Strange Ze on a TN S

[Gringoking88]

Hi all,

Hope you are all well.

I went to do an EICR at a small domestic property supplied by a TN S today, it is wired in singles using the conduit as CPC, however, there are no lugs for fly leads to sockets or light switches. In itself, what does this warrant, C2 / C3, there are only class 2 fittings in use?

Also, upon initial check the incoming fuse carrier was smashed and needed replacing, also Ze was 0.00 with a PFC / PSCC of greater than 10 KA. I advised the client to call DNO to replace fuse carrier, sort the earth and fit an isolator, tbf they atended within a few hours, replaced the fuse carrier, but left the main earth terminal off and said to the client call me back and I can put a clamp on it, rightly or wrong, I revisited and clamped the main earth, however, carried out the Ze again and the same reading is achieved.

Can someone help explain how and why I get that reading and more importantly, how I resolve it?

Many thanks in advance,

 

[Vortigern]

For a start I would call back the DNO to suppy an earth Under Reg 24 they must do so, i.e. maintain an existing earth. They should have put a braided earth on with their spring tension clip. There is now way you should do that. Resolving 10ka PFC not a problem to resolve. CU is rated to 16ka, back up fuse anything from 16 to 33 Ka which covers your upstream OCPD.

 

[Strima]

Very close to the transformer then.

 

[Vortigern]

Did you have att off?

 

[Wilko]

it is wired in singles using the conduit as CPC, however, there are no lugs for fly leads to sockets or light switches. In itself, what does this warrant, C2 / C3, there are only class 2 fittings in use?

Hi, a domestic installation of metal conduit CPC is not common to me. And one that is done properly and in good condition throughout is perhaps rare. But if the conduit is properly bushed to metal back boxes and the Zs at each point is good and there is at least one solid fixing from the back box then it’s sufficient and no code is required in my view. For sockets there must be fly leads though?

 

[westward10]

Please tell me you didn't fit an adjustable earth clamp to the lead sheathed supply cable.

 

[Gringoking88]

For a start I would call back the DNO to suppy an earth Under Reg 24 they must do so, i.e. maintain an existing earth. They should have put a braided earth on with their spring tension clip. There is now way you should do that. Resolving 10ka PFC not a problem to resolve. CU is rated to 16ka, back up fuse anything from 16 to 33 Ka which covers your upstream OCPD.

I will do that as agree. I was just very surprised they said just get the electrician to do it. I was more surprised about the readings tbh.

 

[Gringoking88]

Hi, a domestic installation of metal conduit CPC is not common to me. And one that is done properly and in good condition throughout is perhaps rare. But if the conduit is properly bushed to metal back boxes and the Zs at each point is good and there is at least one solid fixing from the back box then it’s sufficient and no code is required in my view. For sockets there must be fly leads though?

No Fly leads anywhere and no lugs to do so, not on sockets either, just the fixing screws to give continuity.

 

[Gringoking88]

Please tell me you didn't fit an adjustable earth clamp to the lead sheathed supply cable.

I could but I would be lying, I was extremely careful not to damage the supply cable and weiging up the risks, thought it was better to an earth than no earth at all???.... Not saying I got it right but would value wiser words?

 

[telectrix]

I could but I would be lying, I was extremely careful not to damage the supply cable and weiging up the risks, thought it was better to an earth than no earth at all???.... Not saying I got it right but would value wiser words?

hold my hands up. i've done it myself a couple of times.

 

[westward10]

Fitting one of those to a lead sheathed cable is extremely hazardous. Packaging on many of these clamps state they are not to be used on this type of cable, any type of cable for that matter. It is not possible to maintain a tight connection to the cable and you certainly don't want to be in close proximity should the clamp create a fault situation within the cable.

 

[Gringoking88]

Fitting one of those to a lead sheathed cable is extremely hazardous. Packaging on many of these clamps state they are not to be used on this type of cable, any type of cable for that matter. It is not possible to maintain a tight connection to the cable and you certainly don't want to be in close proximity should the clamp create a fault situation within the cable.

Completely agree mate, but given the choice of no earth or an earth, double edged sword?....

 

[westward10]

Completely agree mate, but given the choice of no earth or an earth, double edged sword?....

Not really, never say never. Get the DNO back that is the answer.

 

[Gringoking88]

Not really, never say never. Get the DNO back that is the answer.

Agree completely with that!

 

[Wilko]

No Fly leads anywhere and no lugs to do so, not on sockets either, just the fixing screws to give continuity.

Ok, reading Reg 543.2.7 an earthing fly lead is required in each accessory. And for safety, I would say it really should be there in an outlet. So C2 from the judge with red wine in hand.

 

[Gringoking88]

Ok, reading Reg 543.2.7 an earthing fly lead is required in each accessory. And for safety, I would say it really should be there in an outlet. So C2 from the judge with red wine in hand.

Lol, I like the red wine, however, in terms of resolution, whats the most efficient way to resolve?

 

[Gringoking88]

Also just a note to say thanks for the balanced responses rather than a slatting which can quite easily occur so appreciated!

 

[westward10]

Do the back boxes have earth terminals.

 

[Gringoking88]

Do the back boxes have earth terminals.

Nope, literally just box with conduit feeding.

 

[Wilko]

Lol, I like the red wine, however, in terms of resolution, whats the most efficient way to resolve?

If the conduit and boxes really are in good shape, then perhaps drilling and tapping for a connector would be my pick.

 

[Vortigern]

but given the choice of no earth or an earth

As far as I am concerned they could have no earth. The DNO left them with no earth, that is their problem and liability. It is for you to be on top of them and make sure they do the right thing. Your must understand that as @westward10 said, if the compression of the clamp causes a short it can be fatal to you and is in no way acceptable morally or legally to do that. I have had this before and got them back the same day with the right kit and do the job properly. Don't let them brush you off. If I were you I would take that clamp off in fact, but then I would never put it on in the first place I'd be too scared to. Don't mean to be mean to you, but I feel quite strongly on this subject and feel it is important to say what I said, no offence meant.

 

[Gringoking88]

Do the back boxes have earth terminals.

As far as I am concerned they could have no earth. The DNO left them with no earth, that is their problem and liability. It is for you to be on top of them and make sure they do the right thing. Your must understand that as @westward10 said, if the compression of the clamp causes a short it can be fatal to you and is in no way acceptable morally or legally to do that. I have had this before and got them back the same day with the right kit and do the job properly. Don't let them brush you off. If I were you I would take that clamp off in fact, but then I would never put it on in the first place I'd be too scared to. Don't mean to be mean to you, but I feel quite strongly on this subject and feel it is important to say what I said, no offence meant.

Hi Vortigern,

No offence taken, im new to doing EICR's and have recently passed my 2391-52 but not new to domestic electrics, that said, am continually learning and always want to do things the right and best way, hence the question, appreciate the response. Ive arranged a return visit to remove it and will get onto the DNO, will they talk to me or want to only talk to the occupier based on your experience?

Thanks,

Gringoking

 

[Vortigern]

Hi @Gringoking88 they will listen to you and you are the only one who can talk to them. ESQCR reg 24 absolutely require them to maintain the earth so they cannot wriggle out of that. Not to say I go in with guns blazing, I prefer the charm offensive. Normally the DNO, especially the supervisors and guys on the ground are perfectly ok with supplying an earth and no probs as far as I have experienced. The householder can't really tell them as usually they haven't a clue as to what the hell it is all about. It's when you get the desk clerk who says the computer says no, you have to quote regs to them, and usually you get to speak with someone who knows the runnings and gets it sorted.

 

[Lucien Nunes]

Totally agree with Vort, never put a BS951 type of pipe clamp on a lead service cable. PILC cables are quite durable in some respects but they are ultimately made of soft materials that the clamp can distort and crush. And if there is something you really don't want to have short out in front of you, it's a public supply fused at 400A with a prospective fault current of 10-15kA. And, the lack of permanent spring tension in the clamp means the connection might give out after a while anyway as the lead creeps.

Re. The high PEFC, I don't know that MFT but it looks like it has a test lead resistance null function active. Does this apply to the cable used for loop impedance tests? If so, you might have an incorrect null programmed that is offsetting a higher resistance than the leads fitted. This will cause it to under-read the impedance and overestimate the fault current.

 

[Vortigern]

You know it's bugging me the name of that spring they (DNO) use, something like phosphor tension spring??? Anyone know it?

 

[pc1966]

You know it's bugging me the name of that spring they (DNO) use, something like phosphor tension spring??? Anyone know it?

These things:
https://www.3m.co.uk/3M/en_GB/compa...002385+3292813576&preselect=3293786499&rt=rud

 

[Vortigern]

Well of course, what @Lucien Nunes said, and @Strima i.e. close to the transformer causes low reading. If you look carefully in your manual you will see the figure for potential error in your reading. i.e. it can be (often) 6 figures higher or lower. Meaning, 0 could be plus or minus six, therefore your reading of 0.00 could be 0.06 which can be seen as 0.1 which is not an unusual reading. If you got 0.00 then I can't quite work out your fault current you came up with as 230/0.00 is 230 or in fact cannot be divided by zero so a bit puzzled by that. But if we said 230/0.1 then 2.3Ka is your PEFC which is entirely reasonable. Also as I did ask, was ATT on or off? And knowing how we all like to be acknowledged it is of course @westward10 who also picked up on the BS951 clamp in the first instance and should accordingly be noted in the annals!...that is one n /sigh

 

[Julie.]

You know it's bugging me the name of that spring they (DNO) use, something like phosphor tension spring??? Anyone know it?

CF spring, or constant force spring - spiralflex?



Shouldn't use a manual type tightened clamp as the lead flows resulting in a bad connection over time.

Nope, literally just box with conduit feeding.

What sort of back boxes are they, the round standard circular type with a square box on top? - if so the standard circular conduit fitting has a m4 threaded hole for the earth connection.

If it's a standard steel backbox with the 20mm knockouts, they usually have a brass earthing terminal - so I guess not one of these.

The very old type usually has a clamp type fitting to the conduit, and no earth terminal, these quite often don't have a good earth continuity after this length of time, so some boxes can end up with high r2, others ok; in this case it's almost inevitable that you need to pull a cpc to provide proper connectivity

 

[westward10]

CF spring, or constant force spring - spiralflex?



Shouldn't use a manual type tightened clamp as the lead flows resulting in a bad connection over time.







What sort of back boxes are they, the round standard circular type with a square box on top? - if so the standard circular conduit fitting has a m4 threaded hole for the earth connection.

If it's a standard steel backbox with the 20mm knockouts, they usually have a brass earthing terminal - so I guess not one of these.

The very old type usually has a clamp type fitting to the conduit, and no earth terminal, these quite often don't have a good earth continuity after this length of time, so some boxes can end up with high r2, others ok; in this case it's almost inevitable that you need to pull a cpc to provide proper connectivity

If there are no cpcs through the conduit the BESA boxes are not likely to have metric threads, probably BA and they may or may not have a threaded earthing point in the back.

 

[Julie.]

If there are no cpcs through the conduit the BESA boxes are not likely to have metric threads, probably BA and they may or may not have a threaded earthing point in the back.

The imperial boxes used 5/8, 3/4 " main threads and usually 2BA threads for earth and cover ( though some had 4BA)

But yes even older ones didn't have a threaded earthing hole, but at that time, most of the conduit used for residential used clamp type boxes.