Towel Rail Bonding

[sirsnortalot]

Hi Guys, looking for a bit of advice as I think I'm starting to confuse myself with this one!

We have just finished a new bathroom for a client and I was informed by the plumber everything would be piped in plastic. It turns out however, that the towel rail is on old pipes and he has no idea where they go or what they consist of.

All circuits in the bathroom are rcd protected, as far as I'm aware the main bonding is in place, a 10mm earth can be seen leaving the consumer unit and testing between the pipework at the boiler back to the met gives 0.02ohms. I'll try and locate the connections tomorrow. Everything else in the room is piped in plastic.

What at I can't understand is that testing the rail back to the met gives around 189ohms. I'd have thought if it were plastic, it would have been miles higher and if copper, then the same as the heating pipes around the boiler?
Having disconnected its electrical element, testing between the rail and the earth terminal on the shaver point and also to the shower give the same 189ohms so sub 1667 and in my mind fine. The rail is pretty much
Am I thinking about this the wrong way or does the rail in fact maybe need supplementary bonding?

All thought are appreciated!

Graeme

 

[Midwest]

The metal towel rail (forgetting the electrical element connection for the moment), is not the extraneous conductive part but potentially, the copper pipes connecting to it, are. That is what you should test to. You could conceive that as the towel rail has an electrical element, that it is an exposed conductive part. However, as the element is a sealed unit and inserted into said towel rail, the towel rail is not an exposed conductive part. The high resistance, is probably due to the ptfe tape around the gland of the element. Test the pipes to see if they are extraneous or not.

 

[sirsnortalot]

Cheers for the input bud, my apologies as I should have given a bit more detail in my first post. All readings taken from the rail were taken on the chrome pipe tail as it comes out of the wall. Definitely shouldn't have been any thread tape causing the issue.
I even tried scraping the pipe a bit underneath to try and make a better contact but it didn't make any difference to the numbers..

 

[Midwest]

Your never get a very good reading off a chrome pipe. If you've not seen it, here's a very good vid by Christopher Kitcher on the subject;
https://www.youtube.com/watch?v=v38PfIVy2rI

 

[sirsnortalot]

That is an excellent video, cheers for the link

Looks like I was indeed thinking about it the wrong way... As the measured value between a known earth in the room and the rail was well below 1667 ohms, no additional bonding is required. Hope I have that right!

Thanks again for your help on this.

 

[Midwest]

Yep sorted then, alerted to the vid by another member, can't remember who, so can't take the praise there. Another one by John Ward on earthing & bonding, that's very good as well.

 

[Lucien Nunes]

I'd have thought if it were plastic, it would have been miles higher and if copper, then the same as the heating pipes around the boiler

One possible cause of a middling figure is a branch added to the copper pipework with plastic tees. The metallic continuity has been broken but there's only the resistance of half an inch of water column inside the tee between the two sides. Still, 189 ohms seems low, I don't know what the presence of inhibitor does to the resistivity of water though.

 

[Deleted member 26818]

Just about any impurity in water will lower resistance.