Magnetic switch started TS when I dropped wooden legs on table

[Dave Cohen]

Is this normal?

I have a JET cabinet saw and I was making some legs for my out feed table, when I dropped (maybe slammed a bit) a set of 4 on the table saw surface...and the table saw started up.

Also I have a 220V switch in the line for the saw (on the wall, so kids cannot turn on the saw) and when I turned on the saw this morning the saw started too.

Any suggestions? Is the switch on the fritz? If so can I replace it with something simpler that is more reliable and not magentic? This is a used saw no longer under warranty.

[Allen Bookout]

Sounds like a bad switch to me. I would replace it with another magnetic one. You sure do not want that thing coming back on if you get a surge while you are using it.

[Greg Robbins]

That's always been a problem with magnetic switches. I watched a guy hit a table saw motor with just his hand, and time after time the motor would start.

[Bill Spievak]

I had a similar problem with a Griz shaper, didn't happen all the time, and the first time or two I put it down to dust or something in the switch. I cleaned the switch and looked for broken parts, no luck, so when it happened again I replaced the switch, no problems since. I never did find out why or what was happening, keep us informed.

[Steve Clardy]

I had one of the griz switches that the cover was too tight around the on button. It would partially stick, sometimes causing the saw to come on again after I had just shut it off.
I removed the cover and enlarged the hole with a file.

[John Stevens]

A nice explanation by Rick Christopherson, who posts here from time to time:

http://www.waterfront-woods.com/Articles/magneticstarters.htm

Hope this helps.

Regards,

John

[Dave Cohen]

Thanks that was an excellent article...now what to do? if anything, perhaps find a way to secure it more rigidly to the saw?

[Jack Hutchinson]

I've used them for decades and never knew a magnetic switch could turn on this way. Might be one bad accident averted. Never before in the habit of unplugging, that just changed!

- Jack

[Tyler Howell]

This is Too Scary!!!
Play Safe

[fred woltersdorf]

I've used them for decades and never knew a magnetic switch could turn on this way. Might be one bad accident averted. Never before in the habit of unplugging, that just changed!

- Jack
i'm glad i read this post, i too am(was) guilty of not unplugging the ts to change a blade.

[Andrew Shaber]

That is damned scary! None of my tools have magnetic switches but I always thought they were a safety device. I never change cutters with the tool plugged in but I can think of other scenarios when setting up for a cut that could be quite hazardous.

[Al Willits]

I'll thank ya too, while I'll unplug about 90% of the time, that will now change to all the time.

Al...who uses enough band aids as it is....:)

[Dan Barr]

i'll be unplugging all of my tools. I'm guilty of not unplugging for a blade change too.

pretty darn scary is right. i would have nightmares if i lost fingers to a saw that started itself. i might not be able to woodwork for fear of my tools trying to get me.

ciao,

dan

[Jerome Hanby]

I've got the willies too. My saw doesn't have a magnetic switch and the switch is pretty well shielded from accidental contact, but once something slips or falls over, who knows what could happen. I'll unplug from now on!

[Allen Bookout]

It helps to have a plug in box mounted on the saw where you can reach it from the operating position. Since it is so convenient it almost becomes second nature to unplug it when changing blades.

[Larry Frank]

The incidents described here are extremely scary. I guess that I should follow what I have heard over and over at work - "Lock it Out". I am going to be looking for a s 220 switch with the ability to lock it out. Probably will mount it on the wall where I plug my Jet Cabinet Saw in. Might cost a few buck but I think that it is the safest way to handle the situation. I do not like the wear and tear on unplugging and plugging it back in every time. It will be easier to use a switch.

[Dave Cohen]

I actually have an external 220 switch too, and I posted another thread* on the woodnet forum about that...asking other folks' opinions about using that in place of unplugging the saw for blade changes, and consensus was that you still should unplug...and after today, I think I definitely agree.

http://www.forums.woodnet.net/ubbthreads/showflat.php?Cat=&Board=UBB2&Number=3335346&Forum=,,All_Forums,,&Words=&Searchpage=1&Limit=25&Main=3335346&Search=true&where=&Name=43277&daterange=&newerval=&newertype=&olderval=&oldertype=&bodyprev=#Post3335346

[Rod Sheridan]

Well, I tried all my magnetic starters on the General equipment I own and they don't start when you hit them with a stick.

Most of the push buttons are Allen Bradley, and the springs are so strong, you would probably have to hit them with a hammer to get any contact bounce.

I'll continue to unplug the machinery when adjusting it however, just in case..........Rod.

[Matt Meiser]

Lowes carries (or at least carried 2 years ago--they change products like socks) 30A Siemens disconnects which are relatively inexpensive. I have one by my table saw and one by my compressor. They are fusible, so you need to buy fuses too, but I just put 30A fuses in mine, which should never blow since both motors have built-in thermal protection and are on 20A breakers.

[Robert Strasser]

My "Made In The USA" PM 66 table saw had a magnetic starter that was made in China. Guess what failed on my saw. The magnetic switch of course. Inside the switch there is a really cheap plastic plunger that went through another poorly made plastic part. They did not fit properly which would not allow the plunger to move freely. These parts were the most critical parts of the switch. One day, I could not shut down my saw when I hit to stop button. It would momentarily disconnect the power, but after I took my had off the stop button the saw would start back up. The plunger had finally bound up tight.

It is my opinion that most magnetic switches are less safe than the simple on-off switches. Now I'm talking about the Chinese-made switches that come on most "American-style" table saws and jointers, etc. These switches give you a false sense of safety. I know that they are supposed to save you from the machine starting up unexpectedly after a power failure. But the poor quality of the switches out weigh this safety feature.

I feel much more comfortable with my Felder table saw switches, but I still cut off the power at the circuit breaker before changing blades. I highly recommend everyone should do the same.

[Steven Wilson]

If you have your saw hardwired then you need a manual disconnect so that you can lock out your saw. If your saw plugs in then keep your pigtail fairly short and just unplug it and drape the pigtail over your fence or on top of the saw so that you can see it.

[Dave Cohen]

Just to take it to the nth degree...what about a circuit breaker failure...still safest to unplug

[Pete Bradley]

I believe this depends on the quality of the starter also. Note that Rick whacked a plastic cover during his test. I've heard of this behavior quite a bit with the low-cost mag starters that come on a lot of modern offshore machines. However, I could hit either of my two GE mags with a sledgehammer without starting the machine. There's no reason impact sensitivity can't be designed out, but it's clear that not all manufacturers are delivering the same level of safety.

Pete

[Rick Christopherson]

Yes, the quality of the starter may play a role in the likelihood of this happening, but that is only as it relates to the spring force of the relay contacts, and the inertia of the contacts themselves. However, I don't know if there is a direct correlation between contactor quality and the strength of the return spring. This spring force would be more of a function of the solenoid coil pull-in strength.

Under the right (wrong) circumstances, any contactor could self-start, regardless of quality, but that doesn't necessarily mean those circumstances will exist with a proper design.

To me, the most significant aspect of this is the method for mounting the contactor. On a flimsy bracket, with heavy (weight) contacts and a light force spring, the contact's own inertia is what will cause them to close if the bracket is bounced. This was the case on my 15 inch planer.

Some manufacturers have beefed up their mounting brackets, but I haven't looked into this for several years.

[Andrew Nemeth]

I know you said the saw was no longer under warranty but you may contact Jet about this problem anyway. This type of safety problem may actually be something they want to know about and maybe even take care of for you. I actually have an extra Jet electromagnetic switch from my planer that they sent me to correct a wiring problem. Turns out they just had wired it wrong at the factory, so now I have two. PM if you are interested, I know they are pretty expensive from Jet.

-Andrew

P.S. Add me to the list of people who are rethinking their unplugging habits.

[Dave Cohen]

Called JET and tech support had me reposition the mag switch inside the box and strengthen the retaining clip, so that it is now being held fast to the back of the box. He thought that the switch had moved too far forward and was contacting the buttons too much.

[Dave Cohen]

Thanks Andrew, I will see if this fix fro JET works, if the problem recurs I may get in contact

[Brett Baldwin]

Thanks for the thread. This is the first time I've heard of this even though I've worked around mag starters for years. Very important bit of info to be aware of.

[John Gornall]

I am familiar with magnetic starters and have them on all my major machines. However my Nova lathe has a different type of starter that I haven't had time to understand yet. There is definitely no magnetic solenoid but if I pull the plug with the motor running and then plug it in again it doesn't start. It appears to be solid state but I'm just not sure. The motor is 110 volts and reversible by a switch. Anyone know about this?

[Tony Baideme]

All good points have been made. But are we not forgetting the thing that "controls" the "magnetic controller/contactor"? The remote start - stop buttons.

There is perhaps just as much, if not maybe more, possibility of the start button contacts being too close and it only takes a spit second of contact to energize the contactor magnetic coil and the machine starts right up. If it doesn't take much pressure or movement to engage the start button, I would look at that as a possible culprit.

The start button spring should be fairly firm and need a good amount of travel when pushed in to make contact. If you merely have to touch the start button, it is way too sensitive and could lead to the problem we are discussing here now. The stop button, on the other hand, can be a lot more sensitive, so as to need only a light touch to disengage the contactor.

And as far as an additional disconnect, the best, in my estimation, would be a lever activated knife switch type. These have an "over-center" spring loaded set of knives that make the contact. It's really hard to get these to accidentally engage. And, they can be locked out too. This may be what Matt was referring to.

Work safely.

Aloha, Tony

[Tom Veatch]

...There is definitely no magnetic solenoid but if I pull the plug with the motor running and then plug it in again it doesn't start. It appears to be solid state but I'm just not sure. The motor is 110 volts and reversible by a switch. Anyone know about this?

Issue 89 of Woodwork Magazine has plans for building a current controlled magnetic switch. Those plans use a triac, or solid state relay, to switch the coil current in a regular electromechanical relay, but a triac with the proper current rating could just as easily be used instead of the relay. In fact Issue 91 has plans for a device that uses a 25 amp triac to directly control a dust collector motor. Don't know if your system bears any resemblance to the ones described in those articles, but they certainly demonstrate that solid state devices can perform the function of an electromechanical relay.

And such a device wouldn't be subject to inadvertent starts due to weak contact springs. Might have its own set of problems, but that wouldn't be one of them.

[Chris Foley]

The following is a good explanation of SSR. It is taken from Lessons in Eletrical Circuits, Volume 4 - Digital


Solid-state relays

As versatile as electromechanical relays can be, they do suffer many limitations. They can be expensive to build, have a limited contact cycle life, take up a lot of room, and switch slowly, compared to modern semiconductor devices. These limitations are especially true for large power contactor relays. To address these limitations, many relay manufacturers offer "solid-state" relays, which use an SCR, TRIAC, or transistor output instead of mechanical contacts to switch the controlled power. The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED light source inside the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power. This optical isolation between input to output rivals the best that electromechanical relays can offer.

http://www.cybermike.net/reference/liec_book/Digital/04054.jpg

Being solid-state devices, there are no moving parts to wear out, and they are able to switch on and off much faster than any mechanical relay armature can move. There is no sparking between contacts, and no problems with contact corrosion. However, solid-state relays are still too expensive to build in very high current ratings, and so electromechanical contactors continue to dominate that application in industry today.
One significant advantage of a solid-state SCR or TRIAC relay over an electromechanical device is its natural tendency to open the AC circuit only at a point of zero load current. Because SCR's and TRIAC's are thyristors, their inherent hysteresis maintains circuit continuity after the LED is de-energized until the AC current falls below a threshold value (the holding current). In practical terms what this means is the circuit will never be interrupted in the middle of a sine wave peak. Such untimely interruptions in a circuit containing substantial inductance would normally produce large voltage spikes due to the sudden magnetic field collapse around the inductance. This will not happen in a circuit broken by an SCR or TRIAC. This feature is called zero-crossover switching.
One disadvantage of solid state relays is their tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail "open." In either case, it is possible for a relay to fail in the other mode, but these are the most common failures. Because a "fail-open" state is generally considered safer than a "fail-closed" state, electromechanical relays are still favored over their solid-state counterparts in many applications.

[Tom Veatch]

One disadvantage of solid state relays is their tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail "open." In either case, it is possible for a relay to fail in the other mode, but these are the most common failures. Because a "fail-open" state is generally considered safer than a "fail-closed" state, electromechanical relays are still favored over their solid-state counterparts in many applications

That's what happened with the triac controlling my DC. It failed closed and the DC wouldn't shut off normally. Replaced the triac with an electromechanical relay.