New cable crossing existing gas pipe - pg18 of on site guide unhelpful

[mrstitt]

I am proposing to run two T&E cables past an existing surface mounted gas pipe (22mm).

pg18 of the on site guide says that "Seperation of at least 25mm to be provided for domestic pipework up to 35mm. For pipework over 35mm then 50mm separation is required. The separation distance can be reduced if the gas pipe is PVC wrapped or a pane of insulating material is interposed.

I am therefore considering using PVC conduit to run the new cabling. I can obviously bend this such that I know I've got 25mm of clearance from the gas pipe where it bridges - and restrain it on either side. But its going to be very proud from the wall and hence not look very tidy.

Alternatively I could consider the PVC conduit to be a "pane of insulating material" and then only give a nominal clearance? but the OSG isn't as clear as it could be, and to me the word pane doesn't remind me of round conduit. I guess that the regulations are trying to prevent cables from rubbing against piping and causing damage to the cable - a phase conductor repeatedly arcing to a copper gas pipe would eventually cause a hole in the pipe and end in tears - or more likely would cause the RCD and/or MCB to operate.

I could obviously ask a gas engineer to visit and wrap the pipe with PVC, as it wouldn't be practical to do this without removing the pipe, so would have to be done by a gas safe engineer.

Anyone else got a view on this, I've searched high and low but surely has pipes and electric cables crossing paths is a common problem?

 

[JohnW2]

Alternatively I could consider the PVC conduit to be a "pane of insulating material" and then only give a nominal clearance?

FWIW, that's probably what I would do (if anything!).

I guess that the regulations are trying to prevent cables from rubbing against piping and causing damage to the cable - a phase conductor repeatedly arcing to a copper gas pipe would eventually cause a hole in the pipe and end in tears - or more likely would cause the RCD and/or MCB to operate.

Your guess as to the thinking behind it as good as mine! If the cable is clipped (or whatever) properly, there is not going to be any 'rubbing', and, in any event, T&E provides two layers of insulating material between the live conductors and the outside world. I don't personally think that it is anything to get too excited about!

Kind Regards, John

 

[stillp]

The OSG does not contain regulations, so you can do what you propose.

 

[Taylortwocities]

I wouldn't worry too much about it.
Look under any gas boiler and you'll see cables and gas pipes in very close proximity!

Unnecessary bonding too!!

 

[flameport]

Never understood where the idea of bonding under a boiler came from.
Even if it was necessary (it isn't), all the pipes go into the metal boiler just a few inches above so are already electrically connected together.

 

[JohnW2]

Never understood where the idea of bonding under a boiler came from.

It's not only under boilers - I've seen plenty of cases of a whole pile of pipes rising through an airing cupboard being bonded together.

Even if it was necessary (it isn't), all the pipes go into the metal boiler just a few inches above so are already electrically connected together.

I suppose that in the minds of those who feel that such 'cross-bonding' is necessary, the existence of PTFE tape etc. might have something to do with that.

Kind Regards, John

 

[ban-all-sheds]

I could obviously ask a gas engineer to visit and wrap the pipe with PVC, as it wouldn't be practical to do this without removing the pipe, so would have to be done by a gas safe engineer.

Or you could buy some slit conduit and put that on the pipe yourself.

 

[ban-all-sheds]

Unnecessary bonding too!!

The person who installed my boiler did similar.

And ran the bonding conductor back to the MET, tie wrapped to the gas pipe....

 

[mrstitt]

I don't know where that bonding cable to the right is going (I presume to the main earthing terminal), but its ironic that all that bonding isn't going to the adjacent socket outlet. So basically won't protect someone who's using an appliance next to the boiler. So many plumbers seem to be equipotential addicts - without taking a second to think about the hazards that equipotential bonding is supposed to address.

Thanks for the comments on the gas pipe issue. I've not got a fully copy of BS 7671 to hand - my former employer did - but I don't remember this issue being mentioned in it either. BS 6891 apparently goes into more detail, this is obviously more about protecting the gas pipe then protecting the electrical installation. If there's any gas safe engineers out there who may shine more light on the issue, would be grateful.

I'm considering documenting it as a departure for the sake of belt and braces - and then justify the departure on the basis that PVC conduit is used. The conduit installation in the vicinity of the gas installation will be airtight to mitigate the risk of it filling with gas.

 

[Taylortwocities]

So many plumbers seem to be equipotential addicts - without taking a second to think about the hazards that equipotential bonding is supposed to address.

Probably because they do not know/understand the difference between bonding and earthing.

 

[JohnW2]

Probably because they do not know/understand the difference between bonding and earthing.

Most of the "plumbers' cross-bonding" I've seen has been true bonding, in that it has connected a number of pipes together (presumably with the intent of minimising pds between them) without any connection to 'earth'.

Kind Regards, John

 

[mrstitt]

Probably because they do not know/understand the difference between bonding and earthing.

Exactly, in the balance this installation would probably be safer unearthed then being earthed separately to its adjacent socket - or earthed but not bonded.

If equipment already has a high impedance path to ground, its normally safer to leave it that way such that nobody can get a nasty shock between different limbs. If its already got a low impedance path - such as a water pipe with a continuous copper run - then we need to think about bonding.

Many modern plumbing installations are largely plastic anyway. You need copper near the boiler due to thermal issues, but once its plastic then its basically high impedance. Tap water isn't very conductive unless you pore lots of salt in it. Not sure about the conductivity of water with central heating inhibitor in it - might take some water out of my central heating loop and experiment!

 

[JohnW2]

Thanks for the comments on the gas pipe issue. I've not got a fully copy of BS 7671 to hand - my former employer did - but I don't remember this issue being mentioned in it either. BS 6891 apparently goes into more detail, this is obviously more about protecting the gas pipe then protecting the electrical installation. If there's any gas safe engineers out there who may shine more light on the issue, would be grateful.

BS7671 certainly doesn't prohibit bonding conductors (or any other conductors, come to that) having joins - so joins in the main bonding conductor of a gas service would certainly not, per se, be non-compliant with BS7671. All BS7671 has to say about 'joins' (in any conductor) relates to special requirements if the join is not going to be accessible for inspection, testing and maintenance.

I know nothing of BS6891, but it would be rather odd if two different British Standards said contradictory things about was was acceptable as the main bonding of a gas service!

Kind Regards, John

 

[mrstitt]

My original question wasn't about bonding, it was about clearance.

I'll be using PVC conduit, otherwise if it was Galvanised conduit then perhaps it would be prudent to bond to the gas pipe.

It wasn't written in the on site guide for S&G purposes.

 

[RF Lighting]

No it wouldn't be prudent to bond it to the gas pipe.

Your plan with the pvc conduit is fine.