CPC missing from ring circuit but end to end continuity at C.U!!

[linetech]

Hello All

This will be my first post for a bit of direction sincejoining the forum a while ago, and this issue has us a bit baffled. It is possible me and my mate have overlooked something really daft & obvious! Anyway here it is..

Yesterday we were doing a periodic on my mates flat which is part of a block. These flats were built in 1968 so are near 45 year old. The C.U has labels stating the property was brought up to date in 2009 and next inspection is 2019. The flat has obviously been brought in line with 17th edition as the C.U is a 10 way dual split RCD type, there are two codes of wiring present. I believe initially the flat had one ring and with the 17th in effect the kitchen has had a new ring with new colours.

After deciding to do a full comprehensive of all circuits we checked the C.U for sequence & order of circuits and all looked neat and in order. We started with the first side of the split board with shower, kitchen ring, & boiler which all revealed test values quite normal, however when we got to the 2nd side of the split the ring circuit revealed a CPC end to end of 0.08, Line - 0.3 & Neutral - 0.3, with significant varying results after cross connection of L-N & L-CPC at each socket.
How is it possible that a reading for the CPC, with a reduced CSA, can be significantly lower than the line or neutral loop?? Scratching our heads, further investigation we did, removing the socket outlet with the highest value obtained which was a massive 2 ohm`s. What we found was no CPC. We removed near half of the other sockets to see what was going on and none had a CPCs present which brings me to the question:

How is it possible to get a continuity reading between Line & CPC with use of an R2 Box when there isn’t any CPC provided or more again how the CPC end to end value can be so low!

The only reason I can think of for the continuity at the sockets between L-CPC is that metal conduit is run through the walls and onlyt he screws securing the switchgear is enabling continuity. As for the 0.08 end to end, well I’m hoping we can get some direction right here!

Although it may have a reasonable logical explanation,please be gentle!

Thanks Jay

 

[spark 68]

It is probable that the steel conduit and backbox is used as the CPC (hence the low end to end reading), and as you say the earthing of the socket face plates are relying on the screws to the backbox.
You really need *4mm earth pigtails from the backbox to the socket faceplates *(assuming 2.5mm RFC conductors).

Edit: you also need to confirm that each backbox is earthed and you have the expected low R2 reading at each backbox

 

[linetech]

Hi Spark 68

Yes there are no pigtails. Would that make it a code 1 or code 2?? The readings obtained after cross connection were around 0.28 and higher which is a bit more than expected and outside of tolerance allowance of L-CPC/4, which I believe is 0,05 if I remember correctly . Have not been at the testing long and have not come across these sort of reading before so was a bit baffled to say the least!

Anyway we are gona give it a good look over and see what else it throws up! Thanks for reply.

 

[spark 68]

Reg 543.2.7

As to C1 or C2, you as the inspector would need to decide if it is an immediate danger or not, either way it is unsatisfactory.

You cannot rely on the fixed lug of the backbox in this instance, as the pigtail serves a completely different function in this situation.

 

[Knobhead]

I take it the original (conduit) ring has been extended.

As I see it all you can do is drop the fascia off every socket and identify which is original and what is new. For the new trace/follow the CPC and check it’s continuity, the old if you’re lucky there will be a tapped hole (4BA or 3.5mm) that you can put a fly lead off to the fascia. Where the conduit forms the CPC the fly lead is perfectly acceptable and certainly better than relying on the fascia screws.

Sorry, it’s a ball ache and hopefully you’ll find where the old and new join.

PS the conduit with fly leads will most likely give the best test result.

 

[wildgoose]

Slightly off topic here, but on the subject of pigtails within socket boxes from the earth terminal back to the metal knockout box frame.
With conventional T&E cabling, providing at least one of the socket plate mounting lugs on the knockout box is rigid, then with a modern socket plate, you don't need to have an earth pigtail.

 

[spark 68]

Hi Wildgoose,

Not in this case he can't see post #4

You cannot rely on the fixed lug of the backbox in this instance, as the pigtail serves a completely different function in this situation.

Page 113 GN8, Fig 9.8

The situation in this case is the opposite way round, the pigtail is not to earth the backbox, but to provide the earth to the socket faceplate from the backbox.

 

[wildgoose]

Now read earlier comments and yes your right, this is a different situation.

Did try to stress though on my earlier comment for "conventional t&e cabling". Don't you just hate those uneccessary pigtails everywhere on already overcrowded back boxes - a high level of p/off factor!

 

[onions1066]

i would personally code it as a 2 as no risk is present unless a fault occurs. to put it basically a code 1 is saying risk of electric shock is present, code 2 there is a risk of shock if a fault occurs. so i say its a 2.

 

[GLENNSPARK]

you say the flat been brought up to the 17th edition....yet in the same post you refer to wiring to 2 different codes...i assume thats colours to two different versions of BS7671.....old colours are a departure from the current edition of BS7671.....so how has it been brought up to the 17th edition?.....you have a departure here.....noted?.....

 

[Crosswire]

'2 colours' label on the d.b. makes the installation compliant ( as far as cable colours are concerned)

 

[GLENNSPARK]

'2 colours' label on the d.b. makes the installation compliant ( as far as cable colours are concerned)

but not noted?....

 

[linetech]

Hi Glenspark

No paperwork has been drawn up yet. It’s a favour for a mate and the property is empty so will be done in our spare time. As per original post paragraph 2, line 2 -4 states: brought up to date and brought in line withBS7671.
I am in agreement that the installation can never be a complete 17th edition install as it has the two wiring versions of BS7671,however without quoting regulations the current installation when brought in line with the current edition does not impair the safety any less than that of the original install, and in fact with the addition of the split dual RCD C.U & upgraded main protective bonding provides a resulting degree of safety no less than compliance of the current regulations and offering more comprehensive protection for safety.

As you say good practise would suggest a note of departure perhaps but, compliant never the less. As Crosswire has observed in reply 11 the C.U does have the information label stating this fact.

I guess it’s fair to say should I need any future support on tech issues you will be there to give some direction which is appreciated. As the installation stands at the moment it has been noted that no pigtails are present so this needs to be sorted, and I’m sure you would agree is far more important than whether the installation has two versions of BS7671, departure or no departure.

Thanks for your support on the matter!

 

[linetech]

Well this install has been a pain and I’m glad it’s been aspare time thing! No pig tails anywhere, and the varying results throughout have proven to be loose connections all over the place.. Nice to have the low resistance on the CPC end to ends due to the heavy grade conduit throughout the install, it’s the only plus so far.. Lights have thrown up some surprises, further investigation needed. Be back there in a week so hopefully it won’t be one of those awkward ones... Was supposed to be brought up to date but the deeper we look the worse it gets. :frown2: