Just a thought on gas/water bonding.

[Darkwood]

Just installing some 50mm earth bonding to Gas and Water supplies in a factory, now the factory only has small service pipes for both as they are only really needed for office and works toilets so we have a incoming similar to domestic, im understanding the need re' incomming N size and PME systems and earthing must be large enough to cope with circulating currents that can form between other installations etc....my head scratcher is that to cope with these possible currents my earth to the gas has to be 50mm (N =185mm)but the gas pipe itself has nothing like the copper content or current carrying capacity of the earth im attaching to it, couldn't this cause rise to in extreme circumstances an overheating of the main gas line?

 

[RyanCheetham]

I think you may be overthinking the problem lol but yeah it does make you wonder

 

[Darkwood]

Its happened and took out a block of houses, well according to reports the recent video of firefighters tackling a blaze at some houses after sub station cable theft ... the over voltaged domestic equipment started the fire but the explosion which occured in each house within seconds of each other had everyone scratching their heads... it turned out that a fault current in the PME system ran through the gas pipes overheated them causing multiple explosions across the properties.... other properties were not on fire at the time but still blew out.

Not over thinking its a real hazard how can bonding with such a large cable to a pipe with a fraction of the copper content be safe in say a DNO break in the N/E supply, the high current will try bridge the gap to the next point of earthing which could be through the gas or water of an adjacent building hence the requirements for larger earth bonds in PME systems.

 

[RyanCheetham]

Ok maybe overthinking was the wrong word. But you can't upgrade the gas pipe, so what else can you do in this situation?

 

[Darkwood]

Nothing it was just posted as something to warm the grey matter up, the house example wasn't a true reflection as in this instance the N/E tap in the sub station had been nicked so voltages and currents not normally found were created but it does show you what currents flowing through the gas pipes can do with regards to large PME supplies and domestic size gas and water services.

I suppose they are trying to ignore the risk as its possibly small and the service for gas and water is slowly been converted to plastic.

 

[Deleted member 9648]

All the technicallities aside....clamping a cable the size of a hosepipe to a 15mm water main serving a sink and bog right next to it does seem the daftest thing in the world,and probably is.

 

[Jdsparky]

You could always not bond it at all if your that worried

 

[Knobhead]

Given the wall thickness of BS copper pipe for 15 & 22mm is 0.8mm.

π x (Do/2)² - π x (Di/2)² = CSA mm²
Where:
Do = outside diameter
Di = inside diameter

Therefore:
15mm pipe = 35.69mm²
22mm pipe = 53.28mm²

 

[Darkwood]

lol... you got too much time on your hands Tony ...Its Saturday night!

But it does confirm 15mm wouldn't carry the csa through but never seen any info regarding the situe.

 

[telectrix]

i'm OK . our pipes are lead with walls 1/4" thick, but insides are probably more furred up than my arteries.

 

[Engineer54]

Just installing some 50mm earth bonding to Gas and Water supplies in a factory, now the factory only has small service pipes for both as they are only really needed for office and works toilets so we have a incoming similar to domestic, im understanding the need re' incomming N size and PME systems and earthing must be large enough to cope with circulating currents that can form between other installations etc....my head scratcher is that to cope with these possible currents my earth to the gas has to be 50mm (N =185mm)but the gas pipe itself has nothing like the copper content or current carrying capacity of the earth im attaching to it, couldn't this cause rise to in extreme circumstances an overheating of the main gas line?

One would hope that the protective devices incorporated in a system supplied by a 185mm incomer would limit any fault to a few mS, thereby limiting the rise in temperature. The full fault current wouldn't only just be taking the path of the gas bonding either, it will be taking all the bonding paths that are connected to the MET.

 

[Darkwood]

One would hope that the protective devices incorporated in a system supplied by a 185mm incomer would limit any fault to a few mS, thereby limiting the rise in temperature. The full fault current wouldn't only just be taking the path of the gas bonding either, it will be taking all the bonding paths that are connected to the MET.

You are only relating your post in a fault condition ... if this was the case then the max size of bonding would be 25mm .. the issue with PME systems is you can get circulating eddie currents within the earthing system between your installation and others without fault conditions... hence the raised bonding conductor sized against TNS or TT ... The earth bonding is 50mm in my situe for this very reason but the supply for the water and gas services dont meet the ccc of the conductor attached to them....hence my thread.

Its not just about a fault but in a DNO N/E break the multiple earthing may re-route through the equip' bonding to return to the next earthing point its not a S/C fault current but could be 400+ amps with which a gas pipe at 15mm would overheat .... and kaboom!!!!

 

[Knobhead]

lol... you got too much time on your hands Tony ...Its Saturday night!

But it does confirm 15mm wouldn't carry the csa through but never seen any info regarding the situe.

You asked a question, I answered.

I could have gone in the stupidity of BS7671 and the earthing requirements for services. The IET don’t account for maths and physics in their pontificating. IE, what is the point in running a 10mm alongside a 22mm gas pipe to give an inferior earth bond at 18.67% conductivity of the pipe.

 

[Engineer54]

You are only relating your post in a fault condition ... if this was the case then the max size of bonding would be 25mm .. the issue with PME systems is you can get circulating eddie currents within the earthing system between your installation and others without fault conditions... hence the raised bonding conductor sized against TNS or TT ... The earth bonding is 50mm in my situe for this very reason but the supply for the water and gas services dont meet the ccc of the conductor attached to them....hence my thread.

Its not just about a fault but in a DNO N/E break the multiple earthing may re-route through the equip' bonding to return to the next earthing point its not a S/C fault current but could be 400+ amps with which a gas pipe at 15mm would overheat .... and kaboom!!!!

That's me not fully reading your OP ..lol!!

Again, the 400+A fault current, will not All be going through a single point, (the gas pipe) it will be shared by ALL the parallel earth paths. This is another reason why i always advocate installing a good local earth rod on PME supplied systems connected to the MET, it will go some way in alleviating such stresses in these conditions!!

 

[Darkwood]

The recent vid of the houses blowing out after the N/E tap was stolen from the sub station was investigated and although not the initial cause of the fire (which was started by overvoltages) the actual explosions which hit multiple premises within seconds of each other although individual was attributed after investigation to fault currents heating the gas pipes up, this shows the senerio can occur although this was sabotage it highlights the dangers of having a gas pipe which cant handle the current, and this is where i have my worries; Network circulating current (diverted neutral currents) may see high long duration current flow through the bonding system and yes it will possibly share the load over the whole bonding system but senerio's of say a plastic water mains and only gas bonding needed could see this current solely running through gas line and if its not large enough its going to get very hot

 

[Engineer54]

I really don't understand why all PME installations don't have a local low value earth rod connected to the MET as a requirement from the DNO's. Most, if not all PME installations are installed this way, on the continent. Voltages and Stresses would be greatly reduced in the case of these types of faults, if this would be the case...