Vraiable Transformer Safety Question

Thread Starter


Joined Jan 15, 2013
Hello All!

I have a resistance soldering machine for pipes & small projects. It's estimated to be from the 1930's or so. My question is about grounding the unit.

Primary voltage is 120 vac. Maximum secondary voltage was read at 17.03 vac. (And as low as 0.2 vac at the other end of the winding.)
For this discussion, I'll call the primary tap points H-1 & H-2. The secondary will be X-1 & X-2.
The transformer is an isolation type. From H-1 or H-2 to either secondary, there is no electrical continuity.

To visualize the transformer, picture a horizontal tube with #6 AWG copper wound around it.
There are three insulated wires coming out of the center of the tube: H-1, H-2 and X-1.

X-1 is for the "electrodes". It's connection to the transformer winding is not visible, as it is inside the "tube".
There are two electrodes. One is a cord with a carbon gouging rod holder and the other is a carbon block standing on the cabinet's top. (It is electrically isolated from the cabinet.) As would be the case with an electric arc welder, it is easy to accidentally touch these exposed conductors.

The transformer's X-2 tap is an exposed, #6 AWG copper winding wrapped along the length of the tube.
There is a wiper that slides along the winding where the varnish has been removed.
This wiper is grounded to the case because it is in contact with the cabinet.

The resistance is 0.1 ohms from the wiper to the cabinet.
The 120 volt line cord is not grounded. (2-wire)

Also note that the unit is energized by a line voltage momentary foot switch and will have a portable ground fault interrupting device added.
I'm the only person who uses this unit. It's used for plumbing and heating copper pipe sweating and some hobby work on the bench.

Plan A:
Since this unit is used on installed copper tubing (plumbing, heating, etc.) and that tubing is usually grounded to the building's system ground, to me it seems safer if the cabinet and X-2 (wiper) are at the same ground potential. This would be accomplished by changing to a 3-wire line cord and bonding the grounding conductor to the cabinet. If it is safer, a bonding jumper can be installed from the line cord's grounding conductor to the wiper.
Where I got this idea: In the olden days, when I had to weld a rack for a power transformer or AIR switch to building steel, I'd always install a bonding jumper between the welding machine's cabinet and the building steel (since I was usually sitting on it).

Plan B:
Isolate the wiper from the cabinet and still ground the cabinet to the line cord's ground. Isolating the wiper from the cabinet won't be an easy thing to accomplish, but can be done.

After heavy-duty thinking & careful bench meter testing, my mind says Plan A is safest.
Did I get this correct?

Thanks for helping me understand which plan is the best.

Please Enjoy Today!
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