Earth bonding a mix of plastic and copper.

[huw169]

This is an electrical question, so please bear with me.....

I am currently re-plumbing my ground floor. Incomming mains will be MDPE to copper to plastic stop cock to copper to boiler. Then copper from boiler going to plastic underfloor and finally copper up through the floor to each rad.

How much of it all needs earth bonding?
I'll be doing the copper to the boiler and the gas main obviously, but do i need to do each rad as they will be insulated from the boiler by the plastic pipe?
Thanks
Huw

 

[ericmark]

Where the copper is just local to appliance for show and plastic is used between each appliance you do not need to earth the pipes.

Back to basics the reason we earth pipes is so if something goes wrong in one place the result will not effect another place. For example if a standard lamp fell and the line conductor was in contact with a radiator then that radiator would become live. Which would likely be obvious but if it was connected with metal pipes to a radiator in another room that would also be live but in this case you would not have any obvious signs to show it may be live so earthing the system would ensure the protective system would open.

So if you join any two items with metal pipe you should earth the pipe but if every item is isolated from each other there is no need to earth them.

With bathrooms if wired to 17th Edition i.e. all items in bathroom including lights are protected by a 30ma RCD then same applies. If older system then bonding is still required between all metal objects.

P.S. Do be aware in older houses sometimes the water pipes are only earth. This is not permitted with regulations but it does still happen. Make sure the house is earthed and your actions have not removed the main earth.

 

[huw169]

Thanks for clearing that up for me.

Good point about the earth - i recall an earth cable coming from my incomming electrical supply to a connector block, but not looked at it that closely yet. Will be tonight though.
House was built in 1935 so not 'really' old.
Thanks again
Huw

 

[rebuke]

I'm not an expert by any means, so I might be completely wrong here - but is there not a chance that although it's in plastic pipe, the water itself might conduct to some extent?

 

[LiamPope]

I'm not an expert by any means, so I might be completely wrong here - but is there not a chance that although it's in plastic pipe, the water itself might conduct to some extent?

To some extent, but not as much as you might think.

http://www.hep2o.co.uk/bititesguideearth.htm

 

[ban-all-sheds]

House was built in 1935 so not 'really' old.

Easily old enough for the water supply pipe to have been used to provide the earth...

 

[ericmark]

I'm not an expert by any means, so I might be completely wrong here - but is there not a chance that although it's in plastic pipe, the water itself might conduct to some extent?

In fresh water you can get enough voltage differential to electrocute someone swimming and one should not swim to and fro a boat connected to shore power in fresh water. However in salt water there is no danger as it is such a good conductor.

ρ for fresh water is 0.5 - 500
ρ for copper is 0.000000017

15mm dia = 176mm² so 1 meter is around 60Kohms or more so with 1 meter of 15mm plastic pipe you would likely be safer not connecting an earth than to connecting to an earth.

I am unsure of the resistance when ferric oxide is held in suspension so I would guess at more like 3 meters on central heating systems. I stand to be corrected!

The problem is with plastic fitting which are too short to isolate fully and when using plastic fittings to join copper pipes then I would link across between the two with at least 4mm² earth cable.

 

[matt1e]

House was built in 1935 so not 'really' old.

Easily old enough for the water supply pipe to have been used to provide the earth...

Agreed

 

[huw169]

Thanks fellas.
I'll be checking that closely!

 

[ban-all-sheds]

Bonding & plastic pipes