From an insert included with the latest gas/electric bill:
Keep Metal Objects Away From Your Gas Meter
It is important to keep metal away from your gas meter. PSE&G uses metal pipes that are protected from corrosion by a small electrical current running through them. This makes the pipes stronger and more efficient. If attached to these pipes, metal objects (such as a dog's leash or an antenna) strip the corrosion protection and cause problems with the electricity running through the pipes. When pipes become damaged, they may not work as well, and we may need to fix them.
This is called cathodic protection. It is a way of preventing corrosion on metals. It is a specilized circuit that some where has a sacrifical metal that is supposed to corrod (rust away) before the metal bieng protected. It's an intersting subject, but expensive to install and requires regular testing.
#38784 - 06/02/0402:28 AMRe: Electric Current in Gas Pipes?
Ahhh... This reminds me of something... A few AHJ's out here want gas to be bonded back to the panel, along with the normal things... I have (had... I gotta look for it again,) a formal letter from So Cal Gas Co. stating they DO NOT want their system bonded to the electrical grounding system... I get fun looks when I pull this out during inspections! A couple places want it anyways... Could this have something to do with that?? I always thought it was better if a hot wire hit a gas line that it didn't spark!
#38785 - 06/02/0406:09 AMRe: Electric Current in Gas Pipes?
Randy while the NEC does not allow gas lines as a grounding electrode the NEC requires the gas piping in the house to be bonded. Around here gas lines are threaded or welded black iron pipe.
So in the structure the gas lines are bonded.
I imagine they install a dielectric coupling at the meter in order to provide the cathodic protection.
250.104(B) Other Metal Piping. Where installed in or attached to a building or structure, metal piping system(s), including gas piping, that may become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 250.122 using the rating of the circuit that may energize the piping system(s). The equipment grounding conductor for the circuit that may energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.
Unless you are installing dielectric fittings on the gas line at the hot water heater the gas line ends up bonded anyway.
Bob Badger Construction & Maintenance Electrician Massachusetts
#38786 - 06/02/0406:59 AMRe: Electric Current in Gas Pipes?
This goes to different authorities with different (and conflicting) requirements. Metal piping systems in the house should be bonded, both IMHO and by NEC. But plumbers hate underground water pipe as the grounding electrode, the NEC prohibits the use of underground gas pipe as a grounding electrode, and clearly the gas company doesn't like it either.
IMHO the 'best' way to balance all of this is to have dielectric couplings in any underground metal piping systems where they enter structures, to bond this piping internally, but to explicitly not use this piping as a grounding electrode. This would help eliminate the problems of neutral currents flowing on metal piping, something that happens in urban areas where houses that use the same water pipes all bond them to neutral in their main panels; net result is multiple bonds between neutral and a continuous metallic system, and thus significant parallel paths.
This would require a change in the NEC, since an underground water pipe of sufficient length is _required_ to be used as a grounding electrode.
#38787 - 06/02/0407:17 AMRe: Electric Current in Gas Pipes?
OK, a dielectric at the meter makes sense. However, the pipe that enters the meter is coming from under the ground.
How do they keep the electric current on the pipe if it's in contact with the ground? I know for certain that under the street the pipe is in direct contact with the earth, based on some emergency digging once done around here.
Maybe this should be moved to theory...can somebody explain the cathodic protection in terms of how the current doesn't just go to ground?
#38788 - 06/02/0408:35 AMRe: Electric Current in Gas Pipes?
A very interesting topic. In my area, everyone gets flexible plastic pipe, yellow in color, with a stranded yellow tracer wire of about 8 gauge from the main to the building. The wire isn't connected to anything. It's just there to aid in locating the pipe underground.
#38790 - 06/02/0404:15 PMRe: Electric Current in Gas Pipes?
Around here all new and recent gas services are poly pipe. AND I have noticed rectifiers in cabinets mounted on poles apparently for cathodic protection but this seems to be for the larger distribution lines. Don't know if it would still energize the smaller mains.