On/off switch question

[Rusty Eads]

If I am in the wrong forum I apologize. Wasn't sure if this belongs here or in the workshop section.

Is a double knife throw disconnect an acceptable on/off switch for a table saw that is wired for 220? That is what came with the saw and looks like it has been ran that way for a while but I was afraid of the long term dangers of using it frequently. Any advice would be appreciated.

[ray hampton]

I believe that this switch is the best switch except for one fact ---are the contacts enclosed in a metal box or exposed to nosy fingers

[Dave Cav]

Knife style disconnects (I'm assuming this is an enclosed switch) are just that, disconnects, used to disconnect the motor from the main power for maintenance or repair. They are not supposed to be used for routine switching on and off. It probably won't hurt the motor, but they aren't designed to be operated under load and will arc and pit the contacts and may eventually probably weld shut. The pitted contacts can also cause voltage drop, which eventually WILL damage your motor. I would get a regular start/stop switch or push button contactor. Grizzly has a good selection at reasonable prices; I have used a couple of their 220 volt "paddle switches" and they work fine.

[Ken Fitzgerald]

For regular turning off and on I use the switch on my tools. Most of them have built in electromagnetic switches.

All my tools except my DC have plugs on them. Typically I unplug tools each evening when I leave the shop.

My DC is the exception. It is seldom that I am not in my shop on a daily basis. If I leave town for more than 48 hours, I turn the DC off at the breaker. The DC is on a remote starter and I would not want someone's garage door opener or the remote failing and having it run for days on end.

Why use plugs on tools?...I can move them where I want them...and lightning can jump the contacts on a breaker or switch. I have seen the results personally.

[ray hampton]

Knife style disconnects (I'm assuming this is an enclosed switch) are just that, disconnects, used to disconnect the motor from the main power for maintenance or repair. They are not supposed to be used for routine switching on and off. It probably won't hurt the motor, but they aren't designed to be operated under load and will arc and pit the contacts and may eventually probably weld shut. The pitted contacts can also cause voltage drop, which eventually WILL damage your motor. I would get a regular start/stop switch or push button contactor. Grizzly has a good selection at reasonable prices; I have used a couple of their 220 volt "paddle switches" and they work fine.


Dave, color me confuse, don't the common switch work the same way as your knife switch / when the handle are moved to another position , the contacts also moved

[Frank Drew]

My understanding is that magnetic switches are considered safer than some other kinds of on/off switches for machinery because they have to be reset to start after a power interruption.

[Dave Cav]

Dave, color me confuse, don't the common switch work the same way as your knife switch / when the handle are moved to another position , the contacts also moved

Knife switches have sliding contacts and open and close gradually as the parts slide together or apart. As the contact area gets smaller (but is still carrying current), arcing can occur as the contact opens or closes. Start/stop switched have flat contacts that snap together more or less instantly and either have full contact or no contact, and are less prone to arcing if sized correctly.

As another poster mentioned, a safety feature of a magnetic switch is if you loose power the machine will not restart when power is restored; you have to turn it back on.

[ray hampton]

Dave, many thanks, I remember older cars that use points-- the common light switch at home would never take the punish of being turn off-on that QUICK

[Jarrett Vibert]

Dave couldn't have explained it better, but just for a point of reference. A switches designated for breaking load, are called "current interrupting", and are designed specifically to inhibit/extinguish the arc created by the opening of their contacts. "Isolation" switches with the same voltage/current rating will quickly fail if used in such a manner repeatedly.

[Chip Lindley]

Is a double knife throw disconnect an acceptable on/off switch for a table saw that is wired for 220? That is what came with the saw and looks like it has been ran that way for a while but I was afraid of the long term dangers of using it frequently. Any advice would be appreciated.

Rusty, that's the trouble with old tools that have come from old shops. Out in the boonies, any old method might be used to turn a machine on/off. Some just use what is available, either out of necessisty or ignorance.

As said, your knife switch is only a "disconnect". Those are included when machines are hard-wired into a circuit rather than plugged into receptacles. Some disconnects include fuses for each leg which protect the motor. The disconnect would be opened to change a blown fuse or work on the motor.

Your tablesaw motor needs protection even if it has a reset button on it. Those may become unreliable after they have tripped a few times. If no reset button on your motor, overload protection is a MUST! Your 2hp Baldor needs a Size 1 starter with at least one overload heater sized for your motor's amperage on 230v. There are usually "manual" and "magnetic" starters to be had pretty reasonably on ebay. Here are a couple of manual starters I found that would serve you well!:

http://cgi.ebay.com/SQUARE-D-MO574-A-STARTER-INSIDE-2510-CG-3-CONTACTOR-/150373501558?pt=LH_DefaultDomain_0&hash=item2302f58a76 (http://cgi.ebay.com/SQUARE-D-MO574-A-STARTER-INSIDE-2510-CG-3-CONTACTOR-/150373501558?pt=LH_DefaultDomain_0&hash=item2302f58a76)

http://cgi.ebay.com/Allen-Bradley-off-switch-enclosure-3-phase-nice-/150515432794?pt=LH_DefaultDomain_0&hash=item230b6b3d5a

Personally I prefer magnetic starters just for the added safety factor. In the event of a breaker trip or other power interruption, the machine must be restarted by pushing the ON button again. Manual starters stay "on" during an outage, and the motor will roar to life as soon as power is restored. That could be scary!