Well, it's that you cannot use a gas pipe as a grounding electrode.

I wished they would have used different words for the idea of "grounding". Grounding to earth has no real bearing on the day to day workings of the electrical system.
BONDING is what gives us the safety path for fault current. Fault current, like all current, is seeking it's source, NOT the earth. This is what perpetuates the myth that a ground rod is providing a safety ground the same as bonding.
What is confusing is that an "equipment grounding conductor", the ground wire run with a branch circuit, eventually goes back to the main panel where the grounds are bonded to the service neutral. It really should be called the "equipment bonding conductor".

The thing with gas piping, just like any other metallic piping system, is that it must be bonded to prevent fault current from electrifying a gas pipe. With gas piping though it is specifically stated that the "equipment grounding conductor" run with the circuit conductors feeding a gas appliance shall be accepted as the means to bond the gas piping.
If you run a dedicated external bond to gas piping, and it is installed in a certain way, such as with a heavy wire and right near the pipe entrance, it can act as a grounding electrode which is expressly forbidden.

Petey racer. Again i must enfisize, how can anyone follow
advice on this site when every code is different ? Prime
example. Our gas pipes are electrified for cathodic protection. They are insulated from the gas meter to the home appliance. Even if you connected a ground to the pipe,
which is forbidden, you will never get your ground through the pipe and the bonding will do nothing. That was the third thing our instuctor taught us in the piping aspect of
our training.NEVER connect a ground or bond wire or any
wire to a gas pipe. We now run plastic gas pipe and that
eliminates the gounding and bonding of the gas pipe question.

"...the bonding will do nothing..."

The bonding will do exactly what it is supposed to do, prevent there from being any voltage between the two items that are bonded.

A problem does arise when you have steel gas lines and copper water lines and they are bonded.
You have just made an iron-copper battery with the water in the earth and minerals present being the electrolyte.

That is why under the NEC you are not allowed to use the gas lines as part of the grounding electrode system, but if they may become energized they must be bonded.

The bonding will still create some galvanic reaction (the pesky electrons and ions do not understand they are not supposed to flow that way) but at least it is not a very large low impedance path intended to carry lightning and high voltage leakage from pole transformers like the grounding electrode system.

When you say every code is different, which codes are you referring to? Outside the United States?

The gas pipe is bonded through the appliance. If you run a branch circuit to a hot air furnace for example and a metal gas pipe is run to the same furnace then in fact that gas pipe is then bonded. The point is to bond any gas pipe that has the possibility or likelyhood of ever becoming energized.

You cannot use the metal underground gas pipe entering a building as a grounding electrode, but that is not what we are talking about here. I think you may have misunderstood your instructor.

Flexible gas lines (CSST) are not actually plastic, it is plastic coated couregated metal. In fact, Manufacturers and many ahj are now requiring bonding of the manifolds, valves and any other metal parts of a csst (flexible gas line system) with, in one example, a #6 copper wire. This is in an effort to reduce the risk of lightning traveling the line and jumping to a point of ground, causing holes in the line itself.

-The following is a quote from: http://www.toolbase.org/pdf/techinv/csst_lightningconcerns.pdf

It is worth checking out if you install CSST gas lines.

"Although only four of the six CSST manufacturers were parties to the Class Action suit, all six have informally agreed to upgrade their bonding requirements for the CSST system. Generally speaking, the new bonding method requires the attachment of a bonding clamp to either the CSST fitting or to a piece of steel pipe located near the service entrance to the building. The bonding jumper shall be no smaller than a 6 AWG copper wire (for residential applications). Although the final bonding solution is essentially the same for all six manufacturers, the six technical pronouncements appear to be quite different."

CSST has become a very contentious issue lately.

I am still trying to grasp the difference, and I will read and re-read your replies until I do understand. Until then, is running a #6 copper wire from the gas pipe to a water line, both close to where they enter the building, bonding? Thanks for all your replies. It is not black and white for me yet, but I am trying.

If the gas pipe is connected to any appliace there is a bond but the bond stops at the gas meter.The gas pipe and the gas meter have a rubber bushing insulating both pipes.
There will never be a bond or ground to the in ground gas
pipe. Our plastic gas pipe is plastic. And i mean plastic.
That is why we have to run a #10 wire with the pipe to
find it if and when there is a dig up. This is Canadian code for Manitoba. We have 10 provinces. There are about
60 different gas codes for this country. You have 50 different states. God help you.

Ok. I think I have it.

Bonding is connecting a path back to the service panel.

Grounding is connecting a path to the Earth.

Right?

I am in Ontario, Canada, and all I can find in the CAN/CSA-B149.1 Natural Gas and Propane Installation Code book is page 42 6.14.6 "Piping or tubing shall not be used as an electrical ground". Where does it say that if the piping is attached to an appliance, that the piping is considered "bonded"? Thanks.

I realize there's a Canadian question here, but here's the NEC verb-age

2008 NEC®© 250.104 Bonding of Piping Systems and Exposed Structural Steel.
Other Metal Piping. Where installed in or attached to a building or structure, metal piping system(s), including gas piping, that is likely to 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 is likely to energize the piping system(s). The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.

Fittings that insulate metal pipe so that there isn't a connection from the main gas line and meter to the pipe inside the house are common here. The idea is to prevent current from flowing on the gas line that eventually eats holes in it (electrolysis). The (hopefully)insulated pipe that runs from the meter into the house can be either metallic or non metallic. The parts that are metal need to be bonded so if the pipe for some reason contacts a hot wire it will trip a breaker rather than shock someone.

The reason you bond around CSST is simply that lightning doesn't care if the gas pipe is insulated, it will jump the gap seeking ground. If CSST is used the lightning will usually follow a straight path rather than the internal metal spirals and blow holes in the covering in the process. (cheap dirty explanation)

Maybe the best way to keep things straight are to think about it this way:

The grounding electrode (the metal water pipe that goes outside, ground rod, UFER, etc)and the associated wire (the ground wire) the grounding electrode conductor (GEC) is there specifically to give lightning an easy path to ground.

Bonding wires (equipment grounding conductors or EGCs) are there so that should the pipe somehow become energized, a breaker trips rather than someone touching the pipe getting shocked.

In the case of a house with all metal water pipes, the grounding electrode conductor also serves as the bonding conductor. Metal gas lines would be bonded because there is (hopefully) no metallic connection from the inside pipe to the (grounded - because it is buried)outside metal pipe.

There are a BUNCH of people who don't understand this and why you get people who should know better telling you to use a gas pipe as a GROUND

Clear as mud yet?

Thanks. It was getting clearer until you said this:

"In the case of a house with all metal water pipes, the grounding electrode conductor also serves as the bonding conductor. Metal gas lines would be bonded because there is (hopefully) no metallic connection from the inside pipe to the (grounded - because it is buried)outside metal pipe."

There would be a metallic connection from the inside pipe to the (grounded - because it is buried)outside metal pipe via the metal water line which is buried in the soil outside. It is not connected to the outside GAS pipe, but it is connected to the outside water pipe.

I am not trying to beat a dead horse here, just trying to clear the muddy waters a bit. I really appreciate your time.

Gas lines are dangerous, explosive type dangerous and therefore we do not want lighting being transeferred via a dangerous and explosive gas line.

Water lines are not explosive and therefore we want the lighting transferred via that pipe.

You are correct that the metal water pipe is connected to the exterior water pipe - that is what we want.

"Bonding is connecting a path back to the service panel.

Grounding is connecting a path to the Earth."

Almost.

Bonding is connecting two conductive items so that no voltage can exist between them.
All the metal around a swimming pool is bonded together, and then typically connected to earth ground to prevent any voltages differences from occurring.

Grounding is a specific path tied to the earth as a 'zero volt' reference.
It is for lightning and high voltage transformer leakage.
The earth is an effective conductor (in most places but not all) at these voltages (7kV+).

Lightning strikes are trying to discharge the buildup between the clods and the earth.

There is a capacitance between the primary and secondary of pole transformers that results in some leakage from the high voltage (primary) side to the low voltage (secondary) side.
This is why the secondary of transformers must be tied to ground. The entire secondary (for a 120/240V service ALL the conductors, both hots and 'neutral') can charge up to the primary voltage if nothing breaks down before then.
You would still have 120/240V, but riding on many thousands of volts (of AC or even DC).

Transformers designed for isolation use a metal screen to try and reduce the capacitance, but they still need a ground connection on the secondary.

Grounding is intended to carry fault currents to trip an OCD.
Bonding is intended to prevent voltage differences.

By bonding the gas and water lines together and to the ground of the electric service ground, it prevents either of these conductive systems from becoming energized until some un-suspecting home owner comes along and touches the gas line, for instance, and another metal device or object that IS grounded and you know who completes the circuit to ground. If, however, the gas line is BONDED to the electric service ground, when it is exposed to fault current it will (should) trip the breaker.

Sorry. Thought I made it clear the idea was to have a ground to a metal water line that ran outside and to interrupt the metallic connection in the gas line so you DID NOT have a ground connection to the metal gas line buried outside).

'Tis a bit of a complicated subject

No need to be sorry. I was the one with the problem of wrapping my head around this one, but I think that I get it now. Thanks to everyone who responded. I was thinking that is was more of a complicated issue than it really is. Thanks again.

Brickeyee - Excellent post!