Author Topic: Single Pole or 1P + N circuit breakers  (Read 14045 times)

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Offline soldar

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Re: Single Pole or 1P + N circuit breakers
« Reply #50 on: March 30, 2019, 09:39:37 pm »
I believe the advice to "follow the code" has been repeated several times in this thread. 

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.

I was just watching Mains Supply And Consumer Unit and that is very different than what we have in Spain. Here the "Mains Supply" cabinet is outside the housing unit and not accessible to the tenant, only to the power company. So the main breaker in the "consumer unit" is what would be used to isolate the installation from the outside.

ETA: BTW, using wood in the mains panel would be totally illegal here. Sounds crazy.

« Last Edit: March 30, 2019, 10:15:58 pm by soldar »
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Offline Monkeh

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Re: Single Pole or 1P + N circuit breakers
« Reply #51 on: March 30, 2019, 09:43:51 pm »
"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.

I can't speak for the rest of the EU (and probably really can't soon), but here it is a.. grey area. Unless they've tightened their grip more since I last checked.

They are trying very hard to make it illegal or impractical for people to perform their own work, but this is mostly driven by several large private organisations with immense vested interest in their own profits. But that is very much outside the scope of a technical discussion.

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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

From my perspective it appears to simply be poorly thought out standards. But they are old, and improving them or adopting anyone elses ideas is heresy there.
« Last Edit: March 30, 2019, 09:45:28 pm by Monkeh »
 

Online Zero999

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Re: Single Pole or 1P + N circuit breakers
« Reply #52 on: March 31, 2019, 08:47:40 am »
To answer the question: why 1P+N, rather than 1P? The main reason is sometimes it's necessary to isolate an entire circuit, so it can be maintained, without shutting everything down. Granted, an isolator can be used as well as a circuit breaker, but if the two are combined into one, it saves cost. It's also safer because two contacts will break the arc more effectively than one and for the reasons mentioned above regarding broken neutrals.
 

Offline Monkeh

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Re: Single Pole or 1P + N circuit breakers
« Reply #53 on: March 31, 2019, 01:43:52 pm »
To answer the question: why 1P+N, rather than 1P? The main reason is sometimes it's necessary to isolate an entire circuit, so it can be maintained, without shutting everything down. Granted, an isolator can be used as well as a circuit breaker, but if the two are combined into one, it saves cost. It's also safer because two contacts will break the arc more effectively than one and for the reasons mentioned above regarding broken neutrals.

So the first point would be the non-safety maintenance point I admitted. No cost is saved by using 1P+N breakers as you still need to isolate the supply to them, so you have an upstream isolator anyway. This is not really a useful thing in a domestic or small commercial environment, just a waste of space and additional points of failure.

The second.. the breaker is required to safely clear arcs up to its interrupt current rating, and I can't see how you can say it's objectively safer to have a breaker which meets a standard than another breaker which meets the same standard.

As for the third point, I still do not see a fault scenario in which it is objectively safer to disconnect neutrals in an overcurrent condition. A different device is required to protect against lost neutrals.
 

Offline Cubdriver

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Re: Single Pole or 1P + N circuit breakers
« Reply #54 on: March 31, 2019, 02:56:11 pm »
Certainly in north america standard circuit breakers only interrupt the live conductor.  Neutral is bonded to ground in the panel, so there is no need or desire to break it.
Yes, that's mostly the case here too. I say mostly, because I can think of instances where that may not be the case. For example, if there's an outbuilding with a thick cable running to it from the main property, it will have its own distribution panel and will not bond the neutral to earth again there. In that case, to mitigate the danger posed by a poor neutral connection, which would make the neutral float at a dangerous voltage, the breaker should disconnect both the neutral and phase conductors.

Is there any reason why I wouldn't be able to bond the neutral and earth in this sub panel?

Since I didn't see this answered subsequently in the thread, I'll chime in here - this is one that will depend on your local electrical code. 

For instance, here in the US, you CANNOT by code bond the neutral and earth at a SUB panel - earth and neutral are to be bonded together at the service entrance/main disconnect (where the mains enter the building, usually the main breaker panel, but not always if for some reason it is located more than a few feet from the service entrance).  Any panels that are remote from the main disconnect/earth-neutral bonding point must have separate neutral and earth conductors run to them, and their neutral and earth buses must be kept isolated as well.

So in short, for this answer check the applicable code for your area.

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Online Zero999

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Re: Single Pole or 1P + N circuit breakers
« Reply #55 on: March 31, 2019, 07:13:40 pm »
To answer the question: why 1P+N, rather than 1P? The main reason is sometimes it's necessary to isolate an entire circuit, so it can be maintained, without shutting everything down. Granted, an isolator can be used as well as a circuit breaker, but if the two are combined into one, it saves cost. It's also safer because two contacts will break the arc more effectively than one and for the reasons mentioned above regarding broken neutrals.

So the first point would be the non-safety maintenance point I admitted. No cost is saved by using 1P+N breakers as you still need to isolate the supply to them, so you have an upstream isolator anyway. This is not really a useful thing in a domestic or small commercial environment, just a waste of space and additional points of failure.

The second.. the breaker is required to safely clear arcs up to its interrupt current rating, and I can't see how you can say it's objectively safer to have a breaker which meets a standard than another breaker which meets the same standard.

As for the third point, I still do not see a fault scenario in which it is objectively safer to disconnect neutrals in an overcurrent condition. A different device is required to protect against lost neutrals.
  • You've missed the point. What if you need to work on the circuit connected to the other side? Suppose an outbuilding is wired on a separate 1P+N breaker and you need to work on a circuit in there? No need to turn off the whole house, just isolate that circuit. You've saved money by having the breaker and separate isolator.
  • Did you know that connecting two relay contacts in series can double the voltage rating because it quenches the arc more effectively?
  • Just because you cannot see it, it doesn't mean it isn't so. Reread the thread.
 

Offline Monkeh

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Re: Single Pole or 1P + N circuit breakers
« Reply #56 on: March 31, 2019, 07:34:44 pm »
You've missed the point. What if you need to work on the circuit connected to the other side? Suppose an outbuilding is wired on a separate 1P+N breaker and you need to work on a circuit in there? No need to turn off the whole house, just isolate that circuit. You've saved money by having the breaker and separate isolator.

I have not missed the point, you simply see a huge inconvenience where most people do not.

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Did you know that connecting two relay contacts in series can double the voltage rating because it quenches the arc more effectively?

And under which standard is this tested and acceptable? Again, the device is designed to be capable of clearing any fault it should reasonably expect to encounter in service.

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Just because you cannot see it, it doesn't mean it isn't so. Reread the thread.

I have repeatedly asked for you to actually detail the scenario, instead of just repeating "broken neutral" like a mantra.

I shot down the theory of a high fault current through neutral shorted to earth, as a 1P+N MCB cannot clear it, and likewise it cannot be relied upon to protect from contact with broken neutral at high potential because this is not what MCBs do, RCDs are the protective device for human contact, as well as neutral to earth fault currents.
« Last Edit: March 31, 2019, 07:45:54 pm by Monkeh »
 

Offline ruggedscot

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Re: Single Pole or 1P + N circuit breakers
« Reply #57 on: March 31, 2019, 07:50:29 pm »
A little knowledge is a dangerous thing - messing with electricity if you don't fully understand or appreciate what you are doing is lethal...

The electrical supply to your house can be given in a number of configurations.

TN-C is a configuration of supply where the Neutral and Earth is combined - in this configuration the neutral must not be switched.

its worth reading up on how electricity is supplied and then checking to see how your installation is configured. Don't assume and don't take advice from forums where there are people out there that don't know how your electrical supply is configured.

With a TN-C-S configuration the earth and neutral are combined and then separated through to the point of connection - this type of supply both the neutral and live can be switched. The main issues around switching a neutral is that if you don't you can have a situation where the live is switched off but the neutral is still connected through to the item that you are working on. Depending on other loads in the area that is being supplied and the impedance of the wiring you could have a voltage drop between the source neutral and the loads. this could then produce a voltage on the neutral that is above the potential of earth. So you could then have a potential for a current flow between the neutral and the earth in the event of an inadvertent contact between the neutral and earth.

https://electrical.theiet.org/media/1549/earthing.pdf

 


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