generally higher.

RCDs necessarily require the circuit Neutral conductor to also pass through them in order to work correctly, therefore they will be double pole for a single phase circuit.

RCDs are now mandatory in many jurisdictions and, rather than using separate RCD and MCB components, many installations now use RCBOs almost exclusively due to their combined MCB and RCD functions and inherent safety features, so what may initially appear to be an ordinary double pole MCB may in fact be an RCBO.

In situations such as portable structures where power is supplied via flexible leads, then all live conductors should be switched to afford protection in the event of reversed polarity in the lead no matter which network is employed.

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#4
In reply to #3

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 11:10 AM

thanks for the well thought out reply

However, now i have more questions then answers :)

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#5
In reply to #4

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 12:20 PM

Again, where are you in the world? This is an international forum and it makes a difference.

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#6
In reply to #5

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 12:49 PM

Im sorry, i missed your reply

I am in the USA.

Thanks for following up

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#11
In reply to #4

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 5:14 PM

Indeed the neutral is earthed and most regulations allow that only the live is switched.

However, regulations and practices are born from long experience of injury, fire and death and what can go wrong to cause that. I have recently personally experienced the electricity supplier replacing his kWh meter and its cables to incomer & dist. board and failing to secure the neutral cable in the incoming terminal (despite it having two screws which had been wound tight).

The neutral became disconnected after some weeks - consequently, as soon as a light switch was put on, every neutral terminal in the installation was at 250V to earth, connected through the lamps.

It becomes clear why the neutral has the same wire & terminal insulation as the live and why it might be a good idea to switch both live and neutral in circuits & at points of use. Also why fuses and single pole switches & breakers in neutral are forbidden.

Regulations here require RCDs on all socket outlet [receptacle] circuits for general use and they all break live and neutral - a common arrangement for domestic supplies is to have two RCDs, with MCB circuits split between them.

RCDs may prevent a fatal shock (if they work, they are complicated devices) but a look at national statistics shows only 30 people are killed per annum by LV 230V supply electrocution (0.5 per million of population) while hundreds of thousands are injured.

Despite this apparent new safety, the premium maker of socket outlet units has recently put double pole [live & neutral] switches into his top range.

I suppose that supplier has noticed that RCDs on each circuit in commercial/industrial applications make it more likely that L & N are swapped, even though it is not supposed to happen. Also, more appliances do not have a real on-off switch, only a button for an electronic control which is actually live 24 hours/day, even if used only a few hours a week and have emc filter capacitors neutral to earth - RCDs can trip for residual current in neutral as well as the live. So it becomes more important that simple wall on-off switches really do that totally, rather than the appearance of it to the untrained.

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#13
In reply to #11

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 6:20 PM

Adding to post #11,

More and more gear is electronic, including time switches and lamp ballasts.

And they come with the caution that you should not "megger" test them!

At least insulation test cannot be done line-neutral.

But if you join L to N and test both to earth, you can test insulation.

So if you have easy access to the neutral of the circuit you can test the insulation of the whole circuit on completion or when checking condition.

When everything is joined onto a common neutral bar it is often difficult to identify the neutral wire of the circuit you want - and you cannot test it without disconnecting it, then reconnecting.

Compared to the cost in time of all that, a double pole breaker or isolating switch is worthwhile.

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#10
In reply to #3

Re: Single phase breaking on a 2 pole breaker?

12/27/2017 4:46 PM

Ahem. The neutral is not <... firmly connected to Earth...in the...TT network...>. The neutral is bonded to earth at the distribution transformer only, with good reason, and the Wikipedia article on the topic explains why.

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#16
In reply to #10

Re: Single phase breaking on a 2 pole breaker?

12/28/2017 7:37 AM

Please explain!

For decades I've put in a ground rod that all neutrals are tied to.

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#18
In reply to #16

Re: Single phase breaking on a 2 pole breaker?

12/28/2017 11:05 AM

Such an installation is not "TT".

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#7

Re: Single-phase Breaking on a Two-pole Breaker?

12/27/2017 4:17 PM
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#8

Re: Single-phase Breaking on a Two-pole Breaker?

12/27/2017 4:20 PM

The biggest <...con...> is British Standard 7671, which says "Don't do it, Ethel!" - one doesn't fit over-current protection devices in a neutral conductor!

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#9
In reply to #8

Re: Single-phase Breaking on a Two-pole Breaker?

12/27/2017 4:40 PM

Well, he said he is in the USA. Despite belief to the contrary by most electricians, NEC 240.22 allows it if the breaker is designed so that all poles open simultaneously and no pole can open independently. Does make for neater wiring, but would cause controversy because few are aware of this section of the code.

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#22
In reply to #9

Re: Single-phase Breaking on a Two-pole Breaker?

12/29/2017 7:51 AM

I have seen a double pole breaker open on 1 pole and still connected on the other with the toggle still "on" with no visual sign of trip. This was on a 240 volt circuit in the USA. If it had been a 120 volt with the neutral going through the open side of said defective breaker then the neutral would have been live. That was a good name brand breaker that had been in service for many years. I would never switch the neutral through a normal 2 pole breaker.

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#23
In reply to #22

Re: Single-phase Breaking on a Two-pole Breaker?

12/29/2017 8:21 AM

First let us get on the same page, as I know what I work with is not your average lighting/receptacle panel, but UL 508A industrial control panels, and the breakers we use are a whole world different than the ones in the "normal" panels. To begin with, they DIN rail mount - not bus mount. They are often individually wired. Cost is a bit higher than a lighting panel breaker as well, so perhaps quality is too.

Don't know where you are getting your breakers, but all the ones we use in our shop have a metal pin through the handle of both sides. The plastic handle would have to break to allow one side to trip w/o the other. If you have a breaker still in service that is broken, then I guess you, or the plant maintenance staff, are at fault.

We actually do do this in our shop here on 120VAC feeds to industrial control panels, where all you can get for a NFPA 79 handle arrangement main breaker is a three pole arrangement. Neutral in through one pole and the 120V through another. Ground lands on a ground bar.

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#25
In reply to #23

Re: Single-phase Breaking on a Two-pole Breaker?

12/29/2017 11:49 AM

This was a Square D breaker factory tied and showed no sign of trip. I tested it before handle was moved. It felt good (normal) when moved to off and when flipped back to on both poles connected. I only ever saw it this one time but once is enough. Of course I replaced the breaker with a new one.

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#26
In reply to #25

Re: Single-phase Breaking on a Two-pole Breaker?

12/29/2017 12:07 PM

I understand that. I was zapped by the load side of an open disconnect switch that was, of course, shut off and locked out. Two of the rotating phases bars had broken and were still in place on the incoming service side. When I touched one of the terminals on the load side that were still connected - zzzzzzzap! My fault for not using my meter to confirm the terminals were dead. Fortunately this was 208VAC so the phase I touched was only 120V. That only tickles me a bit, even if well grounded. (I have extremely high skin resistance) Had that been a 480V disconnect, well, I probably wouldn't be writing this. Also had to blame my self for the poor maintenance - until we got skid building running here, I was also in charge of plant maintenance.

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#24
In reply to #22

Re: Single-phase Breaking on a Two-pole Breaker?

12/29/2017 11:21 AM

Didn't think enough before my first reply to this post. I would say that a breaker, such as you described, clearly does not meet the requirements of NEC 240.22 and thus should never be used as an easy way to run the neutral. I guarantee there are breakers out there that do. Any of the breakers that we use in our UL 508A industrial control panels will meet this requirement, unless they are broken, and that would require some very poor operating practices on the part of the user. Not saying I would do this except as noted in my first reply. The biggest issue is still the inherent belief by USA electricians that it is not per code. You will get much grief if you wire this way.

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#15
In reply to #8

Re: Single-phase Breaking on a Two-pole Breaker?

12/27/2017 10:00 PM

Could you point me in the direction of that particular regulation please?

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#19
In reply to #15

Re: Single-phase Breaking on a Two-pole Breaker?

12/28/2017 1:39 PM

BS7671 : 2008 with Amndmnts 1-3, 2015 has.....

132.14.1 A single pole fuse, switch or circuit-breaker shall be inserted in the line conductor only.

132.14.2 No switch or circuit-breaker, except where linked, or fuse, shall be inserted in an earthed neutral conductor. Any linked switch or linked circuit breaker inserted in an earthed neutral conductor shall be arranged to break all the related line conductors.

That is in Chapter 13 Fundamental Principles.

The second, I guess, is similar in meaning to the NEC article described by Phys - perhaps he can reference the equivalent to 132.14.1

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#14

Re: Single-phase Breaking on a Two-pole Breaker?

12/27/2017 7:50 PM

So as phys said, you are ALLOWED to do it here so long as you follow the rules he laid out.

But as to the comment of "...most firms will use a 2 pole breaker" that is just false. As a general rule, the only time someone uses a 2 pole device on a single phase circuit here in North America is if it is 240V single phase where there are two HOT lines, not a hot and a neutral. Where people who do business on "both sides of the pond" get confused is because elsewhere outside of North America, 230V is line to neutral, not line to line, and in some cases you are in fact required to switch the neutral. But NOT here.

If you have a 120V system here, that is line to neutral and you are NOT required to switch the neutral. You CAN, but most people DO NOT.

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#17

Re: Single-phase Breaking on a Two-pole Breaker?

12/28/2017 10:03 AM

Their panel is probably built to a standard other than the National Electric Code. It would be certified by an outside organization with other standards. The panel then becomes part of a larger piece of equipment. The power and wiring to the piece of equipment is regulated by the NEC (USA) or whatever code the AHJ (Authority Having Jurisdiction) uses.

It is difficult to compare the wiring inside a piece of equipment to the building wiring. For instance look at the size of the service entrance conductors to the building and compare them to the wires from the power company. They use different standards. Ask the panel shop why they do it that way. It may just be that the certifying agency requires it, or they may have another reason.

It may be as simple as making the equipment compatable with American power - 120 on Line 1, 0 on Neutral, 120 on Line 2 = 240 and europen power 220. on line 1 0 on Neutral. (I think, I've never worked outside USA) - JHF

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#20

Re: Single-phase Breaking on a Two-pole Breaker?

12/28/2017 11:16 PM

Simply, Two pole breaker for Two phase feeder and Single pole breaker for Single phase feeder.

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#21

Re: Single-phase Breaking on a Two-pole Breaker?

12/29/2017 7:11 AM

Single phase is broken into two primary voltages in the US

240 and 120 Volts

240 takes a two pole breaker

120 takes a single pole breaker

We should simplify to one but oh well.

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#27

Re: Single-phase Breaking on a Two-pole Breaker?

12/29/2017 12:41 PM

Here is a thought:

A line to line connection lets say is is 240 volts into many homes in the US. Both of those those lines are "Hot". The neutral conductor is used when you want to power a small appliance, such as a 120V microwave oven. The microwave circuit needs a one-pole breaker. Both poles are used to power a large appliance such as an 240V electric dryer in the laundry room. That laundry circuit needs a two-pole breaker.

This 240V service to a home is still referred to as "single phase" by most electricians.

Do not put a breaker on the neutral line. Connect the neutral line to the grounding conductor only at the origin of the power, such as the service entrance.

Provide a grounding conductor separate from the neutral line to all appliances.

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