grounding across dielectric unions
Houghton123
| Posted in Construction Techniques on
Our house uses mostly galvanized water pipe, but, in remodeling our bathroom, my son has used dielectric unions to transition to copper. Am I correct that I need to jumper across the unions to continue the grounding of the pipe? Anything special I need to know? Can I just clamp on grounding clamps on each side of the union and run a wire between them (in air, not otherwise touching the pipes)? If so, should the clamps be of the same material? Or do I need to run a separate wire back to the panel for the copper?
Replies
If you jumper across the unions you will have defeated the whole purpose of dielectric unions. They prevent corrosion of iron pipe connected to copper pipe by interrupting the "return" flow of electrons in a galvanic reaction.
I find this logic hard to understand. Maybe you can expand on this theory.
"Maybe you can expand on this theory."I assume your question refers to my statement that a dielectric union between copper pipe and galvanized steel pipe prevents corrosion by interrupting the flow of electrons in a galvanic reaction. It is a statement of fact, not theory. dissimilar metals in contact with an electrolyte solution (water with any dissolved solids; tap water in our case) will tend to generate an electric current if they are connected electrically. Batteries work on this principal. As current flows, then anode metal (Iron and zinc in our case) is eroded away while the cathode metal (copper) remains undamaged. A dielectric union puts a layer of plastic between the pipes, thus preventing flow of current between them (effectively disconnecting the battery terminals) thus preventing damage to the galvanized steel pipe.http://en.wikipedia.org/wiki/Galvanic_corrosionBruceT
The real problem is with the metal when they are wet.The dielectric union separates the the wet parts of the pipes.Now if the outside of the uniion was constantly wet and you added the grounding clamps then you would have problems..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
"The real problem is with the metal when they are wet.
The dielectric union separates the the wet parts of the pipes."Actually, the dielectric union works because it breaks the electrical connection between the pipes, which are acting as electrodes in a battery. Without that connection on the "dry side", the electrolyte side of the galvanic reaction between the wet inside surfaces of the pipes cannot work. A jumper wire would restore that connection and allow the galvanic corrosion to proceed.
http://en.wikipedia.org/wiki/Galvanic_corrosion
BruceT
But when you apply the dielectric union plus jumper the corrosion is distributed more evenly along the length of the pipe, vs being concentrated at the joint. Hence pipe life is not as severely affected.
If your view never changes you're following the wrong leader
But when you apply the dielectric union plus jumper the corrosion is distributed more evenly along the length of the pipe, vs being concentrated at the joint.
Hence pipe life is not as severely affected.
The difference is negligable - the corrosion will occur between the two jumpered points. So generally, it will still be the joint, and you won't really notice much difference.
If you are going to jumper the dielectric unions, there is no point in putting in the unions - might as well just connect the pipe.
FWIW, though, in many cases the galvanic effect is very small - so it might eat through the copper in 50 years instead of the normal 60 year life of copper pipe.
I think I would just use type L pipe and not worry about the union.
2 comments. Modern galv pipe is nowhere near the same as pipe made 50 - 70 years ago. I'm sure most of the plumbers on the board can tell you of stories of replacing 10 year old pipe that is leaking due to corrosion.
And oxygen needs to be present in the water. Black pipe to copper in heating systems is generally not a problem.
"FWIW, though, in many cases the galvanic effect is very small - so it might eat through the copper in 50 years instead of the normal 60 year life of copper pipe."It is the ANODE that corrodes. The zinc galvanizing and then the steel.http://www.corrosion-doctors.org/Definitions/galvanic-series.htmA friend bought a house orginally built in 1955 with galvanized steel pipes.At sometime a few sections where replaced with copper. Hard to tell when, but I am guessing 10-20 years ago.At one place they ran copper hot water line in a galvanized riser. I replced all of the accessable galvanized and it was as very clean, even after 50 years.I put a dielectric in when joining it back to the copper. It was leaking at the threads. Tried to tighten it up and the galvanized elbow split. It was full of corrosion, as was the first part of the riser..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
In general you need the jumper. It does defeat the purpose of the union to a degree, but not totally.
Why do you need to continue the grounding of the pipe?
I had thought that the purpose of grounding water pipe was to give a path to ground if the water pipe becomes charged accidentally as a result of some failure in the electrical system, so that you don't become the path to ground by touching the pipe.
??
Edited 2/24/2008 10:59 pm ET by Houghton123
This whole grounding question applied when cold water pipes were used as the sole ground for the electrical system in the house. That gave way years ago to a 8' grounding conductor rod driven into the ground right below the service entrance where one finds the electric meter/breakers and the two connected by heavy wire (#4,#6 or#8 copper). That was replaced in new construction (most places) by running the wire horizontally in the foundation ditch parallel to the rebar (at least 2.5ft deep) with enough wire above grade to run to the service panel. If you have galv water pipe then house obviously over 50 years old. You benefit with the dielectric plumbing fittings (corrosion wise) but better upgrade the electrical with a real ground rod. It's also unlikely that you will rewire the entire house to current code with a neutral conductor (bare or green) and two hot wires (one black or red, one white). So accepting that your service is wholly inadequate by todays standards (still works though) jump the dielectric joints with ground clamps and heavy wire so you still have something of a working ground assuming those pipes are at least 10' feet long in the earth underground.The ground rod and water pipe work together to form the grounding system that provides a safety path to earth for the electrical system.
{Remember things like lightning, transformer failures and service drops (coming from the secondary side of the transformer)are outside of your hands so grounding can be very important. Cheers, TyrEdited 2/26/2008 1:31 am by Tyr
Edited 2/26/2008 1:33 am by Tyr
The point is to connect the metal pipe to a ground, in case the pipe becomes energized. It is not to provide a ground for the electrical system. I suspect most localities don't trust the water pipe system to be a ground for the electrical system any longer because of the use of plastic pipe. No assurance of continuity.
You are confusing too different "grounds".There is the GROUND ELECTRODE SYSTEM. The purpos of the ground electrode system is to reference the electrical system to ground and to porovide a discharge path for lighting strikes.Any ground electrodes that you have much be connected to the ground electrode system. The most common for residents is metalic water pipe that is underground and at least 10 ft long. It has to be connected to the ground electrode conductor within 5 ft of where it enters the building. However, current code does not allow it to be used as the sole ground electrode conductor. Not that it not a good electrode, but that the supply line might be replaced with plastic in the future. So an underground water pipe can't be the sole electrode.Other common electrodes are ground rods (usually 2 by current code) and for new construction a Ufer (concrete encased electrode).There are other types of ground electrodes, but not typically seen in a residence.The "other ground" is the Equipment Grounding Conductor. That is the green or bare wire on internal wiring and metalic conduit. The purpose of the EGC is to provide a path back to the panel for any leakage for fault currents.In older codes the the cold water pipe was also allowed to be used for an equipment grounding conductor. That you could run the a ground wire from equpment (receptacle) to the closes cold water pipe. That is no longer allowed for several reasons. But it might be existing in homes and it needs to be reconized and handled.There is also BONDING. It used so that any metal that a person might be in contact with is bonded to the equipment grounding system. Bonding is strickly for internal needs (bypassing any discussions of pools). For metallic water pipes that is done where it enters the structure and wired the same as if it was a ground electrode although it might not qualify for a ground electrode. The gas line and metalic ducts (if exposed) also need to be bonded, but they are often bonded indirectly via grounded electrical equipment that connects to them..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
For what it's worth in this discussion, our house is 110-120 years old. I rewired it in the 1990s (I think that was when I finished it up), and the building inspector required both a ground rod and a connection to the cold water pipe.
Again, I've understood the point of grounding the pipe to be to provide a safe path to ground (rather than, say, me being the ground path) of any electrical charge that happens to get connected to the water pipes.
Maybe I should connect a ground wire from the copper pipe to the ground wire that runs back to the panel. That would put the two sets of pipe at the same potential point, so I'm not sure how any galvanic circuit could be created.
Your ground wire back to the panel would take care of accidentally energizing the pipes from contact with a wire somehow, but would do nothing about a galvanic reaction inside the pipes.That galvanized/copper pipe connection is a battery making DC current that flows a few inches through the water between anode(galvanized steel) and cathode (copper)as long as there is a dry-side electrical connection between the two electrodes (pipes). 50 feet of wire back to earth ground at the service panel probably won't affect that battery circuit.As for connecting the equipment grounding wire of any circuit to the pipes, I'd guess that must be forbidden somewhere in the NEC.
BruceT
At one point at least, grounding to a pipe was permitted when retrofitting a 3-wire outlet. Since prohibited, though, due to the popularity of platic pipe.
If your view never changes you're following the wrong leader
BTW- I believe, according to code, those unions must be accessible- or is that just for gas?
If you are concerned about the copper pipe becoming energized you could ground the copper pipe to a ground. But not to the steel pipe.
I have problems with lightning here. Rex Cauldwell's book "Wiring a House" from the Taunton Press claims lightning strikes sets up electrical fields in ungrounded pieces of metal in a house. These fields resonate into other things in the house.
Rex suggest you ground every big piece of metal.
I believe the the dielectric union prevents corrosion at the joint.
So if you jumper from copper to steel with a piece of copper wire you are going to have corrosion where the copper wire joins the steel. I don't know if corrosion eventually ruins the electrical connection or not. But I think the jumpers are a good idea. FWIW, I think they will help bleed off the fields induced by lightning and nuclear bombs.