How do you handle the bonding of a water piping system that is made up of dissimilar metallic piping and contains one or more dielectric unions. If you bond the various segments of dissimilar metallic piping to each other you will have shunted out the dialectic union thus assuring the galvanic action and resultant corrosion that the dialectic unions were installed to prevent.
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Tom H

Tom- I guess this issue has been around as long as I can remember. I would like to comment that if the piping is to be continuous electrically, I would expect to see a jumper for example between the H and C water pipes so the dielectric unions don't isolate a part of the piping system that may become energized and cause an electrucal hazard. I'm sorry, Mr. Plumber, but the law says I must do this. Now for the real world. I don't think there are 5% of the electrical inspectors out there that check this item out and enforce this item. There are so many more items that are not jumped around to maintain continunity that the dielectric union is not even addressed by the electrician. Then we have the non metallic piping and isolated metal plumbing fittings and faucets. In fact since the code has evolved to where the GEC is only permitted to be connected to the water pipe electrode within 5' of where the it enters the building, the only thing that gets inspected for this issue is the water meter and any water filters or water softeners located at the water service entry. Am I off base with this? Good question might be does the plumbing code require dielectric unions? or Does the plumber only install them to prevent the galvanic action?

Tom
That is a good question, one that I believe someday we will have a proper answer. As for now, the NEC requires this and we will install accordingly. This topic will create "lively" discussions on the job and this site for sometime to come .

Pierre

I think the "gas pipe" standard is most applicable. The pipe should be bonded to the EGC of the circuit most likely to energize it. If you have line powered equipment on isolates sections of metal pipe you should bond to the EGC.

I think we're confused as to what a dielectric union does. It only prevents corrosion at the point of contact, by insulating the one piece from another.
Were you to install a bonding jumper, this would not "defeat" the dielectric union at all. Any corrosion, caused by the connection of different metals, would be at the site of you jumper connections. Since our connectors are usually of copper or zinc plated steel, rather than bare steel, galvanic corrosion should not be an issue.

I agree John, I would further say that the current created by dissimilar metals in contact require that they be in contact. The terminals used for a jumper ,I would hope, wouldn't have the same reaction.

I cant guarantee that I used valid technique but a hall effect clamp on ammeter measured a small current in the water heater jumper were the supply to the water heater was galvanized iron pipe and the hot water line was copper. I turned off the main breaker and lifted the neutral to separate the home from the outside world and the reading of a few milliamperes of DC current did not go away.
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Tom Horne

George,

I look for the bonding jumper on the water heaters and I look for plastic water softners, and plastic water filters. Most times people forget to throw a bonding jumper on thoses pieces of equipment. I once saw a house that kept getting "pin hole" leaks in the house. Long story short, there was a plastic water filter and once there was a jumper on it, no more pinhole leaks. That was more than 5 years ago.

Harold,

I can't let you get away with that.

You're going to have to bring more to the plate, or, at least I, will discount the assertion.

Hi,
This subject just came up on a job I am doing. The plumber said that he was not going to be responsible for what my bonding does to his copper lines. He was surpirsed that I was going to bond the spa pump to the cold water copper supply line. He said that his code did not allow this.

I told him that I was going to bond ACROSS the HW heater too and he said he would disconnect it.

This job has a plastic line coming in that switches to copper. I put in two ground rods and ran a #4 to the HW heater to bond across the piping there. Does the 5 foot rule still apply or am I ok at the HW heater?

Apparently the HW heater is the problem. The unit insulates the hot and cold.

-regards

Greg

Bonding across a dielectric union _must_ to some extent defeat the purpose of the union. You have two _different_ metals in contact with a conductive fluid; net result is an electrochemical cell. Short the two terminals of this cell together, and you will have current flow.

That said, think about the distances involved. Without the dielectric union, you have the two metals in direct contact with a _very_ short path through the electrolyte. Add the union, and now you greatly extend the distance that ions have to travel through the water, increasing the resistance of the electrochemical cell substantially.

Lots of times the hot water and cold water pipes are bonded at the fixtures anyway, although the intentional bond is an especially good idea as more and more fixtures are connected using flexible hose.

I wonder if it would be possible to design an approvable water pipe bond that was something like a self triggering SCR, something that would be an insulator up to about 1V, and then break over and start conducting? This would appear to be a short at electrical system voltages, but an insulator at the potential of the electrochemical processes that the dielectric union is supposed to stop.

-Jon

Hi,
I thought a union is installed for future replacement purposes only, and played no other role.

The HW tank insulates the two connections according to my MASTER PLUMBER firend.

-regards

Greg

Winnie, a silicon diode has a voltage drop of .6 or .7 volts. They don't conduct below that voltage.

Mustangelectric, bonding the cold water within 5 feet of entrance is for use as a grounding electrode. Bonding at the water heater is usually for bonding piping systems in the building, in which case the 5' doesn't apply.

(edited 01-15-2005).]

[This message has been edited by Physis (edited 01-15-2005).]

Admittedly I had not considered a pair of ordinary silicon diodes, 'back to back' for bidirectional current transfer, but I don't think that this would be suitable.

Ordinary silicon diodes have _some_ conductivity right down to 0; the current versus voltage curve is an exponential, so it is reasonable to approximate a diode as a device with no conductivity up to a threshold voltage, and a fixed forward voltage drop...but the reality is that at low currents you have a very low Vf, and you have some reverse leakage current.

Then, when the time comes to conduct a fault current, you will have that 0.5-0.7V...at a whole bunch of amps.

Ideally the device would have zero conductivity up to a threshold of perhaps 1V, and then would 'trigger' and have zero voltage drop for any current above a few 10's of mA, and then once the current falls below the limit, would go into the high resistance state again.

I don't know if a semiconductor could be tailored to meet these goals. Also, I don't know if a simple silicon diode would be good enough for this task without a) introducing safety problems or b) costing too darn much.

-Jon

Physis,

What do you mean? Do you want more information? A friend of mine is a plumber and his customer was having a lot of "pin-hole" leaks in his house. The plumber would fix a leak or two almost every year. He finally asked me to look into the situation. I checked all through the house and all that I found was a plastic water filter in the back of the house. I asked the plumber if the leaks were in the front or the back of the house. He told me that all of the leaks were in the back of the house. I put a jumper across the plastic water filter and there hasn't been a leak since.

Harold,

Yeah, that's all. More information. I don't think it's impossible, just very unlikely. Like, for instance, were any other theories considered? Was anything unusual measured? Is it possible it was a chemical reaction?

Winnie,

What does that? FET's or MOSFET's. It's interesting to think about, but makes a pretty complicated jumper. You could power it with the fault.

Physis,

When I was looking for problems, I tried to get any kind of a reading across the plastic water filter. I put a VOM acrossed the copper pipes, (Both digital and analog) I tried to put am ampmeter around the copper water pipe, but I didn't see any kind of readings at all. My thought was that the current (What ever the type) was intermittant and it just wasn't present when I was looking for it.

I would expect that the current was present when something electrical, connected to the plumbing, was "on". A defective water heater element could put quite a bit of power into the piping without tripping the breaker, the same could be true of a disposal, washing machine or dishwasher. The current would only be there when the faulty equipment was on.
This is the kind of thing that causes those nuisense trips of GFCIs.

Harold,

Your situation sounds to me like what happens in boats in salt water.

Basicaly, any metal you put in salt water turns into 2/3 of a battery cell. The only other thing it needs is another piece of metal that's a different kind.

In boats you put a piece of zinc on the bottom of it to "if I recall correctly" to be more negative than the other metals and therefore do all the oxidizing.

If you put two leads from a battery into water, the water molecules get seperated into oxygen and hydrogen. I think it's the negetive lead that gets the oxygen.

Anyway, I'm thinking your situation is electrochemical. And I don't think you were looking for DC.

Shunting the "electro" half would likely stop it.

Maybe I was miss reading you Harold, because I think we're in agreement.

Ok, the problem with dissimilar metals is when the water is a conductor or a dielectric. So I disagree with a plumber who says the jumper would cause his bimetal unions to not work as intended.

You'll need to set your meters to DC to see this stuff.

An electrochemical cell produces DC, so if you are getting corrosion caused by dissimilar metals in water, I would expect DC current flow.

A 'dielectric' is an insulator, not a conductor. Two dissimilar metals in an electrolyte is a cell; if current can flow than one metal will dissolve into the electrolyte. Two dissimilar metals isolated by an insulator is an open circuited cell, and the only current flow will be the 'leakage' through the electrolyte. Adding the bonding jumper will clearly 'close the circuit' around the cell, and let current flow.

To expand what I said in a previous post, the characteristics of an electrochemical cell depend upon things like the metals involved, the conductivity of the electrolyte, the length of the path through the electrolyte, etc. Even in an electrochemical cell, current still flows in _complete_ closed loops. However the current flow through the electrolyte is in the form of ions physically moving, _not_ electrons alone. If you increase the space between the anode and cathode in the cell, you will greatly increase the internal resistance of the cell.

If you have different metals in direct contact in an electrolyte, you have a _very_ short path through the electrolyte, and thus a very low 'internal resistance' for the galvanic corrosion current flow.

If you have two different metals in an electrolyte, separated by an insulator, but electrically bonded together, then you have a much higher internal resistance in the electrochemical cell, and thus much lower current flow.

But I would still expect _some_ current flow.

Harold,

After you bonded around the water filter, did you measure any current flow on the bond? Here is a possible theory: there was a minute DC current causing the pinhole corrosion. But after you fixed the bonding, you got a bit of AC current flow. AC current electroplates for half the cycle, and probably smooths out any pinhole corrosion into much slower overall corrosion of the entire pipe.

-Jon

Yes Winnie, you are correct. I meant electrolyte.

You are better with batteries than me. I have a clearer picture of them already now just from reading your last post.

Assuming it's electrochemical, how does the current path include metal outside the immediate area? There would need to be another electrode involved. Sort of a bipolar battery. Shunting one half essencially removes the common electrode from the circuit. ?

Edit: I don't see the AC effecting the elctrochemical reaction. If it reduces the effect on one side of the rail then it would increase the effect to the same degree on the other side.

[This message has been edited by Physis (edited 01-19-2005).]

They have done some studies around here pursuant to lawsuits against copper manufacturers and water companies. The contention was that most of the corrosion pin holing was caused by current being introduced into the piping by bad grounding/bonding, not that it was actually coming from the metal chemistry in the pipe.
I don't know if it was ever really resolved.

I suppose that is what Harold was addressing with his jumpers.

In my world, you can bet if there's a lawyer involved, everything that can be done to remove the truth has been done.

You, saddly, can rarely find truth in a court room.

I'll stick to science, physics and text books.

So, a legal entity tells a plumber he "must" use copper pipes or he might end up in jail. The plumber goes to the hardware store to buy some copper pipes. The hardware store puts in an order with the guy who makes copper pipes.

So now some lawyer, with the help of some court (possibly from the same jurisdiction that ordered the plumber to use copper pipes) decides the guy that makes copper pipes has caused harm to nearly everybody.

Nobody will point to the authority who "ordered" the plumber to use copper pipes. And study what? Just because I can't readily answer the question, this whole subject has been well understood for at least 50 years if not more than a hundred.

Lawyers.

Edit: The guy just makes copper pipes.

Edit: I was a little sharp with Harold initially, I should appologize for that.

[This message has been edited by Physis (edited 01-19-2005).]

They were pointing to that middle school science project where you put 2 dissimilar metals in water, applied a current and made oxygen and hydrogen, eating away some of the metal. True or not, I don't know but it did get the <government> water company off the hook. I think they were still going after the plumbers and pipe company, contending that the pipe should have held up better. I guess the only thing more attractive than deep pockets is a pocket that is not protected with unlimited government lawyer money. I have plastic pipe and didn't have a dog in the fight so I wasn't paying much attention. I tried to google it and all I got was a bunch of articles from plastic pipe companies trashing copper.
It was in Cape Coral Fl

I think it was Volta that discovered that reaction.

Edit: "didn't have a dog in the fight"

Edit: imagine that, the government interest vindicated.



[This message has been edited by Physis (edited 01-19-2005).]

Physis,

All chemical reactions involve electrons moving around. In an electrochemical cell, you have two separates types of reactions, physically separated, one happening at one electrode, the other happening at the other electrode. One reaction uses up some of the available reagents, and releases some electrons. The other reaction uses up some of the available reagents and absorbs some electrons.

As an intermediate between these two reactions, some ionized material has to physically move between the electrodes.

When current is flowing, the reagents are being steadily used up. A flow of electrons goes from one electrode to the other via some external circuit, and a flow of ions goes though the electrolyte.

There is no requirement that the electrodes be in the immediate vicinity of each other, nor does the external circuit need to be small. The electrochemical cell has lower resistance if the electrodes have large surface area and are close together, but the cell will work even with high resistance. If you stick dissimilar metals into a lemon, separated by inches, you will have sufficient power available to run a clock

-Jon

Ok, but this is going to have to be some pretty poluted water for one union to take out a bunch of pipe.