The realities of trying to test in occupied domestic properties

[DaveS79]

Hi all

I realise I'm probably going to get shot down for this but still intrigued to know how others manage similar situations.

On job today. Job was to change old Wylex CU and issue satisfactory EICR. Checked previous sparks report prior. No major issues with installation. One of the jobs to do was split an over populated lighting MCB which had 4 circuits in . So split them, sussed out what lights they was serving. Just about got my r1+r2's on two of them, but third one serving big stupid wall lights right above desk where tenant working and dismantling and refitting them to get readings could take easy an hour. I'm back there the weekend so if no choice then no choice. Also one of the 6 amp circuits I couldn't verify for the life of me what it was serving, so again no reading could be taken. I realise on EICR's can put limitations on testing unverified circuits or circuits that require extensive dismantling but my understanding is you can't stipulate this on EIC's? Cheers

 

[ProDave]

If you think that is bad, try a full rewire of an occupied house.

Re unidentified circuits, I tend to leave them disconnected and ask the owner to call me when they find what is not working.  Usually I never get called back.

 

[Murdoch]

Hi all

I realise I'm probably going to get shot down for this but still intrigued to know how others manage similar situations.

On job today. Job was to change old Wylex CU and issue satisfactory EICR. Checked previous sparks report prior. No major issues with installation. One of the jobs to do was split an over populated lighting MCB which had 4 cables in . So split them, sussed out what lights they was serving. Just about got my r1+r2's on two of them, but third one serving big stupid wall lights right above desk where tenant working and dismantling and refitting them to get readings could take easy an hour. I'm back there the weekend so if no choice then no choice. Also one of the 6 amp circuits I couldn't verify for the life of me what it was serving, so again no reading could be taken. I realise on EICR's can put limitations on testing unverified circuits or circuits that require extensive dismantling but my understanding is you can't stipulate this on EIC's? Cheers

check the definition of "circuit" in the BBB

The reality is that you are at the coal face and the powers that be never visit there. Finding "unknown" cables is pretty much par for the course and you have to decide whether to leave it disconnected or connected.

Where you have 4 cables in a breaker I tend to split them on to 2 or more breakers - presumably you have spare ways in your new board and this shouldn't be an issue 

Also do you have a caveat on your quote or estimate which allows you to bill them for extra time when you find unexpected issues? If you don't you need to add one - I use one and have NEVER been challenged on it.

Hope this helps 

 

[binky]

Oh lordy, big light fittings and decorative light fittings - I tend to take a reading at the light switch rather than disturb things that may well take an age to reinstall. Not strictly the correct way to do things, but when you have seen so many rats nests of choc-block behind these things ..... 

 

[Murdoch]

nothing wrong with using your wander lead and taking R2 readings either.

 

[DaveS79]

Oh lordy, big light fittings and decorative light fittings - I tend to take a reading at the light switch rather than disturb things that may well take an age to reinstall. Not strictly the correct way to do things, but when you have seen so many rats nests of choc-block behind these things ..... 

If you think that is bad, try a full rewire of an occupied house.

Re unidentified circuits, I tend to leave them disconnected and ask the owner to call me when they find what is not working.  Usually I never get called back.

nothing wrong with using your wander lead and taking R2 readings either.

Thanks Guys. Think I'm learning more and more that can only go so far in certain situations and rest of it is left down to judgement on where potential risks are. The wander lead comes out on practically every job now especially EICR's where it's a life saver. The thing is though even if R2 test great for proving earthing to class one fittings. Filling in the certs for either EICR's or EIC's for ZS is not possible unless start dismantling stuff. Some of the looks I get from customers when doing a board change can feel them thinking what is this guy on he's changed the CU the powers back on.....thank God, just get out and stop dismantling my house

 

[Murdoch]

Thanks Guys. Think I'm learning more and more that can only go so far in certain situations and rest of it is left down to judgement on where potential risks are. The wander lead comes out on practically every job now especially EICR's where it's a life saver. The thing is though even if R2 test great for proving earthing to class one fittings. Filling in the certs for either EICR's or EIC's for ZS is not possible unless start dismantling stuff. Some of the looks I get from customers when doing a board change can feel them thinking what is this guy on he's changed the CU the powers back on.....thank God, just get out and stop dismantling my house

it is permissible, in real coal face sparky world to complete the R2 if it’s not practical to get a Zs

the scams may suggest otherwise, but in my book it’s AOK

 

[DaveS79]

it is permissible, in real coal face sparky world to complete the R2 if it’s not practical to get a Zs

the scams may suggest otherwise, but in my book it’s AOK

Thanks Murdoch good to get advice from someone on the front line lol. What do you stipulate on cert when doing this? Cheers. 

 

[Murdoch]

Thanks Murdoch good to get advice from someone on the front line lol. What do you stipulate on cert when doing this? Cheers. 

I make use of the inspecting comments box

 

[SPECIAL LOCATION]

If you have measured R2 on a lighting circuit...

Using the resistance tables from the OSG, it is actually possible to calculate an approx length based on an assumption that the cable may be 1.0mm or 1.5mm..

So.. from this length you could also estimate what a potential worst case R1 would have been, had you been able to measure it..

e.g. cables may be..

1.0mm / 1.0mm  T&E or singles

1.5mm / 1.0mm T&E

1.5mm / 1.5mm singles

Using any combinations of these values, added to your Ze,

you could get a rough idea if a potential Zs, was anywhere near the max permissible values..

(Its not hard to knock up a spread sheet where you enter your R2 reading and it will return possible R1+R2's for each of three the above cable options)  

If worst case assumptions are well clear of max permissible.. then Don't worry...  Move on...

But..  If estimated calc's are getting a bit to close for comfort..

Then go and dismantle the fitting and do a proper accurate measurement..

Any decorative metal metal fittings you will need to confirm earth continuity anyway..

unless you know for a fact that they are Class 2 fittings..

So measure R2 from CU to external metalwork of fitting while verifying earth continuity to the fitting..

Note:-

Assuming you did a site visit prior to work commencing..

these type of potential problems would have been identified and costed into the job anyway!

Guinness   

 

[DaveS79]

If you have measured R2 on a lighting circuit...

Using the resistance tables from the OSG, it is actually possible to calculate an approx length based on an assumption that the cable may be 1.0mm or 1.5mm..

So.. from this length you could also estimate what a potential worst case R1 would have been, had you been able to measure it..

e.g. cables may be..

1.0mm / 1.0mm  T&E or singles

1.5mm / 1.0mm T&E

1.5mm / 1.5mm singles

Using any combinations of these values, added to your Ze,

you could get a rough idea if a potential Zs, was anywhere near the max permissible values..

(Its not hard to knock up a spread sheet where you enter your R2 reading and it will return possible R1+R2's for each of three the above cable options)  

If worst case assumptions are well clear of max permissible.. then Don't worry...  Move on...

But..  If estimated calc's are getting a bit to close for comfort..

Then go and dismantle the fitting and do a proper accurate measurement..

Any decorative metal metal fittings you will need to confirm earth continuity anyway..

unless you know for a fact that they are Class 2 fittings..

So measure R2 from CU to external metalwork of fitting while verifying earth continuity to the fitting..

Note:-

Assuming you did a site visit prior to work commencing..

these type of potential problems would have been identified and costed into the job anyway!

Guinness   

Thanks SL that's very helpful. Like said in previous post the R2 test is is in deed a wander. Never thought about using it to equate the r1+r2 but lot of logic in it. Spreadsheet good idea yet to find a decent app that seems to give reliable resistance calculations 

 

[SPECIAL LOCATION]

Thanks SL that's very helpful. Like said in previous post the R2 test is is in deed a wander. Never thought about using it to equate the r1+r2 but lot of logic in it. Spreadsheet good idea yet to find a decent app that seems to give reliable resistance calculations 

You don't need an App,  Table 11, pg 196 gives the resistance values for single conductors and T&E..  

just do a bit of basic maths...

For a single conductors

1.0mm is 18.1 miliohm/meter,   or 0.181 ohm per 10m,  or  1.81ohm per 100m

1.5mm is 12.1 miliohm/meter,  or  0.121 ohm per 10m, or  1.21ohm per 100m

You can calculate the ratio difference between conductors, 

so a 1.5mm is 0.668x a 1.0mm value..      (0.181 x 0.668) = 0.121

So if R2 was 1.0mm & R1 was 1.5mm,  the R1 value will be 0.668 x your measured R2.

e.g.  if you had measured R2 as 0.55ohm..

If both R1 & R2 were the same physical size R1+R2 would simply be double your measured value..   1.1ohms

But if it was 1.5/1.0mm T&E   R1 will be 0.55x0.668 = 0.36

So R1+R2 would be 0.36+0.55 = 0.91

I often also use rough values per 10m to suss out if my test readings equate to expected values for the physical size of the circuits/building..

i.e. for  our 0.55 reading...

If it was a 1.0mm CPC it would be approx 30m (0.55/0.181)x10.

If it was a 1.5mm CPC it would be approx 45m (0.55/0.121)x10.

Which can give some idea if your rough guesstimates of cable size are reasonable or not?

On a similar note, I was investigating a high resistance ring fault on a socket circuit once,

and had to explain to a customer although the sockets were working,

the readings suggested there was just over half a kilometre of cable between the bedroom and lounge below it!!

which needed further investigation for concealed junction boxes, cable damage etc.. 

Guinness