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marcuswareham

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Hi guys

I was putting in a spur socket at a friend's house. when i noticed that the CPC is the metal cable trunking. I understand this is within regulations. However upon testing it had a high earth loop impedance and likewise r2.

My question is, is it acceptable to run in a new CPC from consumer unit MET to the relevant outlets, but using a different cable route (plastic trunking etc)

Thanks Marcus
 
Hi guys

I was putting in a spur socket at a friend's house. when i noticed that the CPC is the metal cable trunking. I understand this is within regulations. However upon testing it had a high earth loop impedance and likewise r2.

My question is, is it acceptable to run in a new CPC from consumer unit MET to the relevant outlets, but using a different cable route (plastic trunking etc)

Thanks Marcus

Why not pull a cpc through the existing steel conduit?

Plus you still need to find out why the R2 is too high as the conduit still needs to be effectively earthed to be safe.
How high is the R2 reading and did you get a Zs reading?
 
Why not pull a cpc through the existing steel conduit?

Plus you still need to find out why the R2 is too high as the conduit still needs to be effectively earthed to be safe.
How high is the R2 reading and did you get a Zs reading?

thanks for the quick reply

Hey i suspect trying to get a new CPC down the same conduit might be a pain in the --- as fair way and lots of sharp bends (it is also all within concrete no timber floors or anything etc)

I did do a Zs reading and was good, i cannot remember the figures but i do have them written down just not with me, i will post them tomorrow

I see what you are saying about conduit needing to be well earthed although with new CPC to each out let earthing the metal back box and conduit attached to it, hopefully that should provide adequate earthing as i cannot get to any of the conduit except a small amount in the consumer unit area (if it is earthed from both ends per say this should protect against any breaks or rusted joints within the wall or floor etc)

I should also mention, the conduit at the CU end seems to be well earthed
 
Hey i suspect trying to get a new CPC down the same conduit might be a pain in the --- as fair way and lots of sharp bends (it is also all within concrete no timber floors or anything etc)

I did do a Zs reading and was good, i cannot remember the figures but i do have them written down just not with me, i will post them tomorrow

You suspect it might be a pain so you don't even bother to do a little investigation to find out for sure?
Have you tried pulling the existing singles to see if they move? These conduits were normally installed before the concrete was poured and then the cables pulled in afterwards so they won't have been installed in a way that cables can't be got through them.

Are you saying that the Zs is compliant? If so then the R2 can't have been all that high can it?
 
As Dave says these systems were installed to enable wiring after the concrete is in situ. You may be surprised how easy it is to pull through another conductor, I did this is a school hall recently and used the existing conductors as a draw wire.
 
If you find the conductors are actually easy to pull through, and the instalation is quite old, it could be worth pulling the old conductors out and pulling in 3 new ones behind them. Might even be less labour than trying to just fish the earth through, or pulling them out and then pulling the old ones back in with a new earth. And it should also be less labour intensive than running a new earth to each point via a seperate route.
 
As Dave says these systems were installed to enable wiring after the concrete is in situ. You may be surprised how easy it is to pull through another conductor, I did this is a school hall recently and used the existing conductors as a draw wire.

Humm, ok although stright after the consumer unit there is a 90 degree bend which looks tricky, I might see how easy it is as you say, only problem is when you get stuck half way and no longer have to old conductors or the new, doh !
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Why not pull a cpc through the existing steel conduit?

Plus you still need to find out why the R2 is too high as the conduit still needs to be effectively earthed to be safe.
How high is the R2 reading and did you get a Zs reading?

Hey,

Sorry I read that as Ze rather than Zs.

So Ze at C/U is good 0.33 ohms it is TN-S

at the outlet in question it is 4.91 ohms and simular at the rest of the socket outlets, So thats a PEFC of 50amp which is enough to trip the RCD, also did RCD disconnection time test and it is work as it should.

I tested a lighting outlet which has conduit as the CPC also, and this was 0.86 ohms so better than the sockets. Although did find that non of the light switches had an earth conductor or conduit.
 
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Humm, ok although stright after the consumer unit there is a 90 degree bend which looks tricky, I might see how easy it is as you say, only problem is when you get stuck half way and no longer have to old conductors or the new, doh !
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Hey,

Sorry I read that as Ze rather than Zs.

So Ze at C/U is good 0.33 ohms it is TN-S

at the outlet in question it is 4.91 ohms and simular at the rest of the socket outlets, So thats a PEFC of 50amp which is enough to trip the RCD, also did RCD disconnection time test and it is work as it should.

I tested a lighting outlet which has conduit as the CPC also, and this was 0.86 ohms so better than the sockets. Although did find that non of the light switches had an earth conductor or conduit.
FAO Marcus, use a draw tape.
 
Had a bit of time today to pop round and investigate a little further, I have found that the bit of conduit from the C/U to the first socket where the ring splits is the one at fault.

If I take a temporary earth cable from the MET and connect it to the ring at that point go round and test all the sockets and then they are all around the 0.45 ohm mark, so much better

the cable route to this socket is remarkably easy. So I am thinking of running in a 4mm CPC conductor to this point from the C/U

Would this be ok, it would not be a ring at the C/U but would connect to a ring at the socket outlet, it is of a size that it should be adequate

Also what are peoples thought on the light switches without a CPC, the cable doesn't arrive in metal conduit therefore there is only the live and the switched live at each switch

Thanks

Marcus
 
Then what containment is the cable in?

Good question, I also wondered, I hahaven't seen yet, but will investigate
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Is continuity okay at the water heater which also uses the same conduit.

The water heater is the same as the socket ring, and has a high CPC impedance
 
Have you tried tightening the conduit termination at the board.

Yes indeed, and checked earth bond
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So after a lot of swearing I managed to get a new CPC down the conduit, draw tape was not much use and there was loads of brick, concrete and plaster that had fallen down conduit over the years. As the immersion heater is soon to be no longer needed due to combi boiler going in I used the neutral conductor for it to pull though the CPC, all good now

Also found out the light switches are earthed via conduit, just the one I picked to look at the conduit was not attached to the back box haha

Thanks for you guys help and suggestions
 

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