Installation resistance test querie

[binky]

For clarity, you say single cable, is that a single cable as in 6242 type or 6491's, are we saying 4 independent radials? 

Is each floor separate electrically and mechanically? 

single cable run (singles) to each floor, then splits to 4 cable runs to cover the parking area on each floor. Each floor is separate electrically and mechanically, 14 floors in all.

 any "issues" to anything else in the property won't show up.....

This is what I'm on about, what other issues do yuo expect the test to show?

 

[Murdoch]

Can't you work that out for yourself ?

obviously not as you don't test properly 

 

[Brobat]

I can't turn the whole installation off as its industrial. They wouldn't be too keen in me shutting the whole place down to test a faulty outside lighting circuit. 

I am aware that the regs state that the CPC should remain connected,   The fault is that its tripping the dedicated RCD as soon as the timer sends power to the circuit. I've disconnected the L & N  coming out of the RCD and applied power to it and it doesn't trip which suggests that the RCD itself is not at fault.  however the CPC in question is the armour of some 2 core armoured flex which is connected on some trunking at a fairly awkward position to get at due to the amount of various bits of other lettuced that have been added over the last 20 years or so. 

 

[binky]

Can't you work that out for yourself ?

obviously not as you don't test properly 

yes thanks, and you know nothing of how I work. But you raised the point so enlighten us with your wisdom if you are able? Failure to do so which point ot you being a person who posts glib statements and fails to back then up with actual facts. Worked out my little puzzler yet? Bet you can't, but it is very relevant to this discussion.

 

[binky]

I can't turn the whole installation off as its industrial. They wouldn't be too keen in me shutting the whole place down to test a faulty outside lighting circuit. 

I am aware that the regs state that the CPC should remain connected,   The fault is that its tripping the dedicated RCD as soon as the timer sends power to the circuit. I've disconnected the L & N  coming out of the RCD and applied power to it and it doesn't trip which suggests that the RCD itself is not at fault.  however the CPC in question is the armour of some 2 core armoured flex which is connected on some trunking at a fairly awkward position to get at due to the amount of various bits of other lettuced that have been added over the last 20 years or so. 

fair enough,  chances are it's water in the light(s). Think I would test at RCD with CPCs connected LN-E

 

[Sharpend]

At the split you had three cores to test LNE?

 

[Evans Electric]

We may be getting closer to it now Brobat .     A timer switches on some outside lights and the RCD trips .

So there is a reading between   L - E   and / or  N - E    and  Binky has just said  " probably damp" or water inside light fittings or junction boxes .   Any joints in the ground ?   

The SWA is the earth , you don't need to disconnect it ,  the fault could be between the circuit earth or the general mass of earth ,   if there is  , say ,  six lights  as a radial ,  try parting the wiring half way  and meggering again  in each direction .   

 

[binky]

depending on the type of lights, the control gear can give false readings, hence I was suggesting LN-E 

 

[Brobat]

Thanks for the help peeps. Traced the fault yesterday to the section of swa cable that runs from the RCD to a ‘junction box ‘ ( screw connectors covered in tape). Just got to take the drive up to replace it now. Thanks again 

 

[binky]

@Sharpend

Puzzler solution. True story this from a job we did a few years ago converting lights to LED. The lights were all metal bases, conduit is metal, screws metal, and the concrete is steel re-enforced, so what we were getting is the odd screw in contact with the structural steel. This was allowing us to get Zs readings at end of line - pretty good readings too. In realty the CPC was broken at a rusted out bit of connector block in the  trunking about 2 floors down. What this demonstrates is 2 things:-

1/ testing IR with CPCs connected at dis board is a good idea because of potential alternative earth paths

2/ disconnecting CPC to do individual cct tests is also a good idea -  we would never have noticed the broken CPC otherwise.

Using structural steel of a building is an accepted earthing practice @Marvo had a term for it that I can't remember. Apparently it is popular in very dry areas ie deserts where a simple earth rod just doesn't provide sufficient good earthing, especially for military experiments, eg American military bases in Nevada desert.

 

[Sharpend]

I was asking how many cables/Ccts you had as I was thinking of the conduit being earthed by default at the board and thus carrying an earth through to fitting assuming fittings were metallic of course. 

I had a similar situation several years ago within a domestic property, tested circuit cables individually no earth however there was continuity to back box, transpired the walls and ceiling were gyproc metal stud frame and the earth was being gained via the socket earth. The metal back boxes were screwed to a metal cross member which was fixed to the frame. The light was screwed to the metal ceiling frame which was attached to the wall framework. 

Just goes to show the importance of knowing and understanding the testing that you are doing. 

 

[Sidewinder]

@Sharpend

Puzzler solution. True story this from a job we did a few years ago converting lights to LED. The lights were all metal bases, conduit is metal, screws metal, and the concrete is steel re-enforced, so what we were getting is the odd screw in contact with the structural steel. This was allowing us to get Zs readings at end of line - pretty good readings too. In realty the CPC was broken at a rusted out bit of connector block in the  trunking about 2 floors down. What this demonstrates is 2 things:-

1/ testing IR with CPCs connected at dis board is a good idea because of potential alternative earth paths

2/ disconnecting CPC to do individual cct tests is also a good idea -  we would never have noticed the broken CPC otherwise.

Using structural steel of a building is an accepted earthing practice @Marvo had a term for it that I can't remember. Apparently it is popular in very dry areas ie deserts where a simple earth rod just doesn't provide sufficient good earthing, especially for military experiments, eg American military bases in Nevada desert.

Are you thinking about Ufer earthing?

https://en.wikipedia.org/wiki/Ufer_ground

 

[binky]

that's the one

 

[Tony S]

Are you thinking about Ufer earthing?

https://en.wikipedia.org/wiki/Ufer_ground

that's the one

You had better get used to it, it's in the 18th

 

[Sidewinder]

You had better get used to it, it's in the 18th

Well the DPC, and it'll only be on new builds, IF they get it right!

I can't see house builders taking the house down and digging the foundations up for the spark coz they messed up!!!

 

[binky]

so the plan is to build houses with a 'permanent' earth rod?

 

[Sharpend]

Well 18th is calling for an Earth rod to be installed regardless of supply arrangement. 

 

[binky]

@steptoe will be happy then 

I've often wondered about conductivity of building materials. Solar Inverters can detect earth leakage on the DC side ( ie the panels) of less less than 20mA. Given that roof timbers are ( should be ) bone dry, bricks etc etc bone dry, how the hell does it find an earth path to a slightly damp DC connector on the roof???? It's puzzled me for years.

 

[Sharpend]

Where is steps these days? 

 

[Tony S]

If there is no DPC under the slab then an Ufer earth is a way to go. The rebar in the slab when connected to the MET will provide a better earth than a rod ever could.

The connection to the MET needs to be cad-welded to the rebar to give a reliable connection when buried in the concrete. Only one connection is required as the sections of rebar will interact with each other. There are additives that can make the concrete slab even more conductive such as Bentonite or Marconite.