Question About TN-C-S Earthing

[Gigagator]

Hi all,

First off, I am just a DIYer who only ever does basic work like swapping switches, sockets and light fittings etc.
Not an electrician and am unlikely to train as one (although I came close to doing an apprenticeship when I left school, but I ended up moving into IT instead).
My question is purely out of curiosity.

My understanding is that in a TN-C-S earthing arrangement, the earth for the property is directly bonded to the incoming neutral in the supply/cutout.

I have been told (though never tested the theory) that because we use AC, the neutral conductor will give you exactly the same shock that the live conductor will if the circuit isn't isolated.
I've had plenty of belts off live conductors in my time, but I don't recall ever accidentally touching a neutral to be able to prove or disprove what I've been told.

However, if that is the case, why or how is it one does not get an electric shock from touching the earth or a CPC somewhere in the property, if it bonded to the neutral?

I am likely being daft here, or I am complete wrong about the neutral, but I would appreciate an explanation nonetheless.

Many thanks

Gig

 

[EFLImpudence]

My understanding is that in a TN-C-S earthing arrangement, the earth for the property is directly bonded to the incoming neutral in the supply/cutout.

Yes, if by bonded you mean electrically joined.

I have been told (though never tested the theory) that because we use AC, the neutral conductor will give you exactly the same shock that the live conductor will if the circuit isn't isolated.

It depends what else you touch.
Electric shocks are the result of a potential ( voltage) difference. Two parts at 230V with reference to earth (if you are not touching earth) will not give you a shock.

I've had plenty of belts off live conductors in my time, but I don't recall ever accidentally touching a neutral to be able to prove or disprove what I've been told.

No, because it is joined to earth.

However, if that is the case, why or how is it one does not get an electric shock from touching the earth or a CPC somewhere in the property, if it bonded to the neutral?

You will if you also touch line(live) .


Think birds on power lines.

 

[Harry Bloomfield]

I am likely being daft here, or I am complete wrong about the neutral, but I would appreciate an explanation nonetheless.

The neutral is normally [1] at the same potential, within a few volts, as the earth, so no you should not get a shock touching both neutral and earth. [1] Should the neutral become disconnected, then it can become live, if any appliance or light etc. happens to be turned on.

 

[ericmark]

There are problems with TN-C-S this and this go into the problems.

But in the main it is where TN-C-S is used outdoors. "PEN conductors serve as both a protective earthing and neutral conductor and are used on the PME (TN-C-S) network, which was introduced as an alternative to TN-S and TT in the 1970s." I think now the move is regretted, but a bit late now.

 

[flameport]

I have been told (though never tested the theory) that because we use AC, the neutral conductor will give you exactly the same shock that the live conductor will if the circuit isn't isolated.

Incorrect.

Neutral is the conductor which is connected to Earth at the transformer and usually several other places.
It's at the same voltage / potential as Earth, so it is not possible to get a shock from it.
Earth in this case meaning the mud and dirt ball that we live on.

Line is the conductor which is not connected to Earth, and has a voltage of about 230V relative to the neutral and Earth.

AC does not mean swapping conductors or that each one becomes live for a part of the time.
Neutral is always the same voltage as Earth because they are connected together.
Line changes between +325 volts and -325 volts relative to neutral and Earth. The average value is 230V.

 

[Adam_151]

It's at the same voltage / potential as Earth, so it is not possible to get a shock from it.

Just to add to what flameport has added, in normal conditions, its at the same potential as earth within a small margin, close enough for most installations, one has to think carefully about it if wiring milking palours or swimming pools, and there are some places its not allowed such as caravan park hookups and petrol stations.

If the PEN conductor (which is where it is both neutral and earth) becomes disconnected, eveyrthing downstream will rise up to a voltage that could be hazardous, this is regarded as a very bad thing to happen, there are also some other weird faults that can cause voltages between the physical earth and the PME earthing terminal. We are allowed to use the TNCS earth on EV chargers, only if we install a monitoring device that disconnects everthing in the event of anything nasty happening, but I have my doubts about some of them being able to detect all faults (Particually an issue I went to a few years back that seemed to really confuse the DNO chaps!). We never worry about large lumps of metalwork on TNCS when installing lighting columns though!

 

[Harry Bloomfield]

AC does not mean swapping conductors or that each one becomes live for a part of the time.
Neutral is always the same voltage as Earth because they are connected together.
Line changes between +325 volts and -325 volts relative to neutral and Earth. The average value is 230V.

Under normal conditions, but there are circumstances where it might not be..