to bond or not to bond

[leep82]

I know its been done to death but in going to ask for your thoughts opinions anyway.

Just turned up on a job that ive been on for a while where i have had no communication with the plumber. Everything has been through the customer. The plumber has fitted a new central heating system with a boiler that is oil fed. The bund for the oil sits on a concrete pad and has been in for weeks. I was told by the customer that the pipe to feed the boiler would be surface fixed and of plastic, hence, no need for any bonding. Standing looking at it now and the pipe has been buried for 2/3 mtrs and although it has a thick plastic coating is quite clearly a copper pipe. Is the plastic coating enough to suggest that bonding may not be necessary?

 

[telectrix]

test it to see if it's extraneous. yo may well find that the pipe is bonded at the tank.

 

[Richard Burns]

It is possible that a metal tank on a concrete base may provide enough of an earth connection (especially once it has been the for a while) to test as extraneous. Though if the tank is plastic (as they mainly are now) obviously not applicable.
Damage to the plastic sheath over time in shifting underground material could also permit an earth reference.
However at the present time Tel's advice is the best.

 

[leep82]

Thanks guys

 

[leep82]

Quick update, following on from the advice i tested to see whether or not the pipe was extraneous and it seems it is. I had to run out a wander lead as the part in question is some way from the DB. I disconnected the main earth conductor and then performed an IR test at 500v between my main earth conductor and wander lead. My reading was 0.00Mohms, and so less than 0.02Mohms which would deem the pipe not extraneous. Is this right?? The boiler is all connected and so could i be getting a parallel path through the already bonded water?

 

[Richard Burns]

The test you are doing is to see if the oil pipe is electrically connected to earth and so could introduce an earth potential into the house.

If there is limited electrical resistance (less than 23kΩ) then the pipe is connected to earth and so it is extraneous and requires bonding back to the MET.

There is some consideration that if the resistance is less than 1667 and the installation is fully 30mA RCD protected then it may not need bonding but this is not reflected in the regulations.

If you get a very low reading such as 0.00MΩ then it is easy to change to low ohms resistance testing and if the value is say 0.01Ω to 6Ω then this is very likely to be a connection via a parallel path via other bonding or circuit cpcs and therefore giving you a false reading.

If the pipe is connected to earth then you can think of it like an earth rod which, if just placed in the ground, is likely to give readings from about 50Ω to 2000Ω though there is wide variation possible.

If the pipe is only tenuously connected with earth then the reading is likely to be in the hundreds of MΩ.

if you tested to the disconnected main earth conductor and the bonding is not connected at the service head then you should not be able to get parallel paths from the water bonding or from circuit cpcs.

 

[davesparks]

if you tested to the disconnected main earth conductor and the bonding is not connected at the service head then you should not be able to get parallel paths from the water bonding or from circuit cpcs.

Actually you could get a parallel path I think. If the oil pipe being tested is connected to the boiler, which the water main is also connected to, and if the water main is itself extraneous, then the oil pipe will test as extraneous via the water pipe.
But of course this is getting in to the realms of ridiculousness as far as practicality goes.

 

[leep82]

if you tested to the disconnected main earth conductor and the bonding is not connected at the service head then you should not be able to get parallel paths from the water bonding or from circuit cpcs.

Actually you could get a parallel path I think. If the oil pipe being tested is connected to the boiler, which the water main is also connected to, and if the water main is itself extraneous, then the oil pipe will test as extraneous via the water pipe.
But of course this is getting in to the realms of ridiculousness as far as practicality goes.

This is what i was thinking, there is no way to check the oil pipe on its own as it is connected to the boiler along the the water

 

[davesparks]

This is what i was thinking, there is no way to check the oil pipe on its own as it is connected to the boiler along the the water

It's a fun game isn't it!

 

[yellowvanman]

If there is limited electrical resistance (less than 23kΩ) then the pipe is connected to earth and so it is extraneous and requires bonding back to the MET.

I notice in the May edition of the Professional Electrician & Installer the NICEIC are saying its NOT extraneous if the resistance is > 7.67kohm

 

[Leesparkykent]

I notice in the May edition of the Professional Electrician & Installer the NICEIC are saying its NOT extraneous if the resistance is > 7.67kohm

yes I noticed that....got a copy in front of me now.

 

[Pete999]

Yep I wondered about that, can anyone enlighten me?

 

[davesparks]

Yes well the NICEIC do like to print a lot of bog-roll

 

[Richard Burns]

If we accept 30mA is safe for RCDs, then 30mA should be safe for bonding as well, I suppose.
Though it always assumes that the protective device operates.

 

[davesparks]

If we accept 30mA is safe for RCDs, then 30mA should be safe for bonding as well, I suppose.
Though it always assumes that the protective device operates.

Eh? I'm sure I'm missing something obvious here, but how is the presence of an rcd going to assist with the bonding?