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Table saw and GFI outlet

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Blog entry by Al posted 01-29-2009 08:20 PM 6010 reads 0 times favorited 14 comments Add to Favorites Watch

I joined L J yesterday and already had a great experience with an issue. Need to further tug on you all for another related problem with my table saw. To review, yesterday I blogged regarding my motor on my craftsman table saw and how it struggled to start. After reading the responses I took the path of least resistance and removed the motor and cleaned it (way too much sawdust in it). I remounted it, plug it into an outlet (20amp) which is tied into a GFI outlet and it worked like NEW. I was thrilled. Minutes later, about to use the saw, I plugged it into the same outlet and the GFI tripped. I tested the saw with a non-GFI and the saw worked great again. Back to the GFI and it tripped. I checked for any crimps or cuts and found none. I am now lost. I could only guess that the GFI outlet blew as it’s 10 years old. Any thoughts? Thanks. Al

-- Al Meriden, Connecticut



14 comments so far

View Woodchuck1957's profile

Woodchuck1957

944 posts in 3223 days


#1 posted 01-29-2009 08:35 PM

These kind of posts are really hard to diagnose. If I remember correctly your saw is a older Craftsman Contractors table saw. Is it the one with the motor mount that has a bolt that goes into a slotted bracket ? If so, that bolt only needs to be tight during transport, as in moveing the saw from a jobsite to another. The wieght of the motor should provide enough tension on the belt when in use. Or maybe you need to blow the motor out again, or maybe there is a bunch of dust under the capacitor cover, or maybe your GFI is bad, or maybe your useing too light of an extension cord, or your useing too long of an extension cord, or maybe the switch is full of dust, or maybe the switch is bad. It’s really hard to say without being there to look it all over. Maybe keep useing it on the non GFI, if it keeps working ok, then it’s probably the GFI.

View Scott Bryan's profile

Scott Bryan

27251 posts in 3281 days


#2 posted 01-29-2009 08:53 PM

Mine does the same thing when I plug my Craftsman saw into the existing GFI outlet which is on a 15A circuit. I ran a 20A circuit for my shop tools and put in a GFI breaker and did not have any more problems. I am guessing that the GFI outlet is simply responding to the voltage surge that starting the saw creates.

-- Challenges are what make life interesting; overcoming them is what makes life meaningful- Joshua Marine

View cmaeda's profile

cmaeda

205 posts in 3014 days


#3 posted 01-29-2009 08:57 PM

I had this same problem as well when I upgraded my tablesaw. Check the amps. My newer tablesaw is rated to run on a 15amp circuit but if there’s anything else on that circuit (like lights), the circuit blows. I ended up running a 20amp circuit for the tablesaw and it works good for me.
You could also try plugging in another power tool that uses a lot of amps and see what happens.

View lew's profile

lew

11334 posts in 3215 days


#4 posted 01-29-2009 08:57 PM

It is possible that the GFI is bad/weak.

However, as the GFI is a safety device that protects people from shorts, I have to wonder if there is still a problem in the motor. I would pull the starting capacitor and take it to a motor repair shop to have it tested. They may do it for no charge. My thoughts are that if the starting cap is weak, the motor will start but there may be enough leakage in the cap to “look” like a short to the GFI receptacle.

-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

View rwyoung's profile

rwyoung

388 posts in 2931 days


#5 posted 01-29-2009 09:15 PM

GFI (Ground Fault Interrupter) is supposed to detect leakage current flow as it comes “in” on the hot and “out” on the neutral. Nothing should be flowing in the ground lead. But if the GFI detects something on the order of a few (maybe it is 4 or 6, time for Google) milliamps it will open up. The theory is that the return current has found a path to ground other than back through the neutral.

If your motor keeps popping a GFI, it suggests to me that you may have some failed windings in combination with some “smutz” (technical term) that is just conductive enough to make a path from the winding to the motor housing which should be earth (3rd prong) ground.

Even though you cleaned out the sawdust, there may be residual oil or lubricant that has wept from the bearing races, etc that is making the path. Or it could be that motor vibration over the years has rubbed one or two places on the winding and they are touching or arcing to the motor housing.

Just spitballing these ideas…

-- Don't sweat the petty things and don't pet the sweaty things.

View rwyoung's profile

rwyoung

388 posts in 2931 days


#6 posted 01-29-2009 09:18 PM

Lew -

The ground fault portion of a GFI/GFCI breaker isn’t looking at the total load, only on load imbalance between hot and neutral. If 10A come “in” on the HOT lead, then 10A better go “out” on the neutral lead. There is a limit of detection of a few milliamps, might be as high as 6mA, I honestly can’t remember. The assumption is that if, under the right conditions, you have 120VAC and a few milliamps, you have enough to kill if the alternate grounding path includes you.

However if it is a panel mounted GFI/GFCI it may also have the traditional current interrupter breaker in addition to the GFI function.

Breakers can “wear out” over time. Depends somewhat on what their load lifetime has been and what method they use for detection.

-- Don't sweat the petty things and don't pet the sweaty things.

View Woodchuck1957's profile

Woodchuck1957

944 posts in 3223 days


#7 posted 01-29-2009 10:34 PM

cmaeda also brings up a good point about other this being on that circuit. Like I said, it’s really hard to say what it is. It’s like calling a mechanic about a car problem and expecting a exact diagnosis over the phone, it’s not going to happen.

View EEngineer's profile

EEngineer

1059 posts in 3073 days


#8 posted 01-30-2009 01:04 AM

I think rwyoung nailed it! GFI’s are required to trip at 5 mA. This isn’t much leakage but 10 mA is enough to defibrillate a heart. I have found many motors with much worse leakage than would trip a GFI. They will run for years anyway. Go ahead and run it on a non-GFI circuit.

BUT do, do, DO run a grounded socket and connect the motor and saw frame to ground! My old table saw and motor combination would “tickle” me every time I leaned on the metal saw table until I grounded the outlet and connected the frames to ground. You do NOT want to be the ground path for any motor leakage!

-- "Find out what you cannot do and then go do it!"

View cabinetmaster's profile

cabinetmaster

10874 posts in 3018 days


#9 posted 01-30-2009 02:00 AM

Most older GFI’s are rated at 15 amps. Most saws require a 20 amp circuit. So keep with the 20 amp unless it is wet in the shop than you might want to get a new 20 amp GFI and replace the old one.

-- Jerry--A man can never have enough tools or clamps

View EEngineer's profile

EEngineer

1059 posts in 3073 days


#10 posted 01-30-2009 05:52 AM

Hmmm, let’s be clear here. The title of this post is “Table saw and GFI outlet”. If this is indeed a GFI “outlet”, as opposed to a GFI “circuit breaker”, then there is no current limiting in the GFI outlet itself. GFI circuit breakers have two sections: one that provides overcurrent protection and another that provides GFI protection. I know of no GFI outlets (it doesn’t mean they don’t exist – enlighten me!) that provide GFI AND overcurrent protection.

You can run 20 amps through a 15 amp rated GFI outlet and, as long as the current in both legs (hot and neutral) are balanced, it will never trip! Overcurrent protection to limit current to an amount that the GFI outlet is rated for MUST be provided upstream by a fuse or circuit breaker.

-- "Find out what you cannot do and then go do it!"

View Karson's profile

Karson

35034 posts in 3860 days


#11 posted 01-30-2009 06:03 AM

I was working a new gfi circuit breaker that I installed. (incorrectly I might add,) but hooking up an electric eye would cause the GFI to trip, no other load on the line. So It doesn’t take much to get them to trip. It should not be extra drag of the motor in the saw VS motor only. Something is wrong. Don’t ignore it by not using a GFI.

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Southern Delaware soon moving to Virginia karsonwm@gmail.com †

View Woodchuck1957's profile

Woodchuck1957

944 posts in 3223 days


#12 posted 01-30-2009 06:43 AM

I should know better than get into an electrical discusion, but I will just this one time. If I understand what EE is saying, a GFI doesn’t trip for the same reason that a circuit breaker does. Correct me if I’m wrong.

View EEngineer's profile

EEngineer

1059 posts in 3073 days


#13 posted 01-30-2009 03:12 PM

Google is your friend…

This says it much better than I can. GFI will only trip due to an imbalance in the current flowing through the hot and neutral wires, not due to absolute magnitude of the current flowing through the hot and neutral.

-- "Find out what you cannot do and then go do it!"

View Al's profile

Al

25 posts in 2865 days


#14 posted 01-30-2009 06:38 PM

At this point I think, after reading all the input, that I should upgrade my shop area to accomadate my power tools with 20A service and go to 12 gauge wire. Also change the old GFI to match the 20A. Thanks to all for the great input as it kept me out of “trouble”. In a few short days I’ve been able to realize how much I thought I knew but actually did NOT know. I love this site. Al

-- Al Meriden, Connecticut

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