I'm reading up on some plumbing work I'm considering doing myself. I have galvanized pipes throughout my house. I want to put in copper for the new supply lines I want to install. I understand a Dielectric union will allow me to "join" the copper with the galvanized iron and protect from corrosion.
Great.
Continuing my reading I read someplace that since most older homes are grounded to the plumbing the addition of a dielectic union will undo the grounding.
Fine.
The alternative the person recommended was to use a "brass nipple at least 6" long" and use "brass couplings" for the copper and galvanized areas.
This is totally new to me. So here come my newbie questions:
I looked up brass nipples and I understand the concept there. How do the coupling pieces come into play? Can someone give me a URL to see what they look like?
The dielectric union inserts an electrical insulator in the water line. So if the dielectric union is between the grounding lug and the city water supply, it would indeed disable the ground, as there would not be an electrical connection to the earth, so to speak.
But in my house the grounding lug is attached to the cold water pipe inches from where it first enters the house. I imagine that this is typical. If yours is like that too I can't see how a dielectric union further into the house plumbing would affect the grounding.
I have heard of larger buildings where people have "grounded" any number of stupid things randomly to any pipe they could find, but I hope that doesn't happen much in private homes.
I can even think of a few ways of testing for that, but I think you should get more expert advice.
Actually, the grounding of switch boxes by virtue of the nearest Cold water pipe was very common practice in much of the US from the turn of the century till roughly 1965. After that, Romex w/gnd was in widespread use.
There were exceptions, of course. Big cities mandated the use of metallic raceway, other places used AC (BX) cable as a grounding means. But millions upon millions of houses relied (still rely) on the ground connection at the nearest Cold water pipe for grounding of boxes for: bath switches, bath recepts, kitchen counter recepts and switches, etc.
That's why plumbers have to be extra careful when disturbing old runs of galv iron pipe.
What you suggest is what HAS to be done, because code says that all metallic plumbing in the house has to be grounded.
The perfectly reasonable explanation for that is because is some piece of electrical equipment associated with the plumbing, like maybe an electric water heater, clothes washer or a dishwasher has an improperly connected or broken ground lead and then develops an insulation fault, every faucet in the house could be electrified with 120VAC. And worse, the faulty appliance may well "run OK" without giving any warning of what's waiting to happen. (Think about a kid with bare feet standing on damp soil reaching to turn on an outside sillcock.)
A less likely but not impossible reason the piping may become electrified is when someone drives a long screw or nail into a wall which pierces the insulation of a live wire, touches a hot conductor, and then kisses a pipe.
But, galvanic corrosion is sorely misunderstood by most of the trades, and using an insulated coupling bypassed by that necessary bonding wire won't do squat to prevent corrosion of the steel pipe. The electrons have a fine return path through that bonding wire and the galvanic corrosion of the steel part will take place just as it would have if the two dissimilar metals were touching directly. Well, maybe a little less quickly if the insulating coupling consists of many feet of plastic pipe.
An insulated coupling will prevent galvanic corrosion only if the two disimilar metals are NOT electrically connected together by another conductive path.
The "big boys" use "impressed current" protection of buried tanks and pipes, but these require an electrical supply to create a bucking current. Such systems are used less and less these days as buried street piping switches over to noncorroding plastic.
There is a bit of confusion here. If you have a metalic supply water line it has to be included with the grounding circuit. Beyond that we're discussing bonding, not grounding. The point of bonding the metallic water lines is to provide a path to clear a ground fault and trip the circuit breaker or fuse controlling the circuit. The jumper suggested earlier will work for that. If the ground fault does not have a path to clear, it can leave a section of the piping charged, with possibly tragic results.
Jeff, I agree with everything you said. But the OP's question got me thinking. As I understand it, these dielectric unions are sold for precisely the purpose that the OP is talking about, which is for use to prevent galvanic corrosion where dissimilar metals meet. Yet, code would seem to indicate that all the metal plumbing needs to be bonded together for electrical safety. That seems reasonable, since if you have a section on plumbing which is isolated from ground, then if any spot became energized through a short the entire section of ungrounded pipe would be live. But, as you pointed out, bonding defeats the whole purpose of the isolation.
So, that begs the question, how are you supposed to be able to use one of these dielectric unions and pass code? I even saw Richard Trethewey on Ask This Old House install one on a boiler to solve a homeowner's corrosion problem. He put it in with no bonding, which solves the corrosion, but what about code?
BTW, if you ever see that episode, it has a nice blooper. The grand master plumber has two pipe wrenches on the old union and is trying to unscrew it, but he's pulling the wrong way. The camera cuts away, a second later it's back and now the wrenches are on the right way.
Nope, John. If I read you correctly you are assuming galvanic corrosion ONLY takes place when two dissimilar metals actually touch each other in an electrolyte. That's not right. Think about the simple old fashioned carbon-zinc flashlight battery. The carbon and zinc certainly aren't touching each other. If the battery is left alone will last a long time without "leaking". But, when you let the battery deliver current through a load, by "connecting" the two materials through a flashlight bulb filament or other load, the zinc shell gets "corroded" away pretty fast and will develop leaks right through it.
The reasons I pontificate so much on this subject is because I could understand the principles involved because of an electrical engineering education, and also because I had a first hand experience two years ago which caused me to question and later decry the "conventional wisdom" about using dielectric couplings at the inlets and outlets of home water heaters.
Our electric water heater rusted out so I bought and schlepped home a replacement. The old heater had copper piping running right to it, but I'd heard about dielectric couplings over and over again and they were displaying them right next to the water heaters at Home Cheepo so I bought a couple and installed them when I put in the new water heater, thinking that they would somehow help the new heater tank last longer. I stood them off the top of the heater on 3 inch galvanized steel nipples.
Less than six month's later I noticed some rust on the heater's outlet boss and on closer inspection I saw a bead of water oozing from where the steel half of the dielectric union screwed onto the galvanized nipple. I figured I'd just messed up on teflon taping the pipe threads or didn't tighten them enough, but when I took things apart I was shocked, just shocked, to find the nipple nearly clogged with rust and a pinhole leak corroded through it, breaking through at the root of the pipe thread just outside the portion of the threads which were screwed into the steel half of the dielectric coupling.
I started thinking about what had happened and soon realized that the water heater's tank was grounded via the safety ground lead on the 240 volt circuit feeding it (per code) and the copper piping in our home was also grounded (again per code). Those two grounds provided a low resistance shunt circuit across the insulation of the dielectric couplings, so the galvanic current flowed through that path and the corrosion took place just as though there was no insulation in those couplings.
The reason why the corrosion is worst right at the junction of the copper and steel, even if there's an insulator there, but bypassed by another conductor, is because the electrolyte (i.e. less than totally pure water.) is not a perfect conductor, so the galvanic current density into and out of the surfaces of the dissimilar metals isn't the same all over, it's greatest close to the junction point and gets lower the further away from the junction it travels because it has has to flow through the resistance of the electrolyte to get there. (If you're an electrical engineer, think of a resistive ladder network.)
BTW, the other nipple on my water heater was similarly clogged with rust and also well corroded.
I pulled out both those dielectric unions and replaced them with copper unions standing on "copper nipples".
I was gratified to see that the Rheem Water Heater Company recently published a technical paper advising NOT to use dielectric couplings when installing their products. They say to run copper all the way to the tank shell and the sacrificial anode inside the tank will protect the iron. That paper is at:
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I sawed the leaking pipe nipple I described above in half and snapped a photo of it to email to a buddy. I scraped out the big soft rust clumps prior to taking the picture, so you can clearly see the craters which corroded into the inside wall in less than six months time. See:
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Case closed....
Jeff (Who won't drink the unfiltered tap water in our home )
I'm just a DIY plumber/electrician, who has seen this first hand. I know not to mix metals, and use copper plated stuff on copper pipes, and zinc on galvanized.
I don't know the point, purpose, or intent of the original posters di-electric unions, only how to jump around a break in the plumbing.
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