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#18848 - 12/15/02 05:10 PM Dielectric unions  
elecbob  Offline
Joined: Aug 2002
Posts: 141
An inspector required me to jumper 3 dielectric unions on a commercial job. No biggy but I was wondering: Aren't I defeating the purpose of the union, the installation of which was required by the plumbing code? I've seen plumbers use a brass nipple between the copper and galvanized pipes in lieu of a union. Would using bronze WP clamps avoid corrosion problems? Has anone installed jumpers only to have a plumbing inspector order them removed?
Here's some info I picked off a plumbing site:
CORROSION It is the electrical current generated between dissimilar metals that are in direct contact in a plumbing system which causes destructive corrosion.In order to successfully combat the galvanic corrosion, the metal parts must be separated in such a way that the electrical currents below 1% of the galvanic current which would exist with metal-to-metal contact. This is the function of the dielectric (insulated) union.
BREAKDOWN It is desirable that the dielectric union have sufficient electrical insulation to prevent high voltage breakdown when used in dry lines, such as air or gases. In this case the dielectric union isolates, electrically, one part of the piping system from the other. Successful electrical isolation depends on creating a "wall" of electrical insulation that will stop the largest likely to be encountered at the point of the dielectric union. Since voltage encountered is usually in the range of 220 volts or less, the insulation should withstand this plus a safety factor or a minimum of 600 volts.

[This message has been edited by elecbob (edited 12-15-2002).]

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#18849 - 12/15/02 06:02 PM Re: Dielectric unions  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
The galvanic corrosion occurs at the point of direct physical contact between dissimilar metals that are also in contact with an electrolyte. You have actually made a cell (battery) and one of the two metals will be eaten away as the cell produces current. I don't think that an external jumper will cause the same type of problem.


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