Metal conduit as earth

[maharg]

I 've been asked to do some work on a 1970s flat where the metal conduit is used as the protective conductor, How would I test the continuity of the ring circuits.Any help greatly appreciated.

 

[scotsparky]

a Maharg you have an old PME system

Prove the continuity of P-P and N-N
ensure all flyinf leads are in place and take a long lead round all the points to prove earth continuity R1&R2 then take the Zs on the points. As the Metalwork is the earth to many earth paths to mention so the ZS and Long lead is the clincher.

Alternitavly you could get a conduit tester and test the lot but if you do BE CAREFULL!!!!!!!!!!!!!!!

 

[maharg]

Thanks Scotsparky but this poses another problem.I recognised the PME supply distributors fuse but the main earthing has been disconected from the housing and clamped to the incominig supply conduit which is a copper enclosed in a sheath.The periodic test done last year has it marked as T-N-S.

 

[scotsparky]

send a pic possibly Whatts the ZE?

 

[maharg]

Technical problems with the phone/camera at moment.
I asked the owner for the periodic to check Ze reading but no live tests were carried out as there was no supply to the property on the day of the test-prepay meter-.
I am going back tomorrow to check Ze and do further inspections but it is not the area to be walking around at night with my new tester.
I also want to contact the DNO to make enquiries.

 

[scotsparky]

DNO will give you the highest figure and they probbabbly wonyt know earth type.


Test is yourself.

 

[maharg]

Thanks scotsparky Im going early tomorrow to check this over

 

[trix]

a Maharg you have an old PME system

Prove the continuity of P-P and N-N
ensure all flyinf leads are in place and take a long lead round all the points to prove earth continuity R1&R2 then take the Zs on the points. As the Metalwork is the earth to many earth paths to mention so the ZS and Long lead is the clincher.

Alternitavly you could get a conduit tester and test the lot but if you do BE CAREFULL!!!!!!!!!!!!!!!

Scotspark :
Please let me know how you know that this is a PME system !

Maharg, when testing the conduit, you are only testing R2 not R1&R2
Using method 2, commonly known as the wandering lead method.
As Scottsparky says, connect lead to all points to verify the integrity of tube.

As these flats were built in the 70's it would be recommended to thread a cpc throughout the tube.

By the way, what's a conduit tester ?

 

[scotsparky]

Trix

PME means Protective Multiple Earths. Basically all the containment in the electrical system is used an the earth and equipment is earth by a flying lead from back boxes and such. to that rather than running seperate CPCs. This means you cannot do a tru R1=R2 as there are so many earth Paths. It can still be done but NOT recommended!

IT may be possible to run seperate seperate CPCs throuout the system but as there will be Joints in conduit and possible underfloor boxes Very very difficult.

Sa for a conduit Tester it is a recognised tester which sends 25a throught the conduit to mrove the connection of the joints. Last time i saw one was when i was serving my time in 1990 but as it makes the containment live you have to be VERY carefull and it there is a loose connection in the conduit it could heat up like a toaster.

 

[trix]

No disrespect scotspark, but I still don't get how you assume it's a pme.
By the way, I served my time in late 70's.
You have jogged my memories with the tester

 

[maharg]

Here is the update.
Ze 0.03 ohms.
Pictures to view.

 

[xraytek]

Trix
Sa for a conduit Tester it is a recognised tester which sends 25a throught the conduit to mrove the connection of the joints. Last time i saw one was when i was serving my time in 1990 but as it makes the containment live you have to be VERY carefull and it there is a loose connection in the conduit it could heat up like a toaster.

Could a PAT tester be used for this test?
It wont be that difficult to make adapter leads for the tester and is equipment that you could offer extra services with.

 

[scotsparky]

xraytrek NO a pat tester couldnt do it.!

Trix i just assumed it was PME looking at the Pic looks like a TN-S put the same theory does apply with the testing.

Maharg is that a "Tenby" style earth clamp on the main earth? is so it has to be changed ther are for pipes only not SWA

 

[trebor]

Thanks Scotsparky but this poses another problem.I recognised the PME supply distributors fuse but the main earthing has been disconected from the housing and clamped to the incominig supply conduit which is a copper enclosed in a sheath.The periodic test done last year has it marked as T-N-S.

Are you sure this is PME ?, it was common in the pre early seventies and before for the service to be TN-S and the risers to flats line and neutral singles with the riser conduit being the cpc, another common method was of course MICC riser with the copper sheath being the riser cpc

PME risers which started around 1971/2 are virtually always insulated PVC cables

Just seen your photos, this is TN-S, judging by the solid link isolator this is the top end

 

[maharg]

Scotsparky.I intend to upgrade the main earth and main as they are 10mm and 6 mm.What type of clamp is suitable for the swa.