Electronics > Beginners
Single Pole or 1P + N circuit breakers
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Zero999:

--- Quote from: Monkeh on March 31, 2019, 01:43:52 pm ---
--- Quote from: Zero999 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.

--- End quote ---

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.

--- End quote ---

* 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.
Monkeh:

--- Quote from: Zero999 on March 31, 2019, 07:13:40 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.
--- End quote ---

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


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

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.


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

--- End quote ---

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.
ruggedscot:
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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