main bonding to external boilers

[nick611]

no i want to do it but clients wonder why you want to run an earth wire right through their house or right round it when all they have done is move their boiler

 

[Deleted member 9648]

The pipes may or may not introduce an earth potential.The fuel supply to the boiler may have a bearing.If a metallic oil or gas line from an external source feeds the boiler and does not enter the building then that service would not require main bonding.However the pipes from the boiler into the building may intoduce an earth potential via the fuel service and so would require main bonding at the point of entry.
If the fuel service originates from within the building and is main bonded within the building,and the boiler is on a concrete plinth,then the pipes may not introduce an earth potential and so wont require bonding.
IMO though it would be difficult to establish that the pipes will never introduce an earth potential and so I always main bond the flow and return pipes at the point of entry if they are from an external boiler on the deck.

 

[jam2020]

always a pain when a client gets involved.tough one.

 

[nick611]

yeah real pain anyway thanks for you.re veiws im off for my tea.cheers fella

 

[nick611]

yeah its a tricky one is,nt it really got me thinking.they should put rubber feet on the boiler,save us all some grief

 

[jam2020]

yep,i can see both sides of the argument, end of the day the buck stops with you, as always

 

[WayneL]

Hi,

You can always test the pipes to see if they're extraneous and require bonding.

Rcp> (Uo/Ib) - Ztl


Where:

Rcp is the resistance between the conductive part and the MET in ohms


Uo is the nominal voltage to Earth in volts.(230)


Ib is the value of current through the body in amperes that should not be exceeded. (I would err on the side of caution and use 10 mA)

Ztl is the impedance of the human body in ohms. The value suggested in DD IEC/TS 60479 is 1000 ohms where Uo is 230 V (50 Hz) under dry or wet conditions.

So,

Rcp = (230 / 0.010) - 1000

Rcp = 22,000 ohms (22 kohms)

So, if the resistance between the pipes and the MET is above 22 kohms, they would not be considered extraneous and would not require bonding.

The NICEIC recommend using a 30mA current, which would give you a threshold of 6667 ohms (6.7 kohms) and say that above this wouldn't require bonding - but I'd sooner be a bit more cautious

So, bottom line, if you don't know if it requires bonding...........test it.

http://www.electriciansforums.net/e...ical-regulations/16558-extraneous-yes-no.html

 

[sjm]

Hi WayneL. Very useful post.
When you say test to the MET do you mean with only the main earth conductor connected or with bonding conductors connected as well (I presume that all cpcs are removed to avoid confusion). I ask because if the pipe is connected to another pipe which is bonded you would get a resistance value below 22 kohms. If that was the case what value of resistance would be low enough to not have to bond the pipe.

 

[WayneL]

Hi SJM,

I know the point that you are trying to make.

I think if it's already bonded (indirectly), then you would expect the reading to be really low - you'd be looking for the negligible 0.05 ohms - which would mean that it was already part of the 'equipotential zone'.

I think this test is more for 'stand-alone' metalwork or pipes that you can't decide whether it's extraneous or not.

Could be an idea to do what you said and remove other bonding from the MET first, so that you are just testing the particular pipe that you are interested in - otherwise complex installations could give dodgy readings from parallels.

I personally haven't had to put this to the test - with the jobs I've done it's been pretty obvious on what needs bonding and what doesn't.
I might have a little play with my meter when I get some spare time and see what kind of results I get.

 

[Deleted member 9648]

As above.....testing to establish whether or not an incoming service is an extraneous conductive part is much harder in practice than theory due to parallell paths from connected pipes/wiring.
The only way in practice would be to test the pipes coming from an already installed boiler into the building before they were connected to any other pipework within the building.......not likely to be an option.

 

[sjm]

Thanks for the reply WayneL, pretty much what I thought then. I suspect the test is more directed towards new installations and therefore existing installations give rise to problems, as wirepuller has posted above.

 

[blackiel]

Does anyone agree...Regardless of the regs..If you run a separate CPC and bond all metalwork,then crossbond within the premises,you have covered yourself...bearing in mind,..PME/TNC-S is a money saver for distribution companies...All Earths should be independent,even if connected to Neutral at the Star,they should have their own independent means of flow and escape,without jeapordising or Infiltrating any part of the system..Safety first!!!!!

As said previously...You can always test between main earth and all other pipework/extraneous to see if your continouity bleeps!!!!

 

[Deleted member 9648]

Well you could....but you could make youself a lot of unecessary work as well.
Main bonding is to incoming services is essential,supplementary bonding in special locations (only as defined in 7671) may be required....anything else is a waste of time and money.