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bench safety gear – part 1 – gfci safety box

Some of my recent projects have had me tinkering with AC mains voltage more than usual, so I figured it is time to get serious about my bench safety. My plan is to build several pieces of equipment that should decrease my chances of killing myself.

First, I will build a fused gfci power source that will trip if I do something stupid. Next, I’ll build an isolation transformer and then a current limiter. I’ll probably also need a variac, but I will probably just buy that.

With all of that, I should be quite a bit safer than I was before.

For the gfci (ground fault circuit interrupter) box, the idea is that if I were to accidentally shock myself, the gfci would trip, breaking the circuit. A gfci breaker works by measuring current flowing out from the hot side and comparing it to the current flowing into the neutral side. If they aren’t the same, the assumption is that there is current escaping the circuit through another unwanted path (like through my body) and the breaker trips, shutting off the power.

The parts for this box can be picked up from almost any hardware store. The box itself is a two gang pvc electrical box. It will hold a gfci outlet and a two pole switch. I’ll also add a fuse to the input. That’s pretty much it.

First, I ran a three pronged drop cord into the box. I used a cord from an old UPS. It had a rubber grommet molded into the cable so it fit snugly in the pvc box. If you use a regular cable, I’d suggest picking up one of the screw on strain relief connectors to keep the cord from being pulled out of the box.

Next, I added an inline fuse. If I had to do it over again, I would probably add a panel mount fuse so I can change it without disassembling the whole box. ( See update below: I did swap out the inline fuse for a panel mount. ) I put in a 5 amp fast acting fuse and connected it to the hot wire (black = hot, white = neutral, green = ground).

Next, I wired up the two pole switch. I went with a two pole so it would disconnect both the hot and neutral when the switch is flipped. The end of the fuse is wired into the hot side of the switch. The neutral (white wire) is wired into the opposite side of the switch.

Next, wire the top terminals of the switch to the gfci outlet and then connect the ground wire to both the switch and the outlets ground terminals.

Then shove everything in the box and screw in the switch and outlet.

Next, put on the cover plate.

Then test and reset the gfci outlet. The toggle switch cuts power to the outlet. Next time, I think I’d go with a switch with a pilot light so I can easily see if there is power to the outlet. The gfci has a little green led in the bottom corner, but a big red pilot light is easier to see at a glance.

Test that you are getting power and then everything should be done.

I’ll be putting this between any circuits I am working on and the wall outlet from now on.


*** Update ***

After blowing a fuse and seeing how much of a pain it is to change the inline fuse, I picked up some panel mount fuse holders and swapped it for the inline fuse holder.

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