Stray voltage on gas service line

[hbiss]

This is along the lines of this topic https://forums.mikeholt.com/showthread.php?t=195503 but I didn't want to hijack it.

This is actually a topic on another board. There are two attached town houses and both are fed from a single service line and regulator with a manifold supplying two gas meters. One meter for unit #1 and the other for unit #2. The outlet side of the meters go off to their respective units.

The problem arose when the owner of unit #1 was having a gas fireplace installed and the plumber, in disconnecting the gas line within the unit noticed a spark. This prompted him to tell the homeowner and call the gas company. Apparently there is an intermittent voltage of between 12-24V on unit #2s gas line after the meter was removed. I haven't been able to get an answer as to where that voltage was measured to from the gas line or what kind of meter was used. It very well may be that unit #1 us causing the problem. But the gas company did leave the meter for unit #2 disconnected and posted a turn off notice on the door indicating that the homeowner of unit #2 is to have an EC remedy the problem before they will re-install the meter. (It seems those people are out of town on vacation.)

My reply was that I don't think 12-24V would be unusual given that the gas lines in each unit is bonded to their respective neutrals and you are seeing much the same as the circulating currents on a common water service. I did say that I would check for a compromised service neutral but that's about it.

What do you think?

-Hal

 

[ptonsparky]

Sounds like the pilot was attempting a relight, or the burner itself with hot surface ignition

 

[LarryFine]

I wonder if a gas furnace control transformer could be the source of the voltage? Turning off the furnace circuits could be tried while observing the voltage.

Also, compare each side of the open gas connection to the electrical and/or water grounds to wee what is actually energized.

It could be that the incoming gas is at zero volts, and a houses GES is what is energized, possibly due to a poor electrical neutral.

 

[hbiss]

Sounds like the pilot was attempting a relight, or the burner itself with hot surface ignition

That's what the OP of the original thread said, I can't see how that could happen especially if the gas line is bonded to the GES. And how would that be a hazard anyway? I don't know where that theory came from. Probably because it's 12-24V. He also thinks his house would have blown up.

I'm sticking to my story about a possible poor service neutral for one of the units, particularly because it's intermittent, it varies in voltage and there is enough current to generate a spark. Other than that I believe it's normal.

-Hal

 

[LarryFine]

Other than that I believe it's normal.

Abby Normal?

 

[hbiss]

Abby Normal?

Dunno, is it? Take a look at this thread and tell me how metallic gas distribution would be different: https://forums.mikeholt.com/showthread.php?t=194935

Generally not a concern if you are an electrician and are not seeing any problems.

But it is a concern if you are a plumber. As mentioned above, common water piping between buildings can form a low resistance parallel path for neutral current. In some cases plumbers have experienced electric shock when cutting water pipes used as ground electrodes.

Current follows all possible paths, the majority of the current takes the lowest resistance path. A network of metallic municipal water piping may very well be just as low of resistance or even lower in some instances than the service neutral and I could see it easily carrying around same amount of current as the service neutral in that situation.

I believe we don't frequently see stray voltage problems on gas lines because gas companies use plastic and dielectric couplings.

-Hal

 

[LarryFine]

A gas piping system could become energized from one of the gas appliances. That's what made me think of a furnace transformer when 24v was mentioned.

I'm thinking of this as a bonding issue.

 

[hbiss]

Usually 24V control circuits float- one side is not grounded. However I would have to look at the wiring diagram of the specific piece of equipment to say that with certainty. But then there is the varying 12-24 volt issue that wouldn't fit that theory.

Also remember that this is all second hand information coming from a customer. I have a feeling that gas companies are overly cautious when they get customer complaints about perceived safety issues these days because of the incident in Mass.

-Hal

 

[ptonsparky]

Usually 24V control circuits float- one side is not grounded. However I would have to look at the wiring diagram of the specific piece of equipment to say that with certainty. But then there is the varying 12-24 volt issue that wouldn't fit that theory.

Also remember that this is all second hand information coming from a customer. I have a feeling that gas companies are overly cautious when they get customer complaints about perceived safety issues these days because of the incident in Mass.

-Hal

I’ve never put a meter on an disconnected gas line to see what the voltage was while the furnace was attempting ignition. Never entered my mind that I should. I’ve never put a meter on one conductor of an open paralleled connection. I suspect that voltage would be close to zero, but would still spark when connected. That leads me to believe that the gas line interior to the units is serving as a ground loop. I didn’t look back to see who already mentioned that. Open one end, you get the spark. The 12-24v would be from one of the furnace ignition. Maybe both.

Customer supplied information, as you know, needs to be taken with a grain of salt. “My husband was hanging these shelves when the circuit breaker started tripping.” was used with no further explanation required. Seldom the case.

 

[hbiss]

Neither have I, you never hear much about gas except for CSST in this trade. This is also the same as all those situations where there is sparking when connecting a cable box or the drop cable burned up. Many, including myself advocate an isolator before the cable ground block.

For gas lines there is this https://www.dresserngs.com/sites/default/files/pdfs/resources/pipeline-solutions/NGS-DPS-016.pdf

-Hal

 

[LarryFine]

Usually 24V control circuits float- one side is not grounded. However I would have to look at the wiring diagram of the specific piece of equipment to say that with certainty.

They started grounding the 'common' (opposite the red wire) of the 24v control circuit years ago.

 

[hbiss]

Ok. So if the gas line was not bonded and the ungrounded side of the igniter circuit faulted to the gas line it could result in the situation described. I see your point. But wouldn't the gas line within the (I'm thinking of furnace) be also grounded to the sheet metal by virtue of it's supports?

-Hal

 

[MAC702]

Usually 24V control circuits float- one side is not grounded...

The only controls I work with are HVAC. Our 24V controls are indeed grounded, usually the blue wire, so that would include a residential furnace.

 

[hbiss]

Can you think of any way it could put an intermittent 12-24 volts on the gas line? I can't.

-Hal

 

[LarryFine]

If one of the other T-stat wires became grounded outside the furnace.

As I said before, you can try killing circuits while monitoring the voltage across the gap.

 

[MAC702]

Can you think of any way it could put an intermittent 12-24 volts on the gas line? I can't.

Nope, but I'm not trying really hard, either.

 

[hbiss]

If one of the other T-stat wires became grounded outside the furnace.

As I said before, you can try killing circuits while monitoring the voltage across the gap.

But how would you explain the fluctuation? Certainly killing the power to the furnace, stove or dryer will either prove or put that theory to bed right away.

But given the facts presented the most likely cause is a high resistance service neutral in one of the units. We are essentially measuring the voltage between the neutral bus in the panel of unit #1 and the neutral bus in the panel of unit #2. Different loads in the unit switching on and off cause different voltage drops across the poor connection as referenced to the solid ground of the other unit.

-Hal

 

[LarryFine]

We are essentially measuring the voltage between the neutral bus in the panel of unit #1 and the neutral bus in the panel of unit #2. Different loads in the unit switching on and off cause different voltage drops across the poor connection as referenced to the solid ground of the other unit.

That's why a couple of us have suggested a neutral issue in one of the houses.

 

[JR Fox]

Bonding the gas line story!

Bonding the gas line story!

Years ago in the 1980s in NYC the gas utility installed isolation couplings before the meter in everyone’s homes & business service. This was to prevent electrolysispreventing erosion at the gas main. A couple years when I was working at JFK airport a contractor installing a lightning rods on the roof of a building and bonding everything also put a bond across the gas main isolation coupling. I called the gas company and there was this big meeting with the contractor, gas company and port authority. The contractor won the argument and the jumper bond remained across the isolation coupling.

 

[hbiss]

Lightning is unpredictable and with a system that you designed to carry the energy of a strike to ground, you want to take every effort to keep it contained on your conductors. You don't want the possibility of side flashes. Bonding the metallic gas service is like bonding CSST taken to a higher level. CSST can pinhole, I can see a large enough flashover burning a hole in iron pipe or the coupling.

-Hal