E&N Wired Incorrectly - Socket Outlet

[steve100]

Last week I came across a double socket that had the neutral wired into the earth terminal and the the earth wired in the neutral terminal. I understand the concept of why it would still work, however because the return current from that particular load plugged into that socket would flow back through the cpc, would this make the cpc live throughout the ring circuit? And thus any exposed conductive parts would in affect be live while the load was operating e.g. Metal sockets? It was a TNCS system and they had no RCD's fitted. I understand that because it is a TNCS system at the intake it then becomes a combined E&N protective conductor. But that would be at the intake and in the installation it is slit hence my questions.

 

[Sintra]

No it wouldn't make the CPC live throughout the circuit as it would be a continuous path back to the point were it joins the neutral anyway. However if the CPC was broken before it reached the cu then yes it is possible that metal sockets etc could become live.

 

[steve100]

Thanks, So the current would still flow from the load in the cpc back to the cu and PEN point?

 

[Sintra]

Correct :lol:

 

[johnboy6083]

Even on a TNS system it would still operate, as the neutral is earthed at the star point of the supply transformer.

 

[steve100]

Thanks, so if I was to touch a metal socket outlet then I wouldn't get a shock even though the cpc would be carrying current? Sorry for what may seem a silly question.

 

[Sintra]

Providing the cables where intact back to the source then you would not get a shock.

Try drawing out the circuit with the N-E in the correct terminals then do the same with them reversed.

 

[steve100]

Thnx, I can see how this would work if all the sockets where wired incorrectly. But want I can not seem to get my head around is if only one socket was wired incorrectly, then why wouldn't you get a sock from the socket plate. So, if the first socket in the ring had the cpc connected in the N terminal, and the next socket the cpc was in the E terminal, then the current would flow from the load out of the first sockets N terminal along the cpc to the next socket where the current would then flow through the E terminal to the next socket and so on.... If you have the socket plate earthed at the seconds socket from the socket earth terminal, where the cpc is connect correctly, then wouldn't the current also be present in the conductor connecting the metal face plate to the cpc, so if I touch the metal face plate and created a potential difference, wouldn't I get a shock? Or an I missing something obvious?

 

[Marvo]

In an ideal world the resistance between the CPC of your socket circuit and the actual earth is 0 ohms, in other words the CPC of your socket circuit will always be at the same voltage as earth because it's connected directly to earth.

Obviously in real life there's actually a slight resistance between the CPC and earth so when a current flows in the CPC it assumes a slight voltage with respect to earth. The larger the current flowing in the CPC the higher voltage it becomes wrt earth. This voltage that developes on the CPC is called the 'touch voltage'. So called because it's the voltage that causes anyone who's touching an appliance chassis to get a shock.

The whole CPC, earthing and bonding design of an electrical installation is designed around keeping this touch voltage within safe limits even when a massive fault current of thousands of Amps flows through the CPC momentarily when there's a L-E short circuit fault.

 

[steve100]

Okay thnx, just so that I understand you correctly, what you are saying is that...... Even though the socket has been wired incorrectly meaning that the return path for current will travel in the cpc from the load (this wouldn't be a fault current) and as such the metal socket plate would have current flowing through it (because it is connected to the cpc), that you wouldn't receive a shock?

 

[Marvo]

The CPC is connected through the CU directly to an earth.

If a circuit is wired to use the CPC as the return path then current will flow through the CPC but the CPC will remain at 0volts (or pretty close to 0V). The socket covers are connected to the CPC so they would stay at 0V as well and you wouldn't get a shock if you touched them.

Just because current is flowing through a wire it doesn't necessarily mean you would get a shock from that wire. The thing that causes you to get a shock is touching a wire that is at a higher voltage than earth. If you touch a wire or component that's at an elevated voltage and your feet are on the ground (earth) then this causes current to flow through you.......which is by definition you receiving an electric shock.