Will turning off an individual breaker cut

[bjouis]

Being from France i'm having a bit of a brain fart when looking at UK wiring systems. In our systems, neutral and live come out the top of a breaker and go directly to the circuit (we have radials here, no rings, not allowed in our regs) and we also have no neutral bar for individual circuits, only for rcds.

I watched a video of a guy who seems very 'engineer-y' saying that isolating a single breaker doesn't really isolate the current because the shared neutral bar is still live, presumably he means this makes the neutral on that circuit live too. Is this true? I swear i've seen people say just turn off the individual circuit and then you can get working and it's fine.

So i'm very confused now. I'm a newcomer to electrics so please be gentle.

 

[bjouis]

I think that possibly the confusion comes from the word "Isolated" as opposed to not "powered".

Could be - thanks for your input mate it's appreciated.

 

[pc1966]

Good point. In many electrical / electronic situations "isolated" means completely disconnected from any conductive path either operationally (e.g. transformer, optocoupler, etc) or for maintenance (all-pole switch, removing plug, etc).

But to an electrician here "isolated" is usually the more specific case of the power (phase conductors) being disconnected in a safe way. Typically this means:

• The disconnection method has creepage and clearance distances sufficient not to flash over on voltage spikes
• It has been locked and labelled so others will not accidentally turn it back on
• You have proved the intended circuit really is dead

On any training and examinations you do this is a major step and if you get it wrong they will fail you! It is drummed in to folk as a high priority because it is so important from a safety point of view that all of those points are adequately addressed before work commences.

 

[davesparks]

This is why i'm confused. If the neutrals are carrying voltage and the N of your socket is connected to them all, why doesn't the neutral electrocute you when just the MCB is turned off?

I always assumed it was live into a component like a socket, neutral comes back out carrying the same voltage and then goes into the N tail where it's taken away outside of the property but i'm obviously missing some key fundamentals.

Does anyone have a good video or resource that can explain all this? Electric for 1st graders perhaps?

Before you go too far in to the practical aspects you should work on understanding the basic science of electricity, this is fundamental to everything else. You need to understand what voltage and current and resistance are and their relationships.

Voltage is not carried by anything, voltage doesn't flow, current flows.
Voltage is a measure of the potential energy between two points, if there is a difference in voltage between two points joined by a conductive path then current will flow.
It may help to think of voltage as the pressure which pushes the current around a circuit.
Resistance tells you how much something resists the flow of current.
To take the pressure analogy a little further imagine connecting an open length of pipe to a pump delivering 10 bar of water pressure, water will flow through the pipe and out of the open end, there is nothing stopping the flow of water so the pipe has zero resistance.
This would represent a cable joining live directly to neutral, the maximum amount tof current available will flow. (don't do this, there will be fire, injuries and possibly death)

In terms of pressure (voltage) the pump is delivering 10 bar of pressure and the end of the pipe is open so has zero pressure, this gives a pressure difference of 10 between the pump and the open end so water flows.
With the conductor joining live and neutral the live has a voltage (pressurel) of 240V higher than neutral so a lot of current flows.

Now if you put a blanking plug on the end of the pipe and turn the pump on no water will flow because the blanking plug is resisting it, it has infinite resistance.
In this setup the pipe still has 10 bar of pressure applied to one end, but because of the blanking plug it has 10 bar of pressure at the other end too, there is no pressure drop so no water flows.


In the UK there is a voltage difference of 240V between the line and neutral. The neutral is connected to earth at the substation so there is (in basic terms) no voltage between the neutral and earth, and there is 240V between line and earth.

 

[Ray Pooley]

Being from France i'm having a bit of a brain fart when looking at UK wiring systems. In our systems, neutral and live come out the top of a breaker and go directly to the circuit (we have radials here, no rings, not allowed in our regs) and we also have no neutral bar for individual circuits, only for rcds.

I watched a video of a guy who seems very 'engineer-y' saying that isolating a single breaker doesn't really isolate the current because the shared neutral bar is still live, presumably he means this makes the neutral on that circuit live too. Is this true? I swear i've seen people say just turn off the individual circuit and then you can get working and it's fine.

So i'm very confused now. I'm a newcomer to electrics so please be gentle.

An MCB only has two terminals one at the bottom that connects it to the LIVE bus bar and one at the top that connects to the LIVE (brown or red) wire in the ciruit. It only connects to the incoming load because that's what it measures in order to decide whether to trip or not. The neutral wire in the circuit is, in effect, spliced with all the other neutrals wires in all the other circuits by virtue of the fact that it shares the same connection block with them. The same can be said of the earth wire.

 

[pc1966]

When replying to your question about the UK final ring:

Why does the UK use rings for sockets? > on Free Electrical Advice - Electricians Forums

Find Why does the UK use rings for sockets? Advice and Help. How-to Why does the UK use rings for sockets? in the Electrical Wiring, Theories and Regulations advice boards on Electricians Forums. Why does the UK use rings for sockets? on ElectriciansForums.net Electrical Advice Forum.

www.electriciansforums.net

It occurred to me that our single-pole MCB probably dates back to that sort of time and the decision to have phase-only fusing of plugs. In order to do that safely you need to make sure the polarity is correct (i.e. fuse is always in the "live" feed) and if you are mandating correct polarity as an key point of all installations then going to single-pole fusing at the board (which were replaced by MCBs by around the 1980s) is a logical step.

Another difference between the UK and much of the rest of the world is that even before WW-2 we had polarised 3-pin plugs with a separate earth (OK, we had at least 3 sizes of them!) whereas most EU and USA style plugs were reversible 2-pin ones, and even when an earth was added it more recently to the design was often as a reverse-compatible move and many do not polarise the plug insertion.

Having said that, we often has fused neutrals on the main supply feed, now known as the Distribution Network Operator (DNO) cut-out, to the 30s or so (can't remember that far back on account of not being born) and that is not a good thing!