gas pipe sparking!!

[Edd]

Sent to a property where there had been a small fire! They have had a new 17th CU fitted a lighting rewire and all Bonding brought up to Regs.
I get a call saying the plumber has gone to connect the gas meter to the incoming pipe and a spark was seen.
I test between MET and incoming gas pipe and show 10V. Ze is 0.23 and all seems OK. Thinking its just the gas fitter over reacting and thinking it just requires bonding so everything is the same potential, the little old dear shows me this.
After being shown the picture below I took a cable from the MET to the incoming gas and touching on the pipe there was a substantial spark almost like sticking a wire across a small car battery. I pulled the service fuse and still there was 10V and a big spark was created. At this point I was at a loss the fault was external and I passed it on to the Gas and DNO. Any Ideas?

 

[Plonker 3]

What is that pipe? Is it extraneous?

 

[Guest111]

For a spark like that theres obviously an issue somewhere,did you do a Ze by chance? just wondering if it could be a bad joint on the incomer,if that was the case and the cable fed more than one property then that could possibly cause what you are getting if it was a TNCS earthing arrangement.

 

[tazz]

First of all the pipe needs cleaning, before bonding.....the paint a dirt, do not conduct.

 

[Lucien Nunes]

...And then we can have the discussion about BS951 clamps on lead.

What exactly is the earthing configuration?

 

[driverman]

For a spark like that theres obviously an issue somewhere,did you do a Ze by chance? just wondering if it could be a bad joint on the incomer,if that was the case and the cable fed more than one property then that could possibly cause what you are getting if it was a TNCS earthing arrangement.

Op said Ze was 0.23

 

[davesparks]

Well that clamp shouldn't be on lead pipes should it?

 

[Edd]

That's the old lead pipe, that caused the fire.. She walked past it when it was apparently flaming out like a blow torch. Loop at mains was 0.23. The sparking 10v was still there with service fuse out. TNS earthing...

 

[davesparks]

Why pull the service fuse out? The main switch is a much better option for switching the installation off!

 

[Edd]

Why pull the service fuse out? The main switch is a much better option for switching the installation off!

Because I can! And I also switched the main switch off I also disconnected the earth system just to make sure I wasn't missing something...............

 

[davesparks]

Because I can!!

But to what end? What did you hope to achieve by doing this that couldn't be achieved by switching off at the main switch

 

[Deleted member 9648]

It's not unexpected that a TNCS system neutral current wll partly divert down a bonded metallic service that is at earth potential, even on a healthy system. However in this instance I would be looking for a high resistance connection somewhere on the supply neutral,probably in the head,which would result in a substantial neutral current down bonding wires.

 

[Lucien Nunes]

He reckons it's TNS

 

[Deleted member 9648]

He reckons it's TNS

Oops....missed that. Still possible that its a N current if there is a N-E fault somewhere and a high resistance on the supply N.

 

[Knobhead]

This is a post I made in 2010

This reminds me of a fault I had many years ago in my own house. I’d just installed gas central heating, the very last job for me to do was connect the gas pipe to the gas meter. Touched the two ends of the pipe and got a spark! Not good thought I, well to hell with it jammed the pipe together and soldered the joint. On putting a clamp meter round the pipe found 25A flowing through it Turned off supply to my house, still 25A, I’m getting confused at this point I’m confused, the wife is frantic, no gas or electric now! Phoned electricity board, and after speaking to one of their engineers was told to keep both supplies turned off, they’ll get back to me. Not 2 mins later phone rings, we’ll be there in 20 mins! 4 vans and 2 overhead line wagons showed up! I did live in a rural area so supplies to the road were overhead. Much head scratching followed, at one point we disconnected the gas meter and slung that out in the back yard. In all seriousness this was the point the gas meter reader showed up It was decided to disconnect the entire road from the supply. By this time I’m not the flavour of the month with the entire road, we’ve now got 12 houses with out supply. The OH crew found a burnt line tap on the neutral. Further tests showed a neutral earth fault some where in one of the 12 houses. As I knew all the neighbours I got delegated to got to each house and isolate the supplies, how come this ended up as my job? Eventually there’s a shout from the OH crew as I isolated the furthest house on the road, “faults gone”. Electricity board senior engineer then goes in to “Hitler” mode, tells my neighbour “unsafe installation your supply will be disconnected until rectified”. While the OH crew are repairing the connections to the road I hijacked the apprentice that had come with the board and checked through the house. The apprentice found the neutral fault in a socket, the facia screw had bitten in to the core. OK now neighbours supply can be reconnected as well.

Now think about it the neutral for 12 houses was going through one 3.5mm screw. The house was bonded to the water pipe only. The current travelled along the water pipe until it got to my house, I’d just rewired and fitted CH. So it was a case of if it doesn’t move bond it, the current then went through my house and out through the gas pipe and nest. (I’ve never understood why a steel gas pipe is a better earth than lead water pipe, but it always is)? Just how in the name of god my neighbours house didn’t burn down I’ll never know. The fault must have been there for years.

Part of the reason for the electricity boards panic was there was a government fuel storage dump about 100m from the road. They use cathodic earth bonding which can cause fun and games. But had the fault been from there it would have been DC and not shown on my meter.

 

[Knobhead]

Looks like there's a N→E fault and a dodgy neutral somewhere.

 

[Edd]

Thanks Tony, just for people's intrest, it's a 1920's mid terrace house with underground mains, the Earth strap to the sheath is a nice new braded type. And as for pulling the main service fuse, I was at a loss, so why not pull it? I may have missed something really stupid. But really this is one of the reasons I stoped posting here..

You post an interesting fault about a fire and sparking earth, and all someone's intreste in is that I pulled the service fuse... Really helpful in the question I was asking NOT!!!

 

[Deleted member 9648]

Looks like there's a N→E fault and a dodgy neutral somewhere.

I already said that and didnt get a flickin thanks!.....:mad2:....

 

[Edd]

I already said that and didnt get a flickin thanks!.....:mad2:....

sorry, I've had many N-E faults and loose N on PME systems.. Just lost me with a TNS system and supply to property off (OMG I pulled the fuse as well) and still got a low voltage but high current fault.. I'm still trying to picture Tony's explanation in my head, but I'm getting there...
Thanks
Edd

 

[Newbyspark]

Saw the same thing the other week. Loose bonding connection on gas was sparking, over how ever long it was doing it it melted through gas pipe and ignited gas. Dno came out and even with main fuse out it was still sparking. They were scratching their heads a lot. In the end they think it was a fault in another house but as this house had probably better bonding, the fault was coming down this bonding. Probably would never known if the connection hadn't been loose. No one hurt, but very lucky