House Earthing TN-S or TT....

[MartinJ]

....TN-S appears outside, butt TT employed inside.

Good Evening Everybody (This is my first post)

Overview:
The supply to my parent's cottage consist of an overhead line of three individual cables. Originally, the supply drop from the final pole to the house also consisted of three cables (remnants of which remain in a seen in an attached photo).
In 1990, the three individual cables from the final pole into the house was replaced by a Split-Concentric cable. Looking at the cutout in the photo, there is a small piece of plastic which 'wobbles' (I know you shouldn't) over where (when I've seen other cutouts the same style) an Earth block is located, whether this cover is meant to simply meant to slide off or not I don't know, but the Consumer Unit is now in the way.
Prior to the new cable being installed the cutout was much further to the left, with the meter to the left of it with the tails from the meter running to a BS3036 CU.
Now, the old cutout was directly below the 3036 CU and after the current incoming cable and cutout (moved much further to the right) was installed I remember an Earth cable (probably 6mm2) dangling from the 3036 CU connected to nothing-although I'm sure there was a 'pinch' mark on it, therefore suggesting it had been connected to something, although far too short to connect to the new cutout should it have been.
My father claimed when the new cable was installed, he mentioned to the DNO people he himself installed an Earth electrode outside when the house was re-wired in the early 1980's, of which one of the DNO people mentioned not to worry about Earthing, as there was one 'on the poles running to the house'.
Anyway, for a few years this is how things remained (Yes I know, the house been without effective Earthing!!!).
Then we required three extra circuits to be installed and had the 3036 CU replaced with a Wylex MCB/RCD CU with 100mA T/D incomer and 30mA for sockets and outside circuits, (16th Edition applied at this time), with the earth rod (apparently not initially required as above) remaining and regarding the RCD protection in use as a TT system.
This how it is to the present day.
BUT the question has always bugged me 'Is this truly meant to be a TN-S system OR a TT system as is currently the present case.

I did enquire about this with photos on another site, but with no definite answer.
Some suggested 'Split-Phase' but I can tell you it is only a 2 bedroom cottage, no storage heaters (never).
Some say overhead TN-S supplies do not exist, but I have when reading on this site had seen people mentioning to have seen them, although a rarity.
If you look carefully at the rear transformer view, you can see a link between the middle and bottom conductors (N-E link?).
The transformer supplies three properties altogether, one set of three conductors leading to a nearby property, and another set of conductors leading to a pole which in turn supplies to other properties, including my parents.
On a side note, two properties fed from this transformer are right nearby, whilst the supply to my parents is a very long overhead supply (if this is actually a TN-S Earth, if a RF Circuit was protected by a 30A 3036 fuse as was originally the case, how long would it take to rupture and disconnect in an Earth fault with such a very long, thin overhead line?)
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Problems with uploading photos, could someone help please?

 

[pc1966]

If you can put the photos in a PDF document that seems to be working, or put them on some image sharing site (not Facebook!) and post a link to them.

Hopefully it will be fixed soon, they know about it.

 

[davesparks]

If it has been fed with a split concentric cable then it will be TNS.

Overhead TNS distribution is rare but it does exist, so it is possible that this is what you have.

I know the type of cutout you are describing, that plastic cover does come off t reveal the earth terminal, but you do need to get it confirmed that it as a suitable earth before connecting to it.

Your best/safest bet is to contact the DNO to get them to confirm what type of eartjing system is provided. You may have to get them to visit to give a satisfactory answer as they might just say TT over the phone to get away without doing anything.
But remeber you probably won't get anywhere with a non-emergency callout at the moment.

 

[MartinJ]

Many Thanks pc1966
I have no idea how to do so.
Hopefully this will be sorted ASAP.
It's a pity really, although this is my first post on this site, I have visited it many times over and it appears to be a very useful site.
This has come as a great disappointment really since my post comprises of FULL detail and photos that show what I am trying to explain.

Kind Regards
Martin
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If it has been fed with a split concentric cable then it will be TNS.

Overhead TNS distribution is rare but it does exist, so it is possible that this is what you have.

I know the type of cutout you are describing, that plastic cover does come off t reveal the earth terminal, but you do need to get it confirmed that it as a suitable earth before connecting to it.

Your best/safest bet is to contact the DNO to get them to confirm what type of eartjing system is provided. You may have to get them to visit to give a satisfactory answer as they might just say TT over the phone to get away without doing anything.
But remeber you probably won't get anywhere with a non-emergency callout at the moment.

Many Thanks for your reply davesparks
Regarding the 'split concentric cable', if you look at the final pole supplying the drop to the house through binoculars, on the top you can see a red cable, the middle a black cable and on the bottom bare copper stranded cable.
Regarding the cutout, does the plastic cover come off altogether? if it does then it would slide off in the left direction. Also why cover it, especially if the DNO chap said there was an Earth available, since covering only causes confusion.
Regarding this, I have read in at least one (old DIY) book that although any Earth provided by the Electricity Board provides an excellent Earthing facility, the actual connection must only be made by the Boards' Engineers.
If it still is meant to be TN-S after all this time, then just disconnecting it (as explained by me previously) and leaving the house effectively un-earthed was a bit irresponsible wasn't it? (although I know they aren't/weren't obligated to provide an Earth).
Regarding the suitability of the Earth, whilst since many years the installation is MCB and RCD protected, at the time it was all 3036 (I am referring to my above post regarding it as a long thin line)

If that cover does come off though, there is no way it can now since the CU is in the way right against it.

Many Thanks once again!

 

[davesparks]

Regarding the cutout, does the plastic cover come off altogether? if it does then it would slide off in the left direction. Also why cover it, especially if the DNO chap said there was an Earth available, since covering only causes confusion.
Regarding this, I have read in at least one (old DIY) book that although any Earth provided by the Electricity Board provides an excellent Earthing facility, the actual connection must only be made by the Boards' Engineers.
If it still is meant to be TN-S after all this time, then just disconnecting it (as explained by me previously) and leaving the house effectively un-earthed was a bit irresponsible wasn't it? (although I know they aren't/weren't obligated to provide an Earth).
Regarding the suitability of the Earth, whilst since many years the installation is MCB and RCD protected, at the time it was all 3036 (I am referring to my above post regarding it as a long thin line)

If that cover does come off though, there is no way it can now since the CU is in the way right against it.

Many Thanks once again!

Without a picture I cannot be certain that the cutout you have is the same as I think it is, and I'm not going to advise any further in case I am wrong and you put yourself in danger by attempting to follow my incorrect advice.
That must be a very old book as the electricity boards haven't existed for around 40 years now, electricians can make that connection these days.

If the customer said that they didn't want to be connected to the suppliers earth then that is their choice, it sounds like this is the case from your OP.
The covers were left in place whenever the earth connection wasn't to be used, either due to it being unsuitable or there being nothing there to connect to it. The cover has knockouts so it could be replaced after the electricity board operative made an earth connection. These days the electrician will often make that connection.

The suitability of the earth can only be established by enquiry with the DNO and by testing it.

If it is the type of cutout I think it is then the earth terminal cover can be removed even with a CU tight up against it.

 

[MartinJ]

Without a picture I cannot be certain that the cutout you have is the same as I think it is, and I'm not going to advise any further in case I am wrong and you put yourself in danger by attempting to follow my incorrect advice.
That must be a very old book as the electricity boards haven't existed for around 40 years now, electricians can make that connection these days.

If the customer said that they didn't want to be connected to the suppliers earth then that is their choice, it sounds like this is the case from your OP.
The covers were left in place whenever the earth connection wasn't to be used, either due to it being unsuitable or there being nothing there to connect to it. The cover has knockouts so it could be replaced after the electricity board operative made an earth connection. These days the electrician will often make that connection.

The suitability of the earth can only be established by enquiry with the DNO and by testing it.

If it is the type of cutout I think it is then the earth terminal cover can be removed even with a CU tight up against it.

Without a picture I cannot be certain that the cutout you have is the same as I think it is, and I'm not going to advise any further in case I am wrong and you put yourself in danger by attempting to follow my incorrect advice.
That must be a very old book as the electricity boards haven't existed for around 40 years now, electricians can make that connection these days.

If the customer said that they didn't want to be connected to the suppliers earth then that is their choice, it sounds like this is the case from your OP.
The covers were left in place whenever the earth connection wasn't to be used, either due to it being unsuitable or there being nothing there to connect to it. The cover has knockouts so it could be replaced after the electricity board operative made an earth connection. These days the electrician will often make that connection.

The suitability of the earth can only be established by enquiry with the DNO and by testing it.

If it is the type of cutout I think it is then the earth terminal cover can be removed even with a CU tight up against it.

HI
Just to let you know, it is a black cutout with the split con entering the top, (with a cover secured by two screws) over it with 'LUCY' on it and the bottom part containing the main fuse to the right with the Line tail to meter emerging from the bottom and to the left the Neutral tail emerging to the meter emerging from the bottom. Immediately to the left of this is the cover (possibly containing Earth Block).

I certainly won't just try and do anything whatsoever.

Just wish I could get these pictures to upload, as I know they will say thousands of words.

Regards

 

[pc1966]

Do you have some sort of usable earth just now? It sounds like you have the rod your father installed, presumably it was tested when the new CU went in?

As you point out, if it is/was a long LV feed it might take a long time to blow a fuse. At least these days it is easy to have the PFC/PSSC measured with any MFT so you know what the clearance time ought to be!

 

[MartinJ]

The book is old, probably from the 1970s, but it was many years ago I read it.

The split-con and cutout, was installed in 1990 (as a replacement for three individual cables for the drop from the final pole to the house).
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Do you have some sort of usable earth just now? It sounds like you have the rod your father installed, presumably it was tested when the new CU went in?

As you point out, if it is/was a long LV feed it might take a long time to blow a fuse. At least these days it is easy to have the PFC/PSSC measured with any MFT so you know what the clearance time ought to be!

HI
Yes, the Earthing method actually in use is the Earth Rod my father installed (i.e. TT), but as mentioned before, apparently my father said he mentioned it to the DNO chap that he put it in when the house was rewired (1980ish), but the DNO chap apparently mentioned 'There is an Earth on the poles supplying the house' although has not been used (well not connected to actually) since the split-con was installed.

When the house was rewired in, a small 3036 board was installed, then 10-15 years later due to new circuits, new CU with MCBs/RCDs.

I cannot help regarding testing etc, since the new CU and circuits were installed 25+ years ago. Saying that, we have had a faulty metal light fitting trip the 100mA T/D RCD once and several instances of faulty/leaky appliances trip the 30mA, and I operate the test buttons quarterly* as recommended.
(I know this does not compare with proper testing, but thought I'd let you know)
*Is this still the recommended interval or has it changed/going to be changed?

The overhead LV supply has always been the same (long and thin), only alteration been the split-con replacement in 1990.

Like I said, just wish the photo's would simply work.

Regards

 

[pc1966]

I cannot help regarding testing etc, since the new CU and circuits were installed 25+ years ago. Saying that, we have had a faulty metal light fitting trip the 100mA T/D RCD once and several instances of faulty/leaky appliances trip the 30mA, and I operate the test buttons quarterly* as recommended.
(I know this does not compare with proper testing, but thought I'd let you know)
*Is this still the recommended interval or has it changed/going to be changed?

If the lights were not on RCD (as specified now in 18th regs) then they would trip the main 100mA incomer on a fault as you saw, which is a bit inconvenient.

Even if you have all-RCBO set up if they are not switching neutral then an E-N fault can also trip the main RCD incomer (though you should at least see which circuit was responsible as it ought to have tripped as well, just it would not isolate the N line to prevent the incomer getting triped). Not many RCBO switch neutral I'm afraid, though the compact Wylex/Crabtree ones now do.

As for periodic testing, these days they tend to say every 6 months in the hope folk will do it when the clocks change, but some devices say test quarterly or even monthly. I doubt many ever get tested at all!

 

[pc1966]

Today I could trial-attach a .jpg image, but not .png image, so some progress is being made.
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No, I take that back.. Still not working, but not sure why - might be an image size limitation.
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If the image is less than 1000 x 1000 maybe it works?!

 

[MartinJ]

Evening All

Just managed to insert images, just hope they work.

I will, if possible try and take a clearer, closer view of the Cutout including the Earth block area (Bottom Left as said above)

Regards

 

[MartinJ]

Great!!!

The photos are actually working

If anybody (especially pc1966 and/or davesparks, who have been very helpful so far) could have a look at them, that would be great.

Regards

 

[pc1966]

It certainly looks as you described, though the details of the split concentric cable are not really obvious on that photo. What now, are you wanting to use it as your earth, or are you happy enough with the TT rod?

If it were me and I had a little money spare to improve things, I would look at putting all circuits on dual-pole switching RCBO after a 100mA S-type incomer so faults were clearly restricted to the circuit they occurred on.

But then, most electric stuff is very reliable and years can go by without anything tripping!

 

[MartinJ]

It certainly looks as you described, though the details of the split concentric cable are not really obvious on that photo. What now, are you wanting to use it as your earth, or are you happy enough with the TT rod?

If it were me and I had a little money spare to improve things, I would look at putting all circuits on dual-pole switching RCBO after a 100mA S-type incomer so faults were clearly restricted to the circuit they occurred on.

But then, most electric stuff is very reliable and years can go by without anything tripping!

Hi
Regarding the split-con cable, if you look at the photo 'Old Redundant Cables' carefully, you can see the from top to bottom, top sticks out more, middle sticks out a bit lees and bottom does not stick out much, but there are 3 cables.

Looking at the 'Transformer back view', you can see a faint link of cable between the middle and bottom conductors (N-E Link?).

Regarding 'tripping', in the 25-28 years of the CU set-up, (installed to 16th Edition, so no earlier than 1992) the only time we have had a total loss of supply due to the main 100mA incomer tripping is one instance a few years ago from a faulty light fitting as mentioned previously.

Regarding the 30mA tripping, has happened a few times (can count on one hand), including kettle lead put down on wet/damp surface (not my doing personally).

I know the current layout of CU is not fully compliant to today's standards, but there is no plan for extra circuits etc to be installed.

the CU layout (conforming to 16th Edition Standards), is functioning as was initially required, as a TT installation.

However, if the TN-S was use-able, would it be a superior Earthing method?

Like I said, although it is functioning fine as TT, What really has been 'bugging' me is what Earthing system should it be?

Regards

 

[pc1966]

Looking at the 'Transformer back view', you can see a faint link of cable between the middle and bottom conductors (N-E Link?).

I thought I saw that, but not very clear.

Regarding 'tripping', in the 25-28 years of the CU set-up, (installed to 16th Edition, so no earlier than 1992) the only time we have had a total loss of supply due to the main 100mA incomer tripping is one instance a few years ago from a faulty light fitting as mentioned previously.

Regarding the 30mA tripping, has happened a few times (can count on one hand), including kettle lead put down on wet/damp surface (not my doing personally).

I know the current layout of CU is not fully compliant to today's standards, but there is no plan for extra circuits etc to be installed.

If it is working fine then no need to change!

Usually what forces your hand is if a MCB or RCD fails and you find you can't get replacements for an older model and (of course) the busbar arrangement is such that others do not fit safely/properly.

However, if the TN-S was use-able, would it be a superior Earthing method?

Like I said, although it is functioning fine as TT, What really has been 'bugging' me is what Earthing system should it be?

If available it is certainly a lower impedance path, so in that sense superior as it might allow a MCB's "instantaneous" trip to clear a fault fast enough to avoid the main incomer time-delay RCD to trip.

However, others might argue that a TT earth rod in good condition has the advantage of being under your control and not having other voltages (e.g. lightning surge) superimposed on it.