Electronics > Beginners

Single Pole or 1P + N circuit breakers

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Brumby:

--- Quote from: Monkeh on March 27, 2019, 11:22:48 pm ---If earth and neutral are not connected then by definition there is no neutral. Note that I'm not American, also..

--- End quote ---

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

I think the point being made is more:

--- Quote from: soldar on March 27, 2019, 11:09:57 pm ---Note also that in Spain earth and neutral are not connected as they are in America. Another case of doing things differently.

--- End quote ---

Zero999:

--- Quote from: Benta on March 27, 2019, 08:34:28 pm ---
--- Quote from: Monkeh on March 27, 2019, 07:49:59 pm ---
--- Quote from: Zero999 on March 27, 2019, 01:25:57 pm ---It still has to make the circuit safe, i.e. isolate it.
--- End quote ---

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

--- End quote ---

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.

--- End quote ---
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:

--- Quote from: soldar on March 28, 2019, 09:02:30 am ---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).

--- End quote ---

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..


--- Quote from: Zero999 on March 28, 2019, 08:48:30 am ---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!

--- End quote ---

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:

--- Quote from: Monkeh on March 28, 2019, 01:49:57 pm --- Nobody in their right mind uses TN-C outside of distribution.
--- End quote ---

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.

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