Fitting an emergency ‘off’ button to workshop machinery

If you’re like me, you’ll have a range of power tools in your home workshop. And, if you’re also like me, some of those will be decades old.

The trouble with some of these older machines – and also some brand new ones for that matter – is that they don’t come fitted with emergency ‘off’ switches. You know, the sort of switch where when something is going wrong, you just slap your hand on the button and the machine immediately shuts down.

But the good news is that these switches are now available quite cheaply. And, if you know your way arounds mains power, fitting a switch can be done in only 10 minutes or so.

The switch

You want to buy a switch that is specifically for the function of switching a machine off in an emergency. These switches comprise a large pushbutton that, once pushed, stays locked down in the ‘off’ position until the knob is rotated to release it.

Inside the housing are two switch mechanisms, one marked ‘NO’ (normally open) and the other ‘NC’ (normally closed). If the wiring is made to the NC switch, when the button is pushed, the circuit will be broken, switching off the machine. And that’s just what we want.

The ones I used are rated at 600V and 10A. Note that if you wish to switch off a high current welder or similar, you may need to use a switch rated to 15 amps.

Since I wanted to fit a lot of these switches, I bought 10 at a neat AUD$100 the lot (including postage) on eBay.

Location

The switches come in a sealed box that is easily mounted on any flat surface. The mounting location is important - you want to be able to easily access the switch from any normal location you’d be standing when using the machine. That’s not quite as obvious as it first looks, though – so think about it carefully before drilling the box mounting holes.

For example, in the case of a table saw that I often use to cut large sheets of plywood or chipboard, I fitted two switches, one at each end of the machine. That way, both the person feeding the sheet, and the person collecting it at the other end of the saw, can access emergency buttons, as required.

Wiring

The simplest way of wiring-in the switch is to insert it in the existing power cord – the machine’s normal on/off switch is not altered. Here’s how to do it – and note, you must be confident and knowledgeable with mains powered wiring before proceeding.

1. Switch off power and remove the plug from the wall socket.

2. Carefully strip off the outer sheath of the power cable until about 100mm of the internal wiring is accessible. Make sure you don’t nick the insulation of the internal wires.

3. In this Australian wiring, the earth (green/yellow) and neutral (blue) remain untouched – they simply loop through the box.

4. The active (brown) is cut and each end is wired to the NC switch. (The unneeded NO switch is easily removed, giving more room).

5. Anchor the cables going to and from the new switch so they cannot be pulled from the box.

6. Screw the top of the box back into position, plug in and switch the machine on. Pressing the button should stop the machine. Always then flick the main on/off switch before turning the knob to release it.

Taking this wiring approach makes installation quick and simple – at the cost of a reduced length of cord available to connect the machine to the wall socket.

Conclusion

At the time of writing, I’ve installed the switches on two bandsaws, a lathe, a belt sander and two table saws. So far, I haven’t had to use any in an emergency, but when I do, the faster and more definite shut-off will be very welcome. Cheap insurance!

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