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Electrical Power Shutoff

Young children are not well known for rational decision-making or obeying the rules given to them by their parents. Even before my two year old son was born, I was rather terrified of the prospect that he could one day be injured by my power tools — something that could easily be prevented. Teaching him tool safety from the beginning is a given, but I wanted to make it impossible for him to ever power up any tools without supervision.

My solution to this problem was to route all power in my shop through a magnetic contactor and put a locking switch in the circuit that supplies power to the contactor's coil. As long as the contactor is open, there's not a single outlet in the shop that has power. If my kid is bent on hurting himself, he'll just have to do it the old-fashioned way and resort to using a hammer or something.

The details

I started by mounting a new sub-panel box and an 8" square junction box in my shop and running a 10 ga. feed from my main panel to the junction box fed by a 60A breaker. The neutral and ground pass through the junction box and connect directly to the sub-panel. I installed an industrial magnetic contactor rated at 65A in the junction box and attached the two hot legs from the main panel to one side of the contactor. The other side of the contactor is connected directly to the sub-panel. That's the extent of the high-amperage wiring.

Next, I wired up a control circuit. The control box consists of a key lock and a couple of cheap industrial control switches. One is a green, normally open 'start' switch, the other is a red, normally closed 'stop' switch. I added an LED indicator light just for the 'wow' factor. When you press the start button the contactor closes, but only if the key lock is unlocked. Pressing the stop button opens the contactor, even if the key lock is locked. (That's so that the shop power can be killed in an emergency even if the control box is locked.) The indicator lamp lights whenever the contactor is closed.

To finish the project, I rerouted all the circuits in the shop through the new sub-panel. The only exception is a handful of lights scattered throughout the shop that are controlled by a switch at the door. This provides adequate lighting to enter the shop to retrieve tools without having to power up the entire shop.

There are a few nice side-effects of this setup:

• Because the contactor is magnetic, it remains open after a power failure. Any tools that were running will remain off until the user has a chance to clear their head and calmly turn the power switches off before restoring power to the sub-panel.
• Since most of the lights are hard-wired to the sub-panel, I won't forget to shut off the sub-panel when I leave the shop.
• It provides some protection from electrical fire hazards. If I forget to unplug the soldering iron, for example, it loses power anyway when I leave the shop.
• Shutting down the sub-panel is a convenient, sure way to "safe" tools when changing blades or performing maintenance.
• I can leave my stereo switched on and it powers up with the rest of the shop.
• It impresses visitors to my shop. ;-)

Photos

Note: These photos are of an old installation of the shutoff switch. My new switch uses the industrial controls mentioned above; I'll try to get new photos online within a few weeks.

	The sub panel and contactor box, closed
	The inside of the sub panel and contactor box
	The control station

I didn't include a lot of details here about the contactor or the circuit to control it here. If you're interested in building a similar setup, please let me know and I can share schematics and additional information.