Tn-S Or Tn-C-S

[Wizbit]

Wonder if anyone can help?... Doing a pfc test on a domestic supply and getting readings of 1.88kA L-N, and 0.86kA L-E.  This surely suggests it is a TN-S system?...and yet the last PIR on the property states it is a TN-C-S supply. The reason for confusion is that the main incoming cable is a lead sheath...the Main Earthing cable exits the Main Cut-Out at a MET block, so at first glance it looks like a typical TN-C-S supply...BUT, there is also an older green earthing cable leaving the bottom of the Main Cut-Out going to a clamp on the lead sheath of the Incoming cable. (NOT a BS951, but an older style clamp?)

Photo attached...I hope!

My thinking is that it USED to be a TN-S supply and was changed somehow to TN-C-S, and that cable to the lead sheath is simply acting as a Bonding..BUT...why would I be getting such a difference in the two pfc readings if it IS now a TN-C-S supply?....any thoughts out there gratefully received! :C

 

[NozSpark]

Just because the head has an earth block on the side of it does not mean that it is TNC-S, the earth wire from the sheath just passes through the head on its way to the block. .

I suggest that the previous person who stated that it's TNC-S is a numptie and almost certainly didn't carry out the PIR correctly

 

[Technician]

Could you be getting the earthing conductor and bonding conductors mixed up?

 

[SLIPSHOD & SLAPDASH]

Those readings & the pic tell me its a TNS supply. I have seen many of those 'PME type'  heads used on TNS supplies when the old cast ones were changed, it seems they fitted anything to hand. They were also a bit better then labelling up the supply PME if it was.

 

[Wizbit]

Many thanks NozSpark..just what I thought.. & I agree with the numptie notion..some of the other things on the last PIR from just 3 years ago are scary...even to my newbie eyes...I wonder if he even came into the house!...hmm, and he was a registered spark with an organisation that vaguely rhymes with 'rabbit'...ho hum...

Thanks SLIPSHOD..that's good to know..

Something else on this installation..one first glance there appears to be no Main Bonding coming into the CU, and certainly can't trace anything visually on the Gas main. But...the water pipes into the house are all Lead , and I have found a bare (painted) 4mm wire clamped to one of the lead water mains in an outside loo (same clamp type as in my pic of the Cut Out) which then disappears into the kitchen wall!.. could this be an old earthing to the water mains?...or could this be some kind of older Bonding? it could possibly be linked into the house Lighting circuit from the water pipe. Could this make sense in any way? !

 

[kerching]

GPO were fond of getting an earth to the house phone system by running a wire through the WC overflow and clamping it to the water pipe

Bloke around here thinks the CS in TNCS stands for Cable Sheath

neither of the above points will probably have no relevance to your situation......

just saying

 

[ProDave]

Just to a continuity measurement from the N incomer to the CU, to the E terminal in the head. That will tell you if it's TNS or TNCS

 

[Wizbit]

Thanks ProDave...So a TNCS should read continuity between N incomcer in CU and the head E terminal yes?...But in a TNS won't you still get continuity as they are connected at the transformer end?..and if so is it just then a case of tiny reading (TNCS) versus big reading (TNS)??...or have I lost the plot?!

 

[ProDave]

That's about it.

you can't see what goes on inside that head. you can't see if the N terminal and E terminal are one solid block of brass, or two separate terminals.

So a simple continuity check should tell you.

Pretty dam close to zero hms and it must be TNCS, a little higher and it's likely to be TNS

 

[Evans Electric]

Just a couple of extra points , Wizzer,  the bare earth wire on the lead pipe could well have been the original main earth .  That head was probably just L&N    nothing on the lead sheath at all .  Supply authorities  ( well this area anyway which was MEB)   were not obliged to supply an earth connection to domestic premises......commercial yes .  

    So houses had lighting & maybe some 2 pin (round) X 2A  and 2pin X 5A  sockets to start with  then if a 15A X 3pin  was fitted, the beare earth wire went to a lead water pipe . 

The other point is , those heads you show in pic often had a connection hidden inside from the sheath to that earth terminal , BUT THE CONNECTION WAS NOT ALWAYS MADE.    Back in the day when hardly anyone tested anything , many blokes assumed that was a usable earth terminal when it was ****** all really.

History lesson finished till next week .  

 

[Nicky Tesla]

i have taken the heads apart to check n basically be nosey before.   If the fuse is pulled out there is a screw terminal behind  it that holds part of the front plastic on.  If that is removed it reveals the earth and neutral connection leaving the live covered.

But dont go doing that cos your not a meter maid or DNO sparky,  H&S and all that.

 

[Wizbit]

Thanks evans electric...you learn something every day!.. That would well make sense in that house...i discovered a radial socket circuit that used to be an old cooker circuit. The cabe was twin only, so when it was changed to a socket, someone ran a 4mm earth cable all around the kitchen and clamped onto the incoming Lead Water Pipe..I'd always wondered about that!..

keep the history lessons going!