Why Do We Not Get A Shock From The Neutral Conductor?

[andrewedwardjudd]

Hi I was having a go at explaining grounding and after some discussion and a few arguments I came up against some problems because the internet was providing what appeared to be a totally wrong conceptualisation of the nature of our supplied power supply.

Ie many professional electricians are talking about an electrical flow to ground or neutral. That must be conceptually wrong since we are talking about alternating current?

As far as I can see we do not get a shock from a properly wired neutral or earth because the surface area of the so called live conductors exposed to air is very small compared to the very much larger surface area of the so called neutral/earth bonded conductors that is exposed to the air?

So in an unusual situation where the neutral was not connected to earth at your location, and the property was some distance to any other grounded neutral or it was unusually dry, and there was a large appliance with a live unpainted chassis or unbonded live sink that was near you, it seems to me you would get a shock if you touched neutral?

I am right or am I talking rubbish?

Cheers

Andrew

 

[ProDave]

Current should flow from L to N. Yes being ac, it will reverse direction 50 times a second, but convention says it flows from L to N which is all you need to understand to make things work.

In a correct installation, no current flows from anywhere to earth. Earth is just there to protect you when things go wrong.

Neutral is grounded at your substation so it will only ever be a few volts above earth potential even if you are a long way from the substation. That's why you won't normally get a shock from it. It's nothing to do with exposed areas I don't know what you were talking about with that bit.

 

[ParkySparky]

If neutral on a circuit is open the it will become 'live'

 

[andrewedwardjudd]

Current should flow from L to N. Yes being ac, it will reverse direction 50 times a second, but convention says it flows from L to N which is all you need to understand to make things work.

In a correct installation, no current flows from anywhere to earth. Earth is just there to protect you when things go wrong.

Neutral is grounded at your substation so it will only ever be a few volts above earth potential even if you are a long way from the substation. That's why you won't normally get a shock from it. It's nothing to do with exposed areas I don't know what you were talking about with that bit.

Thanks for the reply.

As you agree, in reality the current is not flowing from L to N, and it is reversing, so if a person talks about current flowing from live to neutral it is not going to help another persons understanding of a technical issue which needs a knowledge of actual processes rather than only a convention which simplifies for those not needing a more in depth knowledge.

My question is related to getting a shock when touching a so called live conductor when we are isolated from any obvious contact with the neutral other than via the air.

As you mention it is standard practice that at frequent points around the system the neutral is connected to the ground where it is important this connection is sufficiently deep that it is connected to a moist earth.

If we consider a very isolated transformer and a very long cable from that to our supplied location which has no earthing at the supplied location then the neutral and the live are tending to become electrically identical. Neither wire can produce a shock unless current flows to the other wire

Once the earth is connected to the neutral at the supplied location, something must therefore change.

 

[binky]

you can touch live if suitably insulated - rubber boots, dry wood floors etc, etc. Seen a few sparks do it to scare apprentices, but really goes against the grain with me!

 

[ProDave]

Okay if we are going to be pedantic, with neutral bonded to earth, the live will alternate between +325V and -325V 50 times a second. But neutral will remain at or very close to earth.

So touching L will give you a shock, touching N won't normally.

but you only get a shock touching L because some part of your body has a path to earth, through your shoes for example, or any other part of your body touching something.

IF you are stood on a perfectly insulated rubber mat, then in fact you won't get a shock (DON'T TRY THAT AT HOME)

If neutral were not bonded to earth somewhere, then you could get a shock from that, but the system we have in the UK always references neutral to earth somewhere.

 

[Evans Electric]

What Dave says . N is earthed so you are not forming a circuit . Touching N and L ...you will be forming a cicuit , a shock will be experienced , quickly followed by death . Or as Parkysparky says , disconnecting the N, then touching it , you will be forming a circuit again and risking instant death .

DO NOT TRY THIS AT HOME !!!

I think , Andrew , its about potential difference . There isn't one between N and E normally so you feel nothing or put both ends of a test lamp on a L conductor , it doesn't light up , although its live theres no potential difference . Move one probe to another phase and of course it lights up , theres a PD .

However , stick around and a Forumite with a fully functioning brain will probably explain it better.

 

[andrewedwardjudd]

What Dave says . N is earthed so you are not forming a circuit . Touching N and L ...you will be forming a cicuit , a shock will be experienced , quickly followed by death . Or as Parkysparky says , disconnecting the N, then touching it , you will be forming a circuit again and risking instant death .

DO NOT TRY THIS AT HOME !!!

I think , Andrew , its about potential difference . There isn't one between N and E normally so you feel nothing or put both ends of a test lamp on a L conductor , it doesn't light up , although its live theres no potential difference . Move one probe to another phase and of course it lights up , theres a PD .

However , stick around and a Forumite with a fully functioning brain will probably explain it better.

As far as i can see so far we are all agreeing apart from me believing the circuit with live was created via the moist air.

Where i am confused/lacking knowledge is the nature of neutral when connected to earth. My understanding is that neutral/earth wires have no potential difference meaning without reference to the other wires associated with it, and since we have an alternating current there is no fundamental difference between the neutral and the live.

Ie it is my understanding that if you connect a high current device to the ground it is the ground that changes rather than the high current device changing.

Therefore since your house changes in equal amounts with respect to neutral and they are bonded together. there can never be a significant potential difference with respect to live which is isolated from the house.

Ie you touch neutral and you can only connect to neutral. But if you touch neutral there are significant pathways back to neutral because neutral is connected to the house.

 

[andrewedwardjudd]

As far as i can see so far we are all agreeing apart from me believing the circuit with live was created via the moist air.

Where i am confused/lacking knowledge is the nature of neutral when connected to earth. My understanding is that neutral/earth wires have no potential difference meaning without reference to the other wires associated with it, and since we have an alternating current there is no fundamental difference between the neutral and the live.

Ie it is my understanding that if you connect a high current device to the ground it is the ground that changes rather than the high current device changing.

Therefore since your house changes in equal amounts with respect to neutral and they are bonded together. there can never be a significant potential difference with respect to live which is isolated from the house.

Ie you touch neutral and you can only connect to neutral. But if you touch neutral there are significant pathways back to neutral because neutral is connected to the house.

Sorry i messed up those last paragraphs mixing live and neutral :-(

It should be:

Therefore since your house changes in equal amounts with respect to neutral and they are bonded together. there can never be a significant potential difference experienced, because live is isolated from the house.

Ie you touch neutral and you can only connect to neutral. But if you touch live there are significant pathways back to neutral because neutral is connected to the house.

 

[Manator]

I think you are getting a little confused, or you are confusing us. Under normal operation the neutral will carry the same voltage as the line, if disconnected from a single circuit only, the neutral will not return any voltage. Some 20 years ago I got a blast from a distribution neutral that was still connected at the sub main, the explosion destroyed the 3 phase board, left me blind for some 20 minutes or so, and left me very wary of the neutral conductor.

 

[Evans Electric]

I can't really grasp the moist air and the "House" references Andrew, we can't tell from your profile if you are an Apprentice /trainee sparks or what .

Go back to basics , forget about moist air and stuff .

Look at the PD aspect . The PD between N and E is usually 0 (zero) so if you touch it nothing happens . Like a bird sitting on a 33,000 V national grid cable . Zero volts pass through your body.

The PD between L and E is 240 , so touch it and 240 of the said volts pass through your body . Its all about the difference in potential.

 

[andrewedwardjudd]

I think you are getting a little confused, or you are confusing us. Under normal operation the neutral will carry the same voltage as the line, if disconnected from a single circuit only, the neutral will not return any voltage. Some 20 years ago I got a blast from a distribution neutral that was still connected at the sub main, the explosion destroyed the 3 phase board, left me blind for some 20 minutes or so, and left me very wary of the neutral conductor.

I am not sure what you are saying. You seem to be mixing up voltage with current or you are using some kind of trade talk i am unfamiliar with?

 

[Qui-Gon Jinn]

andrewedwardjudd,

We are a UK forum, just wondering why you are posting from an IP registered in Finland, with your location as Wellington New Zealand?

Perhaps if English is not your native language this could be where the confusion comes from?

 

[andrewedwardjudd]

andrewedwardjudd,

We are a UK forum, just wondering why you are posting from an IP registered in Finland, with your location as Wellington New Zealand?

Perhaps if English is not your native language this could be where the confusion comes from?

I have homes in NZ and Finland and it seemed easier to say i was in NZ

I am from the UK. My mother and father spoke only English and I hardly speak any Finnish

I have no formal electrical training and I am asking these questions in the dummies section of this forum

As a point of interest my 1960's Finnish home has no earth connections and all of the electrical sockets are two wire only with just a two inch link connecting the earth and neutral together at the outlet/socket only. The metal of the new cooker has been recently connected to neutral by a qualified electrician. The supply is three phase. The system is totally legal.

Many of the answers here are emphasing a difference between earth and neutral but effectively they are often the same thing anyway where as i recall the earth bar in the UK consumer unit was directly connected to the metal external cladding of the single phase supply, where the metal cladding is the neutral.

My question is really about the nature of the thing we call neutral

 

[ProDave]

Many of the answers here are emphasing a difference between earth and neutral but effectively they are often the same thing anyway where as i recall the earth bar in the UK consumer unit was directly connected to the metal external cladding of the single phase supply, where the metal cladding is the neutral.

My question is really about the nature of the thing we call neutral

That's wrong.

It's common for the earth bar in a metal clad CU to be bolted directly to the metal case. but the Neutral bar is insulated from the case. In a UK install, earth and neutral are NEVER linked together in the installation. Not on the customers side anyway. The ONLY place neutral and earth are linked is at the supply source, usually your nearest substation transformer.

 

[revjames]

In NZ the earth and neutral bars are linked inside the CU and the supply authority do not give you an earth you have to bang a rod in. So technically not TNC-S, TNS or even TT.

 

[andrewedwardjudd]

In NZ the earth and neutral bars are linked inside the CU and the supply authority do not give you an earth you have to bang a rod in. So technically not TNC-S, TNS or even TT.

haha!

It seems my faulty memory and experiences in strange countries is creating a heap of confusion for me and most of the people i encounter!

Thanks to all who are helping me come to grips with this so far

 

[Sidewinder]

TN-C-S is not allowed within the consumers installation in the UK without specific "special" approval.

This could be one source of confusion.

Not many sparks in the UK will comprehend the idea of what it seems you are proposing, plus, it seems from the way you are describing it that it is not acceptable within UK installs.

Being a UK based forum then most members can only comment on UK centric regulations & requirements.

 

[andrewedwardjudd]

That's wrong.

It's common for the earth bar in a metal clad CU to be bolted directly to the metal case. but the Neutral bar is insulated from the case. In a UK install, earth and neutral are NEVER linked together in the installation. Not on the customers side anyway. The ONLY place neutral and earth are linked is at the supply source, usually your nearest substation transformer.

When you said this i was somewhat taken aback that my memory could be so faulty - even though it is 20 years since I was in the UK.

I was not however talking about the earth being connected to the metal cladding of the consumer unit but rather the earth being connected to the external metal sheath of the incoming supply, where as i recall the supply was a single wire with a metal sheath. Ie effectively the neutral and earth are the same thing.

My London house was very old and might have had original cable from who knows when, but it appears the kind of system i described which appears to be known as PME or TN C S is not uncommon in the UK?

That particular UK system and the NZ system are electrically very similar if my understand is so far correct.

 

[revjames]

That particular UK system and the NZ system are electrically very similar if my understand is so far correct.

errr no - TNCS has a linked neutral in the service head. In NZ the link is within the consumer unit and the earth is provided with a rod. In NZ you are also not allowed to break the neutral, so no DP switches on anything either.