Sudden Ground Fault on an Electrical Tool

My Bauer portable power metal saw from Harbor Freight worked well for many months. Then I installed GFCI outlet protection in my workshop. Suddenly my saw tripped GFCI outlets. It is no longer under warranty. I have received no electrical shocks from the ground fault. An Ohmmeter shows no continuity to ground. This Instructable shows the problem and how I corrected it. Hopefully, it will save someone from condemning a good tool, even if it is not a metal saw from Harbor Freight.

Materials—No materials were needed or used.

Tools

• Medium Phillips screwdriver
• 5mm hex wrench (A hex wrench for a 3/8 inch drive socket wrench is very helpful, but not essential.)

This is a spoiler alert. The problem was in the area of the air vent to which my finger is pointing and forward of it. That means you do not need to dismantle more than the plastic castings for the handle and its mounts. Nor should you panic as if the motor or variable speed control failed.

Use a medium Phillips screwdriver to remove six screws. (first photo) Use a hex wrench to remove four large screws. (second photo) Remove two large screws that anchor the lower end of the handle. (third photo) A socket drive for these two screws is really helpful and saves a lot of time.

Tap on the photo and read the text notes. When I opened the handle section, red power wires to the motor crossed over one another. They did not appear to be in channels designed to keep them from being compressed together. One of the challenges for reassembly is to keep wires in their desired places. But, I also strongly suspect the person who assembled this tool just put the wires into the tool without much concern that they would not be compressed against one another.

I have had a couple of previous experiences with ground faults, etc. Everything works fine and appears good initially. But, there is compressive force on wires from other wires or from something metal that is not to be part of the circuit. In time the compressive forces push insulation material to the side and the thickness of the insulation diminishes until there is enough electrical leakage to trip a GFCI or even cause a shower of sparks. Yet, if you look at the wires you may not see any burn marks and the insulation appears intact. But, it is not.

Look for formed channels where wires are supposed to rest and place them in those channels. The insulation on the red lines to power the motor is relatively thin. The pre-formed channels for them keep them separated. (Notice there is a recess for the terminal fitting on the green ground wire, but it is not in place in the photo. Move it into place, too.) Some adhesive, like auto body adhesive, could be put into the wire channels to hold the wires in place, but would require time to dry before it would be effective.

If insulation appears to be compromised and available space inside the tool allows, use electrical tape or shrink wrap tubing to restore the insulation.

Carefully reassemble the tool. Plug it in and turn it on as you go to be certain nothing was compromised at each step.

I use this power metal saw in an upright frame so it functions like a bench top bandsaw. Everything seemed fine after I had closed the handle mount area with its various screws. But, when I put the saw into its stand the GFCI tripped immediately. After some puzzlement I discovered the electrical outlet near the saw’s stand has a ground fault I need to find and fix. When I used the extension cord from another outlet during assembly and testing all was fine and worked well. (Update: I found the ground fault problem in the outlet. In the string of GFCI outlets the outlet next to last in the string had one of the white wires that had slipped out of the terminal hole on the backside of the outlet and was loose in the box. The box is plastic. When I found that and secured the wire in its terminal the ground fault problem disappeared. [That white wire was a bit short and the installer did not notice that it had slipped out before he was able to tighten the screw on the outlet. I was expecting a staple pressing against a cable too tightly, or a clamp screw too tight.])

See the second photo. These connectors for securing a wire or cord where it enters an electrical box can exert too much compressive force on individual wires inside the outer covering. That can cause electrical leakage, even a direct short. Beware of over-tightening. (Update: The temperatures in my garage workshop rose enough to make troubleshooting in its attic more comfortable. But, before tearing up things to get at a cable supplying the final outlet in the chain, I decided to open the ceiling box prior to the final one that trips the GFCI. When I pulled the outlet down out of the box it became very apparent one white wire had slipped out of its clamp terminal on the back of the outlet. The wire was a little short. The original installer did not notice it had slipped out as he tightened the side terminal screw. As soon as I secured the white wire in its terminal, the ground fault was gone. Naturally, I had shut power to this circuit “off” before opening things.)