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I am refurbishing an old Craftsman 10" table saw that has been collecting dust in the back of my shop for years.

I don't remember exactly how I acquired it, but to my recollection the one time I tried to start it sparks flew from the on/off switch. The switch controls an integrated 110vt receptacle which the motor plugs into. All is factory original.

I tested the motor by plugging the motor power cord directly into a wall receptacle and it runs fine. I would like to resume using the factory On/Off switch (because it is a safety switch- pull On, push Off) but wanted to check it first.

Opening the switch box I discover that the switch is double-pole, i.e. opens and closes both the hot and neutral wires that lead to the motor receptacle.

  • why did the manufacturer install a switch that opens/closes both the neutral and hot?
  • can I safely use the switch to only open/close the hot (the neutral side is dead/open)? I can wire the neutral to bypass the switch...
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    Is there any substantial chance that a table saw owner might wire their own plug (or the socket) and accidentally reverse L and N? – Andrew Morton Dec 30 '19 at 19:12
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    Same thought here: in the case of a swapped L and N, this makes sure the motor isn’t energized when turned “off”. – jbeldock Dec 30 '19 at 19:25
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    Just looked it up. This is a safety technique called double-switching. See answer and link below. – jbeldock Dec 30 '19 at 19:30
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The technique is called double-switching. According to this article, on AC equipment the technique is used to avoid dangerous conditions when hot and neutral are reversed, as often happens with outlets that have been improperly wired. It is permitted by this exception in NEC 404.2(B):

404.2(B) Grounded Conductors. Switches or circuit breakers shall not disconnect the grounded conductor of a circuit. Exception: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.

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    Thanks, I had a feeling it was safety related. I will bypass the neutral side of the DP switch and rely on switching the hot side only as I am the only person using the machine and will make certain H&N are correctly wired. – Jimmy Fix-it Dec 30 '19 at 19:54
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    I am the only person using the machine and will make certain H&N are correctly wired - new switches are available for those saws on eBay and from other vendors. Might be worth replacing it vs bypassing the safety feature just so it's next user doesn't get a bad surprise some day down the line. – dwizum Dec 30 '19 at 21:04
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    @dwizum, good call; I will order a replacement switch. In the meantime I had need of the saw and it worked fine with the neutral bypassed. Thanks for the input. – Jimmy Fix-it Jan 2 at 4:42
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Probable reason: they made ONE saw assembly and used a motor that could be configured as 110 or 220. For 220 you would break both lines, for 110 you don't need to, but there is nothing saying you can't (so long as your switch breaks both lines together), so it's just easier to have everything the same.

  • Thanks for the response, I understand your point. However, it seems odd that they would go through the trouble of factory installing a double-pole switch that leads directly to an integrated factory 110vt receptacle, which the motor power cord, equipped with a standard 110vt plug, is inserted into. – Jimmy Fix-it Dec 30 '19 at 18:51
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    @JimmyFix-it they could use a single-pole switch for 120V saws, and save 2 cents per switch, but then, they'd need to maintain two different SKUs. Have you considered just hitting SearsPartsDirect, Galco or Grainger and getting another switch? – Harper - Reinstate Monica Dec 30 '19 at 20:32
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    Another reason is Philippines, where you have to switch hot+neutral because about half the Philippines is center ground. (north American split-phase with neutral eradicated). – Harper - Reinstate Monica Dec 30 '19 at 20:33
  • Hello @Harper - Reinstate Monica. Using a double-throw switch in a gang to create a single-throw switch to allow switching of a 230V higher voltage is unlikely. Both contactors in the gang see the 230V higher voltage and so have to designed to insulate that higher level of potential difference. Therefore, the lower voltage switch is being operated beyond specification, even if ganged. If the part is upgraded to switch the higher voltage then the contactor will only see half the current at 230V than at 110V and so a double-throw would not be required. Best wishes. – vk5tu Jan 2 at 4:22
  • @vk5tu Insulation is not the issue (and I mean double-pole). On a Philippine or North American 240V load, if you don't throw both legs, you have failed to de-energize the machine and lethal voltages are present even with the machine off. That's not allowed. – Harper - Reinstate Monica Jan 2 at 18:02
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I don't think anyone can answer why that it a double pole switch or how the saw was originally wired.

You can remove the neutral wires from the switch and connect them together. Make sure you carry the neutral integrity to the power source so you don't end up switching hot. Also, did you determine why you had sparks flying when you tried turning it on before? Check that on-off switch thoroughly.

  • Thanks, I believe the sparks were from the neutral side of the switch failing. It is open on that pole, checked with continuity tester. – Jimmy Fix-it Dec 30 '19 at 19:56
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    ...which does not bode well for the hot side of the switch lasting much longer. Do yourself a favor and replace it. – Ecnerwal Dec 31 '19 at 3:20
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My understanding is that should a short happen down stream of the motor on either side, the safety switch cuts both sides because you are dealing with 110v AC here.

If there was a short to ground anywhere, the motor could still spin (at half speed) due the fact that AC alternates directions with every cycle.

Breaking both poles at the same time means that no electricity can flow in either direction.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Dec 31 '19 at 18:22
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    If your house is wired correctly, neutral should always be within a few volts of the ground potential, so a short from neutral to ground in the saw will not cause the motor to spin, so that would not be a reason to cut neutral with the switch – cat40 Dec 31 '19 at 20:41
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    Welcome to Stack Exchange. You write: "If there was a short to ground anywhere, the motor could still spin (at half speed) due the fact that AC alternates directions with every cycle." – I don't think that's right. Are you thinking that AC means that power is supplied on the hot wire half of the time and on the neutral wire the other half of the time? It does not; under normal circumstances, power is always supplied on the hot wire and never the neutral wire (except perhaps a little bit of stray voltage) – Tanner Swett Jan 1 at 6:10
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Does the cord have a grounded plug (3 prong?) or 2 prong? If 2 prong is it polarized (one prong slightly wider than the other, which is the neutral prong). If the saw is old enough not to be grounded, nor have a polarized plug, it's really important to have a double disconnect if the saw in plugged in backwards, very easy and "normal" to do if not grounded or polarized. So I agree with the post above, that if the saw is plugged in backwards, with a single pole switch, you'd be cutting off the neutral, rather than the hot, which is NEVER a good idea.

Note that the Neutral is at ground potential, meaning there should be no voltage between the neutral and ground under normal operating conditions.

If I were rebuilding the saw, given it's age, I'd replace the cord with a 3 wire cord with a grounded plug (you can buy those with the plug pre-wired at Home Depot or Lowes, replace the switch with another double pole switch and attached the ground (green) wire to someplace metal on the frame for proper grounding. Be sure to pay attention to the amperage draw of the motor and that your wire and switch are of the proper size. You should be able to find that on the nameplate on the motor. Not knowing that right now, I'd suggest at least 14ga wire and 15-20 amp double pole switch. It wouldn't hurt to go to 12ga wire which is rated at 20 amps (might be hard to find pre-wired, might have to buy your own plug). Hope this helps.

  • The voltage on neutral may deviate significantly from ground if multiple devices are plugged into a power strip that's fed by a long extension lead, and a device with a high startup current is switched on. If for example an extension lead had a resistance of 0.25 ohms and an air compressor required 20 amps starting current, but at the moment that it started up a screwdriver was brushing against ground and neutral on the other device, with a resistance of 0.25 ohms, then instead of the startup current causing 10 watts to be briefly dissipated in the hot and neutral feed wires, ... – supercat Jan 1 at 20:05
  • ...it would cause 10 watts to be dissipated in the hot feed wire, 6.6 watts to be dissipated in the neutral wire, 3.3 watts in the ground, and 3.3 watts in the screwdriver contact point. Even if that wouldn't be terribly likely to cause significant damage or direct injury, it could still be very startling, and that could in turn result in injury. – supercat Jan 1 at 20:08
  • @George Anderson, thanks for the input. It is a grounded plug with a substantial integrated cord (I suspect 12/3 SJO), so no (or very small) chance of switching neutral vs. hot. I also checked the wall receptacle wiring that I intend to use. I suppose a future user could plug in to a mis-wired receptacle so I am ordering a factory replacement double-pole switch. In the meantime, it works great! – Jimmy Fix-it Jan 2 at 4:52
  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, props for taking our tour before posting; few newbies do. – Daniel Griscom Jan 2 at 12:46
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Table saws, can remove a lot of material very quickly. This makes them very effective but it also makes them very dangerous. That is the whole reason your manufacturer used a special switch in the first place.

A double pole switch considerably reduces the chance of the motor turning on unexpectedly due to a fault in either the switch or the wiring.

This is not the place to cheap out, trying to make a faulty original switch work by bypassing one side of it or fitting an inappropriate modern switch. Fit a modern switch designed for the purpose. Preferably one with "no volt release", so the saw doesn't start up unexpectedly after power is lost and restored.

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A table saw motor contains components like capacitors and will switch into generator mode when unpowered. That can leave enough voltage while the saw runs out to cause twitching or flailing, and the vicinity of a rotating table saw blade is a bad location for twitching or flailing. Without a neutral to base that voltage off, it's a lot harder to get that voltage to cause damage as you'd need to touch two rather than one conductor.

There is a reason table saws usually have double interruptors, and it's not a good place to try trading safety against a few dollars.

  • The generator theory is interesting and I can see how, after switching off, an electrical motor could/will become a generator while spinning down. What specifically could this bad bit of induced current in the motor windings cause? What is "twitching" and "flailing"? – Jimmy Fix-it Jan 1 at 16:30
  • @JimmyFix-it: The reaction of an operator touching the wires in question. I don't think cutting hot and neutral is necessary to resolve this particular issue, but it may be good nonetheless. – supercat Jan 1 at 20:09

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