Overhead TN-C-S Supply and Incoming Main Power Query


Postby collectors » Wed Apr 22, 2015 3:47 pm

Hi, i live in a rural area with an overhead supply classed as a TN-C-S supply. Now at the main intake looks to be wired the same as a PME system.
My question is, all boards are fitted with RCD's but when i cut a cable with the fuse for that circuit off but the main RCD on, it wont trip the RCD. IE:N to Earth
Now! i know the RCD is working as i picked up the damp plug to the cement mixer one day & got a mild shock & the RCD tripped. (not the best way to find out)
So is it the style of a TN-C-S supply that makes it not trip between N & earth?.
Also, all gas & water supply's are in plastic, so no other earth's.

Thanks

Chris
collectors
Rank: Foreman
Progress to next rank:
26%
Posts: 278
Joined: Mon Dec 01, 2008 7:26 am

Sponsor

Simply Build It

Postby ericmark » Wed Apr 22, 2015 7:10 pm

Oh I wish I could post pictures but this site is a pain for that. Anyway PME an TN-C-S are in most cases the same.

In the main the earth and neutral are the same wire until it reaches your house, Then they are split. A earth neutral fault needs 30 mA or more to point 15 ~ 30 mA to flow to earth to make it trip.

It is simple ohms law the main bit being the voltage. If there is zero voltage between earth and neutral then even a zero ohms short circuit will not cause any current to flow. There will be a resistance in the cables from the PME head to your consumer unit so a dead short will trip the RCD but much depends on current being drawn.

This is a real problem let me consider a real event and describe what happens.

A current has dislodged itself from a bun in the toaster. The current has landed between neutral and earth on the element and although not being used the switches only switch the line the neutral is still connected. The voltage between neutral and earth is zero so nothing trips.

The kettle is now used and the 3kW load means now there is a difference between neutral and earth. As a result enough current flows through the current to cause the RCD to trip.

To a casual observer it would seem the kettle is the problem where really it is the toaster causing the problem.

This is quite common and it has resulted in many a high power item being blamed when really the fault is in a low powered item.

With a RCD we really want all item to switch both lives. Neutral is considered as a live. Even as an electrician remembering the neutral is live and to call the phase wire line is hard.

The RCD is also called a ELCB-c or an Earth Leakage Circuit Breaker - Current. Before the ELCB-c we had the ELCB-v now outlawed which measured voltage instead of current. To work it measured the difference between house earth voltage and real earth voltage. The problem was if anything bonded the house earth to real earth it stopped working. As a result they were banned and the ELCB-c was used instead known also as a RCD.

At the end of the day what we want is for it to trip before we get a shock. And in the main it does that. OK where one touches a line wire it may take 40 ms before it disconnects but in the main that does not produce fatalities it may give us a wake up call to be more careful but does not kill us.

Clearly we can get far in excess of 30 mA but the time should not be more than 40 ms. In the main that means we survive. It does not mean we do not get a shock.
ericmark
Rank: Project Manager
Posts: 1737
Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.

Postby collectors » Thu Apr 23, 2015 8:04 am

That was a great analogy of the situation explained well. It did through me for a second about the toaster current & that it could of been a raisin. LOL.
Have tried to attach an image .

Chris

[attachment=0]DSCF4857.JPG[/attachment]
Attachments
DSCF4857.JPG
collectors
Rank: Foreman
Progress to next rank:
26%
Posts: 278
Joined: Mon Dec 01, 2008 7:26 am

Postby ericmark » Thu Apr 23, 2015 11:58 pm

OK maybe I should have used a raisin but could not resist having a current allow current to flow through the current!

But the earth is connected to neutral in the head so power must be drawn before there is any voltage differential to force amps to flow.
ericmark
Rank: Project Manager
Posts: 1737
Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.

Display posts from previous
Sort by
Order by



  • DIY How to Project Guides

  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!



 


  • Related Topics