TT or TNS

[Clitheroe]

I know you get many questions on earthing systems, but I would like some comments/suggestions on the following.

My house electrical installation has always relied on a TT system with an earth electrode. Wylex consumer unit with 30ma rcd for all sockets, shed and outdoor lights and 100ma Time delay rcd for lights, cooker and water heater (consumer unit installed summer before last). This I understand is satisfactory.
What I have always wondered however is my supply is overhead with what looks like an earth cable on the poles supplying my home (TN-S).
The installation inside the house as far as I can remember has never been connected to this earth at the cutout, there is no earth terminal in sight on the cutout which has what looks like an armoured cable (installed 1990) to replace the three old fabric cables used previously.
I would like to know why anybody might think the supply is how it is.
I personally think it could be because the line supplying the house (only my house) is about a quarter of a mile long and/or if I remember rightly when the armoured cable was installed prior to me gaining my electrical knowledge the chap putting it in said 'There's an earth in the supply cable' but my father went and said 'umm.. I use an earth rod', perhaps giving the chap the impression that he wanted to continue using a sh***y earth rod instead!. The overhead supply was installed I've been told around 1952.

 

[andy]

overhead lines do not have an earth. if they have 4 cables there L1, L2, L3 and neutral. if they have 5 cables there L1, L2, L3, switched live for street lights and neutral. almost all overhead lines are TT, altho some can be TNCS. you wont get TNS from an overhead due to the lack of seperate earth

 

[Lectrician]

Is it only you served by the network run??

How many cables on the pole?? just 1?? Could be a split concentric from a pole tranny which could indeed supply TN-S. This is seen alot in the sticks and on farms etc.

If you cant get TN-S, there is a good chance you could still get TN-S-C.

 

[ban-all-sheds]

Clitheroe - you say "what looks like an earth cable on the poles supplying my home" - why? What does it look like, and why do you think it's an earth?

Many overhead TT supplies have been converted to TN-C-S - maybe what you can see is one of the PME connections that have to be added to TN-C-S to make it averagely safe...

 

[Spark123]

Are the cables on the overhead lines arranged horizontally or vertically? Is there a transformer at the end of the line?

 

[mapj1]

Hmm.
It is not unknown for the lowest wire, supposed to be the earthed neutral, to be spiked to earth at the foot of the wooden poles. This doesnt mean it is a separate earth though.
It is how the company get their 20 ohms or less to ground that you should add to the 3 terminal measurement of your earth rod impedance, when estimating worst case fult impedance. (Or better, if the power is on then measure round loop, which is your rod plus theirs anyway, and eliminate that uncertainty.)
I'd agree that the 4th wire doesn't normally mean L1/L2/L3 +N+ E, but just that there is something odd, like an extra time switched line derived from one of the phases.
Note also that although N is supposed to be the bottom wire, it sometimes isn't, and also that although all PEN connections supplying PME installations should be crimped thare are still some green and furry bolted joints up there from years back.

 

[andy]

Note also that although N is supposed to be the bottom wire, it sometimes isn't, and also that although all PEN connections supplying PME installations should be crimped thare are still some green and furry bolted joints up there from years back.

the switch wire is usually above the neutral and is normally thinner than the other lines

 

[Lectrician]

3 cores on a pole is usually split phase, and often gets confused by DIY types for L N E.

 

[Spark123]

Are the cables on the overhead lines arranged horizontally or vertically? Is there a transformer at the end of the line?

I mean is it a 400/230v (normally vertical) line or 3.3kv+ (normally horizontal) line? At least that is how it works up here.

 

[Lectrician]

2 or 3 horizontal (and higher) can be 11, 33 or 66 Kv. 132 and above are usually on 'pylons' not poles.

HV are also distinguishable by their insulators on the T bar.

 

[Clitheroe]

Sorry, perhaps I didn't explain it clear enough as I was in a rush when posting in this topic.

The line comes from a transformer which has three wires coming from it which supply two houses fairly close to the transformer pole and my house quite a distance away. The poles supplying my house have three wires on them andwhen I said that my house is quite distance away there are about six poles altogether serving only my house. The bottom wire is thinner than the other two an is earthed on the first four poles via an earth electrode.
There are no street lights whatsoever on any of the poles as they are all (including the transformer pole) in a big field area.

One of the other houses supplied looks like it has not had it's supply cable changed to the armoured type and is supplied with three seperate wires.

One thing I can say is I am fairly convinced that this is not a PME network.
So with two wires of the same thickness and one thinner looking wire earthed at different points is the reason why I thought it could be TNS.
At the transformer pole there looks like there is a wire link between the bottom thinner wire and the middle wire perhaps to earth the neutral, although probably not a PME network.
The three wires are top, middle and bottom, possibly Phase top, Neutral middle and Earth bottom.

Could you tell me if this may help for you to answer my topic.

 

[Lectrician]

Sounds like split phase. The neutral at the bottom, which will be earthed down at the poles.

Can you take a pic of the transformer pole, and the pole supporting the drop to your house??

 

[Clitheroe]

Sorry, I am unable to post any pictures in as I dont have my own computer or any camera, I am only using a computer in the public library which you cannot load your own software onto.

Anyway, The transformer is supplied by two cables on an overhead line horizontally with grey insulators. From the transformer then goes three cables vertically to a pole, and then an armoured cable to one house. There is also another set of three cables from the transformer pole to another pole which supplies another house with three seperate cables but from the same pole are also the other three cables that go onto the poles supplying my house, 1 top cable and a middle and bottom cable, middle and bottom cable closer to each other.

At the pole at the back of my house where the incoming armoured cable comes off, if I look carefully up at it with binoculars, I can see a red cable from the armoured cable connected two the top cable on the pole, a black cable two the middle cable and what looks like armoured sheath to the bottom cable.

Does this help in any way?

 

[Lectrician]

Well, It does sound as if you do have TN-S, with the service drop to your house being a split concentric cable.

Hark at me - post above I reckoned split phase

This set up was used for farms in the 60's, but isn't seen at all now. The earth could drop off, and you would never know!

Umm - You are currently TT - possibly an alteration as the TN-S was no longer satisfactory.

Possibly a call to the lecky board is in order.

 

[ban-all-sheds]

Umm - You are currently TT - possibly an alteration as the TN-S was no longer satisfactory.

From what he said it's the other way round - he's got a TT supply, but that's because 50 years ago when it was converted to TN-C-S his Dad didn't take it up...