Bonding of external water pipe

[bennie]

Hello,
Carrying out remedial work on a commercial site and I've come across a metal out building that has a water supply to it. The water pipe comes out of the ground behind the building in copper but enters the building in plastic. There is conduit on the outside of the building next to the water pipe and if I test between the two I'm getting 0.12 ohms. Internally the pipe work is all plastic. Should the external water pipe be bonded?

 

[timhoward]

The water pipe comes out of the ground behind the building in copper but enters the building in plastic.

If it enters the building in plastic it isn't an extraneous conductive part and doesn't need bonding.
Does the conduit enter the building or is it secured to the building?
Is the structure itself bonded?

There is conduit on the outside of the building next to the water pipe and if I test between the two I'm getting 0.12 ohms.

IMHO testing between the two outside the building isn't particularly conclusive.
There are a number of reasons you could get that reading, the conduit could be bonded or being used as a CPC, it could be touching the metal structure that is bonded; the copper water pipe could be bonded the other end (where it came from).

Your concern is limited to exposed metal structural parts or metal things that pass from outside to inside giving you a second version of 'earth' inside the building. See 411.3.1.2

 

[bennie]

If it enters the building in plastic it isn't an extraneous conductive part and doesn't need bonding.
Does the conduit enter the building or is it secured to the building?
Is the structure itself bonded?

IMHO testing between the two outside the building isn't particularly conclusive.
There are a number of reasons you could get that reading, the conduit could be bonded or being used as a CPC, it could be touching the metal structure that is bonded; the copper water pipe could be bonded the other end (where it came from).

Your concern is limited to exposed metal structural parts or metal things that pass from outside to inside giving you a second version of 'earth' inside the building. See 411.3.1.2

I just wasn't sure, if the reading isn't low enough between the two points that I said wouldn't that be an issue? The issue is the conduit has the tails to the DB in it. An SWA comes out the ground next to the water pipe and is terminated in to a conduit box which has connectors inside. It's a two core SWA so the conduit is being used as the CPC.

 

[timhoward]

I haven't spent hours poring over the regs thinking about this but my thinking is:
The conduit is an exposed conductive part and it is already earthed.
I don't think a copper pipe exiting the ground outside and then become plastic is deemed to be an extraneous conductive part
as you are standing on the same earth that it is coming from. 411.3.1.2 says "within a building"
I'm therefore currently struggling to think of any regulatory reason you would bond the copper (and from your measurement I suspect it is already the other end) or add supplementary bonding between the two.
If everything was inside a bathroom it would be a different story.

(I know you aren't supposed to be able to touch two different earthing systems simultaneously but the copper pipe isn't deemed to be one as it's not part of the electrical installation even though it's effectively the same as an earth rod.)

 

[bennie]

I haven't spent hours poring over the regs thinking about this but my thinking is:
The conduit is an exposed conductive part and it is already earthed.
I don't think a copper pipe exiting the ground outside and then become plastic is deemed to be an extraneous conductive part
as you are standing on the same earth that it is coming from. 411.3.1.2 says "within a building"
I'm therefore currently struggling to think of any regulatory reason you would bond the copper (and from your measurement I suspect it is already the other end) or add supplementary bonding between the two.
If everything was inside a bathroom it would be a different story.

(I know you aren't supposed to be able to touch two different earthing systems simultaneously but the copper pipe isn't deemed to be one as it's not part of the electrical installation even though it's effectively the same as an earth rod.)

Im glad you see my struggle with this, it seems a bit of a grey area. I don't wont to sweep it under the carpet so it may go down as a limitation as I can't visually verify the bond but give the reading i have got . I think my rear end will be covered.

 

[timhoward]

Im glad you see my struggle with this, it seems a bit of a grey area. I don't wont to sweep it under the carpet so it may go down as a limitation as I can't visually verify the bond but give the reading i have got . I think my rear end will be covered.

Unless you can come up with a regs based reason that you think it should have a bond I'd be inclined to say no more about it.
The pipe isn't extraneous if it's outside, and it's insulated before it goes inside. I think all your test showed is that the conduit is a CPC and the copper pipe is bonded somewhere or has another link to the electrical installation e.g. via a boiler.

What can actually go wrong? If the conduit becomes live then ADS should operate. ( Even if the pipe became live your test suggests ADS would likely occur as the path back to the MET will be less than 0.1 ohms so at least 2300 amps would flow!)

 

[bennie]

Unless you can come up with a regs based reason that you think it should have a bond I'd be inclined to say no more about it.
The pipe isn't extraneous if it's outside, and it's insulated before it goes inside. I think all your test showed is that the conduit is a CPC and the copper pipe is bonded somewhere or has another link to the electrical installation e.g. via a boiler.

What can actually go wrong? If the conduit becomes live then ADS should operate. ( Even if the pipe became live your test suggests ADS would likely occur as the path back to the MET will be less than 0.1 ohms so at least 2300 amps would flow!)

Ok. Thanks