How can you tell if your Earthing system is TN-C-S or TN-S

kwikfix328 · 19 May 2008

Going by pictures that I've seen in books, and on the .net I assumed that my Earthing arrangement was a TN-C-S one. However looking more closely at the pictures on this forums Electrical info pages, i see these two pictures below look superficially just the same, save for the internal connections that are not visible under the covers of a service head cutout.

TN-C-S

TN-S

What connection do you think i have here:

New meter and Isolation switch

kwikfix328
19 May 2008
3

Just a shot of my electrical supply cupboard. You can see i have had a new meter fitted along...

I'm only asking out of general interest, no work planned.

JohnD · 19 May 2008

If you have TNC-S there is supposed to be a durable label near the service head said saying "This system has Protective Multiple Earthing" (or similar words.

If you have quite an accurate multimter you could reasure T-N resistance, say, at a nearby plug, or any voltage between them. I can't remember typical figures but someone will have them.

RF Lighting · 19 May 2008

Thats quite a thick incommer, so it may be a split concentric cable, which would mean a TN-S supply.

It is impossible to say for sure without opening the cutout, which you really must not do, or by measuring N>E resistance as JohnD mentioned.

Lectrician · 19 May 2008

You also have economy seven metering, with no storage heater load by the looks of it. If this is the case, you want to consider if having economy seven is actually beneficial to you.

kwikfix328 · 19 May 2008

Two replies and two slightly different answers so far...

OK test meters that i have at my disposposal at the moment are a Robin Continuity/Isulation meter ( KMP3075DL ) and a Megger LT7 EFLI meter.

I do have my own DMM which is of reasonable quality, or i could borrow the Fluke from work tonight, although whenever I've taken my No brand ( rapid Electronics ) DMM into work to compare it to the fully calibrated Fluke i always get the same readings from boh machines. .

What should i be looking for, Remember this is just for personal info and interest, I'm not planning on doing any work, i just like to know this kinda stuff... its the engineer in me

.

kwikfix328 · 19 May 2008

Lectrician said:
You also have economy seven metering, with no storage heater load by the looks of it. If this is the case, you want to consider if having economy seven is actually beneficial to you.

I have taken my own meter readings roughly once per month for several years now and keep a spreadsheet record on my home PC.
I did the sums just prior to the DNO fitting an Isolator and new Meter a few weeks ago and am still about £100 per year better off by keeping Economy 7 .

I am a shift worker , mostly working night shift, so still use more than the average Joe during the night when most sensible people are tucked up i bed...

Lectrician · 19 May 2008

Test between neutral and earth as close to the origin as possible. Look for low ohms. If the reading jumps around, even with your prongs fully pushed hard onto the terminals, it is likely TN-S (the neutral current effects the reading). For TN-C-S I would expect a nice low steady reading (with your CU switched off). Something like 0.05 or below.

You cannot effectivly get onto the earth too easily in the meter cabinet by the looks of it, but the neutral block is handy. You could just stick the end of the sharp meter probs through the earths sheath somewhere

kwikfix328 · 21 May 2008

Lectrician said:
Test between neutral and earth as close to the origin as possible. Look for low ohms. If the reading jumps around, even with your prongs fully pushed hard onto the terminals, it is likely TN-S (the neutral current effects the reading). For TN-C-S I would expect a nice low steady reading (with your CU switched off). Something like 0.05 or below.

You cannot effectivly get onto the earth too easily in the meter cabinet by the looks of it, but the neutral block is handy. You could just stick the end of the sharp meter probs through the earths sheath somewhere

With ref to my photo posted above.
I checked between the black neutral block and earth wire, i used a sharp probe to puncture the insulation on the G/Y cable right next to the Service head, ( this is something that i don't normally do and never do at work). Using a Low Ohm continuity meter & test leads nulled to zero,
I got a steady reading of 0.00 ohms with the CU switched of at the CU Isolator switch, so i guess I have a TN-C-S earth system.

Gonna have another read ( OSG, etc) and make sure i have all of the recommended mandatory labels in all the right places and then that should be me finished.

Thanks for the advice.

Lectrician · 21 May 2008

Sounds like PME (TN-C-S).

The label thats states PME would be provided and installed by the DNO, but they often do not fit them, more so recently.

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How can you tell if your Earthing system is TN-C-S or TN-S

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