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Doc Green’s Woodturning Site

Power Line Switch for PM

The Powermatic 3520B (and other models) does not have a switch to turn off the 240 VAC input power to the lathe. The Start/Stop switch (big red knob) on the control panel only switches a low-voltage control signal to the inverter. This means that as long as the lathe is plugged in, power is applied to the inverter and display.

There is no general agreement as to whether it is best to remove the input power between turning sessions or simply plug it in and forget about it. Some turners unplug the lathe after each session; others unplug it only when thunder storms are likely or when the lathe will not be used for an extended period.

It is a simple matter to add a switch in line with the power cord to eliminate the hassle of dealing with the plug. A large switch is not required.

You do not need a big metal box with a handle (a disconnect) that looks like something you might find in a factory. One type of switch that is suitable looks like an ordinary light switch and fits behind a standard switch plate.

This article describes the switch, connections, and one possibility for mounting the switch. No special electrical skills are required.

The Power Cord

The power cord contains three wires: two hot wires (one white and one black) and a ground. The ground wire is green.

The Switch

A double-pole single-throw (DPST) switch is required and can be purchased at a big box or hardware store. The switch will have four terminals.

Another type of switch having four terminals looks almost exactly the same but operates quite differently and will not work in this application. This is a “four-way switch” and is used where it is desirable to turn a light On or OFF at three or more different locations. (Imagine a workshop having three doors with a light switch at each door.)

The photos below illustrate the similarity in appearance of the two types and the function of a four-way switch.

If you accidentally get the wrong switch, power will be applied to the lathe but it will not turn OFF no matter which way you flip the toggle. The four-way switch simply reverses the wires of the power cord, which makes no difference in the power going to the lathe. So, . . .

Don’t get a four-way switch.

The Connections

The switch is inserted in line with the white and black wires (the two hot wires) in the power cord. That is, the wires are cut and the four loose ends are connected to the switch terminals.

The ends of the white wires connect to one side; the ends of the black wires connect to the other. It does not matter which color goes to which side, and it does not matter whether the wires from the plug connect to the upper or lower pair of terminals.

Mounting the Switch

The switch must be mounted so that no wires or terminals are exposed and the ends of the line cord are firmly anchored where they enter the switch enclosure. As long as these requirements are met, there is considerable lattitude in how the switch is mounted.

My preference is to make a custom enclosure out of wood because dry wood is an excellent insulator and also because you can tailor the enclosure to your particular situation. My enclosure mounts flat on a painted wood surface.  

The sketch at right shows one method for making a wooden enclosure. Four pieces are glued to a plywood base to form a box. The interior opening for the switch is 1 1/2” wide by 2 7/8” long. It is 1 1/2” deep. A standard switch plate should just cover the entire top of the enclosure.

If your work space has open studs, it may convenient for you to purchase a switch box that mounts on the side of a stud. My suggestion is NOT to use one made of metal. Metal is a conductor and a metal box must be grounded. This means you must connect the green wire in the line cord to the box.

One way to do this is to include an extra wire (a pigtail) in the splice of the ground wire and then connect this wire to the box.

Many of the available boxes do not have a good means for anchoring the cord where it enters the boxes. It is a simple matter to anchor the cord outside the box with a wooden clamp secured by a screw. The photo at right shows a cord clamped to a 4x2 with two clamps, one of which is a bit fancier than the other.

What could go wrong?

If, after installing the switch, the circuit breaker trips immediately when you plug the power cord back into the receptical, or if the breaker trips when you turn the switch ON, you have made an error in wiring the switch. Specifically, you have hot wires of different colors going to the same electrical side of the switch. Refer to the diagrams above and then check your wiring. You will find the error.

If flipping the switch toggle does not remove power to the lathe, you have inadvertently gotten a four way instead of a double pole switch.

If you turn the switch ON and nothing happens, like no power is applied to the lathe, you have either forgotten to make one or more connections, or a connection has come loose. Check the connections to see that they are tight.


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