Single Pole or 1P + N circuit breakers

[Brumby]

If earth and neutral are not connected then by definition there is no neutral. Note that I'm not American, also..

If earth and neutral are not connected then, by definition, there is no functional earth, either.

I think the point being made is more:

Note also that in Spain earth and neutral are not connected as they are in America. Another case of doing things differently.

[Zero999]

It still has to make the circuit safe, i.e. isolate it.

No, it has to clear the fault. Isolation is not the same thing.

Agreed.
Let's go back some years: circuit breakers were thermofuses/filaments that melted. They were only placed in the live conductor in the fuse panel.
The resettable breakers have the same function, which is to open the circuit in case of overload.
Fusing neutral is nonsense.

Read the thread again. No one is talking about fusing the neutral, which would be dangerous, because if the fuse breaks, the live would still be connected.

We're talking about a double pole circuit breaker, which is a double pole single throw switch and disconnects both the live and neutral conductors. Soldar is right, disconnecting both conductors is safer, than only one, because an upstream fault could mean the neutral conductor is live.

In situations where there's a risk the neutral might not be neutral, a double pole breaker must be used. If this wasn't a requirement, then they wouldn't exist!

[soldar]

In engineering many decisions have to weigh several considerations and there is no one absolutely correct answer. Different countries with different boards come to different rules and anyone can disagree with the rules. In many cases there are historical considerations.

IMHO polarized plugs are very high up in my list of priorities and yet continental Europe is stuck with the abominable Schucko plug for historical reasons from Germany which today would be irrelevant if we were starting out from scratch.

Historical reasons explain things which are difficult to explain otherwise. I understand in Japan they have a mish-mash of voltages and frequencies.

In Spain the code mandates cutting off all poles and it certainly adds safety. The question is how much safety it adds and whether it is justified in terms of cost and complexity. I guess the answer might depend on whether the person who died of electrocution was your daughter or your mother-in-law.

OTOH, the code in Spain allows flexible multi-strand cables in conduit for permanent wiring. I do not like that and I believe it adds risk but the panel who made the rules gave more weight to the fact that flexible cable is much easier to feed through conduit.

Regarding protective earth there are basically three different systems illustrated in this page. In Spain  TN-S and TT are commonly used. The former most commonly for smaller and residential installations and the latter for larger installations.  America uses TN-C which is I thought was absolutely prohibited in Spain but that page says it is allowed if the common neutral-earth conductor has a minimum section of 16 mm2 (that is a diameter of 4.6 mm). That does not sound too difficult to meet but, still, I have not seen it used. What is absolutely prohibited is using metal water or gas pipes for earth. No way. 

In any case, when doing any kind of work it should comply with local codes, standards and practices so that others can rely on things complying with local codes. When in Rome ...

[Monkeh]

Regarding protective earth there are basically three different systems illustrated in this page. In Spain  TN-S and TT are commonly used. The former most commonly for smaller and residential installations and the latter for larger installations.  America uses TN-C which is I thought was absolutely prohibited in Spain but that page says it is allowed if the common neutral-earth conductor has a minimum section of 16 mm2 (that is a diameter of 4.6 mm).

No, they do not use TN-C. If they used TN-C there would be no separate neutral and earth in the installation, whereas the reality is they generally use TN-C-S or TN-S with an additional bond between earth and neutral at the property. In the UK, you will find TN-S, TN-C-S, and TT. Nobody in their right mind uses TN-C outside of distribution.



Repeat after me: An MCB is not necessarily an isolator. You must isolate an installation before working on it.

Now, if you have appropriate standards in place to use a dual-pole MCB as an isolator, that's fine. But that doesn't make a single-pole device 'less safe', because it is not an isolator..

We're talking about a double pole circuit breaker, which is a double pole single throw switch and disconnects both the live and neutral conductors. Soldar is right, disconnecting both conductors is safer, than only one, because an upstream fault could mean the neutral conductor is live.

In situations where there's a risk the neutral might not be neutral, a double pole breaker must be used. If this wasn't a requirement, then they wouldn't exist!

And in what situation other than one where you must isolate the circuit does it matter?

As a point of terminology, the neutral conductor is always live. That's why it's double insulated and must be isolated before contact.

[soldar]

Nobody in their right mind uses TN-C outside of distribution.

Those diagrams are about distribution. You might want to go back a look at them again. I think you are misinterpreting things.

In America earth and neutral are bonded at the panel.

In Spain this is not allowed and earth and neutral are separate wires all the way to the distribution transformer.

[Monkeh]

Nobody in their right mind uses TN-C outside of distribution.

Those diagrams are about distribution. You might want to go back a look at them again. I think you are misinterpreting things.

In America earth and neutral are bonded at the panel.

In Spain this is not allowed and earth and neutral are separate wires all the way to the distribution transformer.

So now we're talking about distribution side instead of installation side. Those goalposts move fast. And the relevance to the misuse of a single pole breaker as an isolator in fact still escapes me.

[soldar]

It is very simple: faults in the circuit, inside or outside, can make the neutral wire become energized. Having circuit breakers which break all poles offers protection against such thing. The Spanish authorities consider this protection significant enough that they require that circuit breakers to break all poles. I understand it and that is enough for me.

To the OP my recommendation is to follow local code.

[Monkeh]

It is very simple: faults in the circuit, inside or outside, can make the neutral wire become energized. Having circuit breakers which break all poles offers protection against such thing. The Spanish authorities consider this protection significant enough that they require that circuit breakers to break all poles. I understand it and that is enough for me.

In what circumstance does not having the neutral disconnected by the breaker pose a risk? Merely having it above ground potential is not an issue, because it is a live conductor. You don't have access to it.

[Zero999]

The goal posts have not moved. If you reread the the thread, you'll remember, it was about distribution from the first page.

You forget, we're talking about fault conditions. The system is not functioning properly. If the neutral conductor is still live, it still poses a shock and fire hazard.  Under normal conditions, the user will not have access to the neutral, but the case of some equipment could be damaged, exposing it or some insulation might have melted, caused it to be shorted to earth and a high current to flow.

The safety regulations are there for a reason, whether you understand why or agree with them or not.

[Monkeh]

The goal posts have not moved. If you reread the the thread, you'll remember, it was about distribution from the first page.

Distribution = all that stuff outside your house. You know, the stuff controlled by large entities following different regulations. This is about an installation, not distribution.

You forget, we're talking about fault conditions. The system is not functioning properly. If the neutral conductor is still live, it still poses a shock and fire hazard.  Under normal conditions, the user will not have access to the neutral, but the case of some equipment could be damaged, exposing it or some insulation might have melted, caused it to be shorted to earth and a high current to flow.

But you already said it's not a fused neutral. So you're not offering any protection against combined fault currents from other circuits in an open neutral condition.

If the neutral is exposed, you've already passed two layers of insulation (if the upstream neutral is also open, a double fault..). The line could just as easily be exposed - but an MCB doesn't protect against that either, it protects against overcurrent faults, which your body probably will not create. Especially not from an exposed open neutral.

If the installation or equipment is damaged you isolate it. A single-pole device is not an isolator, so you use the isolator.

The safety regulations are there for a reason, whether you understand why or agree with them or not.

Yes, they are, and they are written by people who understand the application of each type of protective device and the circumstances in which they operate, and hopefully applied by people with equal understanding.

[soldar]

John Ward discusses and explains his very issue:

[Monkeh]

TL;DW. I know earthing systems.

An MCB operates on exactly one fault condition. Please propose a scenario in which it can operate and opening the neutral conductor would be beneficial. If this scenario can be resolved by the use of an RCD (re: TT systems.), or it involves manual operation of the MCB, I refer you to everything I've already said.

[soldar]

Yeah, there are none so blind as those who will not see.

Anyway, it is well explained in that video. For anyone who would like to see.

[Monkeh]

Yeah, there are none so blind as those who will not see.

Anyway, it is well explained in that video. For anyone who would like to see.

So explain it.

I could go watch 18 minutes of that but I do have other things to watch, so if you'd like to point to the timestamp where he explains the exact fault scenario an MCB sensing only on line would operate in and benefit from opening the neutral, that'd be great.

[stevelup]

I agree 100% with you.

The sole purpose of an MCB is to deal with an overcurrent scenario. If it opens on overcurrent it has done its job. End of story.

[Monkeh]

Corner case: Broken upstream neutral, damaged cable with both line and neutral shorted to earth, potential risk of overcurrent in the circuit neutral via other circuits.

The protective device for this scenario is an RCD. If line never shorted the MCB could not clear this fault - an RCD will.

Propose other scenarios.

[perieanuo]

Hi, 1p+N if you have triphasic system,otherwise 1P


Envoyé de mon iPad en utilisant Tapatalk

[Zero999]

I don't see how disconnecting the neutral, as well as the phase is any more dangerous, than just disconnecting the phase. There can be an argument made for the case that it's not necessary to disconnect both conductors, in most applications, but it's completely wrong to imply it's more dangerous. The fact is, it's clearly safer to disconnect both conductors, this isolating the entire system, whether or not it's absolutely necessary or not.

Any thread involving safety inevitably turns into a shitfest. One should state, their reasoning for why x is safer then y, cite the relevant regulations, then move on and ignore all the dangerous misinformation which follows: hopefully anyone with any common sense will be able to pick through it all. 

[Monkeh]

I don't see how disconnecting the neutral, as well as the phase is any more dangerous, than just disconnecting the phase. There can be an argument made for the case that it's not necessary to disconnect both conductors, in most applications, but it's completely wrong to imply it's more dangerous.

When have I implied it's more dangerous?

The fact is, it's clearly safer to disconnect both conductors, this isolating the entire system, whether or not it's absolutely necessary or not.

I see no valuable safety benefit to enforcing isolation in an overcurrent device. Only greater expense and an additional point of failure, and yes, MCBs fail. Of course, if you happen to be dealing with a multiple-line scenario with no neutral, the isolation comes for free - but they're still not made for repeated manual operation, and can fail to close again.

Any thread involving safety inevitably turns into a shitfest. One should state, their reasoning for why x is safer then y, cite the relevant regulations, then move on and ignore all the dangerous misinformation which follows: hopefully anyone with any common sense will be able to pick through it all. 

Regulations are behind a paywall, and I am not an electrician. I note you haven't been citing any, either. My reasoning that it is not necessarily and beneficially safer is that there is no guarantee (in the vast majority of scenarios, not even a possibility) of a device triggered only by the line being able to clear any fault scenario involving the neutral.

I am very open to reasoned fault scenarios I've missed. I have been attempting to tease them out for a while now.

[bsdphk]

This is one of the most damaging kind of discussions we have here on EEVblog.

Far too many people seem to assume that in the rest of the world electricity is just like where they live themselves.

Hint: It is not, which is why electrical codes differ.

If you live on a mountain, like a lot of Norway do, there is no way you can get a usable earth potential terminal, "ground" and "mountain" are electrically very different things.

The electrical code of Norway reflects this, having made intelligent choices in the entire system, to provide the best possible compromise for protection.

The "in the entire system" bit is important, personal electrical safety depends on everything the wire is connected to and what kinds of faults and accidents can happen to it.

In USA, a very common failure mode is a car hitting a pole and the 5 or 11kV wires on top shorting to the 110-0-110 wires below.

This is why USA is so hung up about "surge protectors", and rightly so!

In countries where the MV wires are not carried on top of the same masts as LV, class C transient protection is a waste of money because it doesn't provide statistical significant reduction of damage.

(And if you are worried about lightning, class C catches fire like anything else, unless you also have tested and maintained class A and B up-wire.)

These differences in system failure modes are internalized in the electrical code for your country/region/whatever, and even if you truly understand the systems design decision in your local power-grid, you should still do whatever your electrical code says you should do, because anything else is both A) illegal, and B) reason for you insurance adjuster to laugh.

If you want to increase electrical safety in your lab-environment you can do two things:  1) Don't be careless, silly or stupid, 2) Use an isolation transformer until you are sure there are no electrical problems in your DUT.

/rant off

So answer to original poster:  Pick whatever your electrical code tells you to.

[Monkeh]

This is one of the most damaging kind of discussions we have here on EEVblog.

Far too many people seem to assume that in the rest of the world electricity is just like where they live themselves.

The basic principles are no different and in most cases (Norway sounds interesting. You might look up our term, 'equipotential zone'.) the protective systems are broadly equivalent.

But I guess discussing and learning is just too damaging. Especially when I started discussing with a member from my own country, I mean, I really could be mixing things up there..

[Zero999]

I don't see how disconnecting the neutral, as well as the phase is any more dangerous, than just disconnecting the phase. There can be an argument made for the case that it's not necessary to disconnect both conductors, in most applications, but it's completely wrong to imply it's more dangerous.

When have I implied it's more dangerous?

The fact is, it's clearly safer to disconnect both conductors, this isolating the entire system, whether or not it's absolutely necessary or not.

I see no valuable safety benefit to enforcing isolation in an overcurrent device. Only greater expense and an additional point of failure, and yes, MCBs fail. Of course, if you happen to be dealing with a multiple-line scenario with no neutral, the isolation comes for free - but they're still not made for repeated manual operation, and can fail to close again.

Any thread involving safety inevitably turns into a shitfest. One should state, their reasoning for why x is safer then y, cite the relevant regulations, then move on and ignore all the dangerous misinformation which follows: hopefully anyone with any common sense will be able to pick through it all. 

Regulations are behind a paywall, and I am not an electrician. I note you haven't been citing any, either. My reasoning that it is not necessarily and beneficially safer is that there is no guarantee (in the vast majority of scenarios, not even a possibility) of a device triggered only by the line being able to clear any fault scenario involving the neutral.

I am very open to reasoned fault scenarios I've missed. I have been attempting to tease them out for a while now.

I apologise, I didn't post any standards. I thought Soldar did, but I was mistaken.

To clear up the confusion. In many cases a circuit breaker needs to perform both isolation, as well as overcurrent protection, hence the requirement for double pole breakers by some electrical standards. Granted this isn't always necessary, but sometimes it is. If it wasn't, then double pole breakers wouldn't exist.

[bsdphk]

If by "broadly equivalent" you mean "tries to avoid fire and death" then you are entirely correct.

And you are of course welcome to discuss and learn about this topic, which is incredibly interesting to its core, I'm sure there are a lot of failure modes you have never considered.

But giving advice on what type of protection device to employ, without knowing what the relevant electrical code contains, is just plain irresponsible.

[Monkeh]

To clear up the confusion, in many cases a circuit breaker needs to perform both isolation, as well as overcurrent protection, hence the requirement for double pole breakers by some electrical standards. Granted this isn't always necessary, but sometimes it is. If it wasn't, then double pole breakers wouldn't exist.

In which circumstance is this necessary? Honest question. TT installations here use dual-pole time-delayed RCDs to cover the cases I'm familiar with - you can't even get 1P+N MCBs for many if not most domestic and small commercial boards in the UK.

Let's also be specific with terminology: SP or 1P - single pole breaker, 1P+N - single pole breaker with switched neutral, 2P or double pole - dual pole breaker with overcurrent protection on both poles. 2P are used for split-phase or two phases from a three-phase supply. I recall seeing 2P+N and 3P+N as well in a catalogue somewhere.

1P+N breakers are, to my knowledge, primarily used to provide isolation for service without requiring the isolation of the entire installation or equipment. This is a valid use case! But it is an additional one on top of the primary (and in SP devices, sole) purpose of overcurrent protection, and is nothing more than operating it as a switch (re: this is not a functional switch..).

Standard single pole MCBs are not isolators and are not to be used as such, and standard installation practices here, at least, do not call for 1P+N MCBs to provide that capability. If standards elsewhere do, that's potentially a nice bonus, subject to my concerns about resettability (I have seen otherwise good MCBs simply fail to reclose as the mechanism is not designed for operation as an isolator, and is only required to remain capable of opening, not closing). It's also a large waste of space in almost all normal conditions, as most 1P+N breakers used seem to be two units wide, and in domestic situations isolating the entire installation isn't usually a problem.

If by "broadly equivalent" you mean "tries to avoid fire and death" then you are entirely correct.

And you are of course welcome to discuss and learn about this topic, which is incredibly interesting to its core, I'm sure there are a lot of failure modes you have never considered.

But giving advice on what type of protection device to employ, without knowing what the relevant electrical code contains, is just plain irresponsible.

They generally try to avoid fire and death in almost exactly the same way, with few significant variations. American variations are quite interesting (everything is de-rated after the fact, a 20A circuit may not carry 20A, etc), and to my knowledge their degree of testing is limited in the extreme. It's a little disturbing.

I have been asking for conditions I have not considered. Repeatedly. As for giving advice - I haven't! Others already said follow the local code and I considered that quite adequate advice, although I might also say "hire an electrician if you can't answer your own question". That advice often gets me shouted down, however.

[bsdphk]

"hire an electrician if you can't answer your own question". That advice often gets me shouted down, however."

Not only if you can't answer your own question, but also because it is the law in all of EU as far as I know.

The derating in USA codes is very much because people are allowed to bodge their own installation and they are not very good at it