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.

 

[Megawatt]

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.

Through my experience if you take a neutral loose and don’t turn off the corresponding breaker of that particular circuit yes if your hand or any part of your body is grounded you will get bit

 

[telectrix]

in france you use 2 pole breakers. here we use single pole, just the L is isolated. and the N's all common up to an unswitched bar. except with RCBOs where each N is fed from the RCBO. usually again single pole or sinle pole trip with N switched as the RCBO trips.

 

[Mike Johnson]

In France the neutral and phase must disconnect when operating the MCB therefore dual pole, the air gap is required to be a minimum of 3mm, therefore the circuit is dead, in the UK the MCB's are only single pole and only disconnect the phase, so the neutral is always connected, therefore in effect still live, if you need to work on your french installation you only need to trip the MCB or the RCD protecting that row of maximum eight MCB's.

Disjoncteur divisionnaire = MCB
Interrupteur différentiel = RCD
Disjoncteur différentiel = RCBO

Also note that the first generation of UK RCBO's did not switch the neutral sufficiently to be approved in France, easily recognised as they had a blue flying lead out of the unit and only took up a single module space in the board.
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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.

Just re-read that, usually in France the neutral and phase to the circuits come out of the bottom of the DD, power for them being by a comb from the ID across the top, what make of DD and ID's do you have in your board.

 

[bjouis]

In France the neutral and phase must disconnect when operating the MCB therefore dual pole, the air gap is required to be a minimum of 3mm, therefore the circuit is dead, in the UK the MCB's are only single pole and only disconnect the phase, so the neutral is always connected, therefore in effect still live, if you need to work on your french installation you only need to trip the MCB or the RCD protecting that row of maximum eight MCB's.

Disjoncteur divisionnaire = MCB
Interrupteur différentiel = RCD
Disjoncteur différentiel = RCBO

Also note that the first generation of UK RCBO's did not switch the neutral sufficiently to be approved in France, easily recognised as they had a blue flying lead out of the unit and only took up a single module space in the board.
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Just re-read that, usually in France the neutral and phase to the circuits come out of the bottom of the DD, power for them being by a comb from the ID across the top, what make of DD and ID's do you have in your board.

Sorry yes, i meant out of the bottom, the top is connected with the copper bar that links all MCB's to the RCD.

I'm still a little confused.

To my logic if there is another live circuit with an N going into the neutral bar then any wire in the neutral bar has current going through it - so even if you isolate the MCB it will only turn off the L of that circuit but the N would have current running through it back towards say the socket you were working on. Is this right?
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Basically what i'm asking is if we want to safely work on say a socket, do we have to disconnect its neutral and turn off its MCB or how does it work if the N is still charged from the 'live' shared N bar?

 

[Mike Johnson]

Using your logic, any appliance still plugged into the circuit will still work when the MCB is turned off, the circuit is not complete, it's very difficult to push water through a hose. LOL

 

[bjouis]

Using your logic, any appliance still plugged into the circuit will work when the MCB is turned off, the circuit is not complete.

Can you explain it to me then please? (Remember i'm a newcomer so am trying to understand things)

If the N's all carry current and they share a bar then why wouldn't a still-attached N cable on a socket still be live with just the MCB turned off?

View: https://youtu.be/At7AnMV7Xd0?t=527
On this vid i watched he says you're not isolating the circuit because the N is still connected.

So could you explain this to me in a 'for idiots' way?

 

[bjouis]

I guess i am misunderstanding the neutral wire and what it actually carries? I thought it carried the same voltage back to the N tail on the CU but seems i am wrong about this?

 

[snowhead]

The path for the current flowing in the Neutral returning to the Common bar is onwards through the main Neutral and back to the source that the Live comes from.

 

[davesparks]

To my logic if there is another live circuit with an N going into the neutral bar then any wire in the neutral bar has current going through it - so even if you isolate the MCB it will only turn off the L of that circuit but the N would have current running through it back towards say the socket you were working on. Is this right?

If you disconnect the live by opening the MCB then there will not be current flowing through that circuit's neutral as long as there are no borrowed neutral situations.
I think you may need to brush up on the fundamental principles of electricity before proceeding, current will only flow in a complete circuit, it can't flow through a broken circuit.

Single pole isolation is permitted when the neutral is reliably connected to earth at source as the neutral will be held at, or about, earth potential so no shock risk occurs.

 

[Deleted account]

The neutrals are not carrying current. It’s voltage potential.

with single pole breakers all the neutrals are connected to the neutral bar therefor if you turn any one breaker off. All the neutrals still have voltage even the isolated one.

 

[bjouis]

The neutrals are not carrying current. It’s voltage potential.

with single pole breakers all the neutrals are connected to the neutral bar therefor if you turn any one breaker off. All the neutrals still have voltage even the isolated one.

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 did a drawing:

So essentially why i'm confused is, if MCB 1 in the pic is forming a complete circuit, but we switch off MCB 2 thereby killing its live, isn't its neutral just in effect spliced into the circuit from MCB 1?

I'm guessing the neutral bar carries no actual voltage that can harm a human and that's why it's safe?

I get that you can just turn off the MCB and work on the socket safely but i'm trying to understand the actual principle as to why.
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I think you may need to brush up on the fundamental principles of electricity before proceeding, current will only flow in a complete circuit, it can't flow through a broken circuit.

That's what i'm in the process of and is the reason for this question

I'm going to college later this year in the UK but am trying to get a headstart by understanding the principles etc beforehand. I get how CU's work and about how power is carried to sockets/lights etc in the property/how stuff is wired up etc but i'm struggling to understand the actual electrical fundamentals behind it all. I think i'm missing a bit of info somewhere which when i find it out will make it all click for one of those lightbulb on moments.

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?

 

[pc1966]

There may well be historical differences with how France and the UK electrical systems evolved leading to the choice of single-pole or double-pole breakers.

In the UK the neutral is reliably close to true Earth to a couple of volts, and checking this "polarity" is correct is one of the first steps of any electrical inspection. As a result if you isolate the live (phase) conductor you are only going to see a couple of volts at most with respect to ground and that is safe.

As already mentioned, some RCBO do isolate both live and neutral, this is not common and is not essential from a safety point of view but it is a big advantage for fault finding as you can check neutral to neutral isolation and do insulation testing, etc without removing a cable.

Where the UK approach is hazardous is if the supply polarity is wrong, then we have no over-current protection in the event of a "live" (incorrect neutral voltage) to earth fault. If you look at the coding of faults for inspection in the UK that is one of the few that are listed as C1 "immediate danger" (same as exposed live conductors) that must be put right immediately (or put out of use).

 

[Deleted account]

Think along the lines of path of least resistance.

the neutral is connected straight back to ground/ earth.

are you more or less resistive than the the designed path to earth.

if you interrupt the outgoing neutral then you could become the path of least resistance.
just like the dropped neutral principal.

 

[Mike Johnson]

John Ward has a series of vids on U-Tube that will help.

 

[pc1966]

One other thing is in the UK a phase-only switch is acceptable for isolation by an electrically competent person, but not for the general public.

For general isolation where no testing would be conducted it must be all-pole (phase and neutral), so you will find that our main consumer unit switch is always double-pole, as are the switches for socket outlets are also double-pole.

 

[bjouis]

There may well be historical differences with how France and the UK electrical systems evolved leading to the choice of single-pole or double-pole breakers.

In the UK the neutral is reliably close to true Earth to a couple of volts, and checking this "polarity" is correct is one of the first steps of any electrical inspection. As a result if you isolate the live (phase) conductor you are only going to see a couple of volts at most with respect to ground and that is safe.

And i think this is where i've been confused. I assumed the N tail was just carrying the same voltage as the L. So basically the reason why the N doesn't electrocute us in my example drawing is that the neutral bar is earthed and so if i touch the wire it's going to still go through the bar to earth and not through me to earth.

Is that somewhere close to correct?

John Ward has a series of vids on U-Tube that will help.

It was actually one of his vids that confused me about this. Watched it late last night and he made a comment about it not being truly isolated as the N was still connected at the bus bar.

I am going to work my way through his vids.

 

[pc1966]

And i think this is where i've been confused. I assumed the N tail was just carrying the same voltage as the L. So basically the reason why the N doesn't electrocute us in my example drawing is that the neutral bar is earthed and so if i touch the wire it's going to still go through the bar to earth and not through me to earth.

Is that somewhere close to correct?

Basically yes, the neutral should always be at a safe near-Earth voltage.

I think France is always a TT supply (local earth rod) where as the majority of UK supplies are TN-S or TN-C-S supplied by armoured cable.

In our TN-C-S case the neutral is the earth conductor (known as PME "protective multiple earth" or CNE "combined neutral earth"). That leads to a whole lot of debate on this forum due to the implications under fault conditions, but you have plenty to get one with before worrying about that!
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Just search for "earthing-your-questions-answered.pdf" that will provide a bit of useful background (link to IET site seems to get messed up here).

 

[bjouis]

Thanks so much everyone for your patience and understanding. I think i understand it now. I no doubt will be back with more noob questions. Is there a forum section for 'idiotic questions' or similar? I have a feeling i will need to be posting in such a place quite a lot in the upcoming months.

 

[Mike Johnson]

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

 

[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!