position of main bonding of gas & water pipes

[Adam Funk]

For many years, the bonding straps on our incoming gas & water pipes have been in the same place, relatively close to the consumer unit. The one on the water pipe (which runs roughly parallel to the supply cable) is probably within 600 mm of the cellar wall, but the one on the gas pipe (which runs diagonally across the cellar ceiling) was not. When we had the boiler replaced & the gas pipe enlarged earlier this year, the plumber added a bonding strap right over the gas meter with a 10 mm^2 cable back to the earth block beside the CU & the cutout.

I'm just curious: is there a particular reason why the main bonding needs to be specifically within 600 mm of the pipe's entry into the building, even if the cutout, CU, & pipe entry are all in the same room?

[Tim Watts]

IIRC the IET regs (17th) say "where practical" and "before any branches" (implied "must").

My gas bond is right over the meter outside because I expect a branch to be made externally should someone want to connect a gas cooker.

My water bond will be in the ceiling right over where the rising main comes up.

This is because:

a) "where practical" != "must".

b) Taking a 10mm2 wire down 2m of pipe which itself is soldered and runs all the way back up past said wire is pointless in this very specific case but it would be unwise to make a generalisation of that decision.

c) There is almost zero chance anyone could possibly want to make a branch off before that point.

d) If I followed the reg to the letter, I would have a relatively inaccessible bond clamp, at the back of a cupboard. Whereas my way, I will have a very accessible clamp in an attic space.

However, I appreciate it may confuse future people, so I will put a small sign next to the stopcock informing of the location.

Given the words "where practical" I do not feel it necessary to record as a departure on the EIC.

In your case, I suspect the plumber was doing what he felt comfortable with, but it sounds like there wasn't much wrong before.

[John Rumm]

Although it quite possibly was not 10mm^2 before...

[Tim Watts]

This is also a very good point -

[Brian Gaff]

Something to do with a lightning strike I was told once, but I'd be interested. Brian

[Adam Funk]

I just went downstairs to have a look & you're right --- the old one (still in place) is smaller (4 mm^2, I think). Did the cable size requirement change sometime in the past 20 years?

(Curiously, I see that the "sheds" only seem to sell 10 mm^2, at least now.)

[John Rumm]

They changed, I can't recall when though.

Its the typical minimum for any TN style install. Having said that, its too big for supplementary bonding.

[Johny B Good]

I believe it was the result of using the neutral as a substitute for the substation earth (steel wire armouring on the incoming cable) in a system named, afaicr, PME.

[John Rumm]

Its quite possible the upgrade happened around the time PME supplies started to become more common and the norm for new supplies.

(PME is also a TN style install - TN-C-S typically. The need for a substantial main bonding conductor is certainly greater when you consider some of the possible failure modes involving loss of a PEN conductor to neighbouring properties but not yours).

[Adam Funk]

Well, it doesn't hurt anything to use oversized cable for bonding, does it?

[John Rumm]

Its just rather chunky and difficult to work with. (not to mention expensive)

[ARW]

And remember when PME was first installed they used 6mm earth for the main earth.

[johnjessop46]

Gas Safety Installation and Use Regulations GSIUR if I recall correctly require connection to be within 600mm of the meter or point of entry to the building and before any branches.

[Tim Watts]

Was that a question?

The IET Regs 17th state basically that, with the 600mm being "where practical" - but the "before branches" is well stressed.

[Adam Funk]

Why not (if the cutout is some distance from the gas or water entrance) bond both branches close to the cutout rather than the unbranched pipe farther from the cutout? (I'm just wondering why it matters.)

[Tim Watts]

It would be perfectly reasonable.

The regs try to present prescriptive rules for 99% of possible scenarios, so if followed, one can say "it's good enough".

It does not preclude reasonable departures as long as the end effect is maintained.

[John Rumm]

By also encouraging the bond to be done close to the point of entry of the metalwork, you also minimise the chances that someone will at a later date make a change that in effect could leave a branch of pipework unbonded (say by using plastic pipe between the entry point at the bonding connection)

[Tim Watts]

Indeed.

Although, with the removal of the absolute need to have supplementary bonding, plumbers could be doing "bad things" by inserting plastic more or less anywhere.

So they really ought to be checking anyway.

In my case, there's sod all chance of a branch happening on the 2m between the c*ck and ceiling so I'm happy with that.

[John Rumm]

Then again, stick enough plastic in there, and it ceases to be "extraneous metalwork" anyway. (i.e. can no longer introduce a potential into an EQ zone)

[ARW]

Jumping off a wardrobe now has new meaning. Sorry Tim - I'll get my coat