Jeff Wisnia wrote:

John Hines wrote:

(Chet Hayes) wrote:


Jeff Wisnia wrote in message ...

John Hines wrote:

(coolneo) wrote:



Hi,

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.


Put a clamp on the old work, and a clamp on the new work, and run a
heavy gauge wire between the two clamps, to jump the union,
electrically.

This is identical to what is done for water meters to insure ground
conductivity.

This would work if you put in a short plastic piece, or something like a
water filter, which is plastic.

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.

Just my .02,



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.


A copper clamp on fitting on the copper pipe, and an iron clamp on
fitting on the iron pipe.

Only dissimilar metal would be iron clamp to copper wire, but corrosion
there, wouldn't impact the pipe.

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.

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.