[BobVilaDIY]

Just to recap: American electrical system; the outlet has GFCI - tested it using receptacle tester, like the one jgrickett51 (comment #18) mentioned. Line and neutral are properly wired; tested the overload button on the outlet, and the socket tester. Both tripped the line - as expected.

GFCI are deigned to prevent injury and death, so they won't trigger for small currents that can cause a tingling sensation but are not dangerous currents. There is a current threshold that needs to be exceeded for a GFCI outlet to trip. This is why I mentioned that you need to measure the current from the metal parts to ground. This is step 1 to figuring out what you are dealing with. It's like taking a temperature of someone who appears sick and complaining of having a fever.

 

[highfigh]

Good afternoon!

I get a mild electric shock when I touch the metal part of my table saw (DWE7491). This only happens when I turn ON the saw i.e. when the blade is spinning. I confirmed by using a receptacle tester - it shows mild to high AC current.

There are no burnt smell when the machine is turned ON or when it's cutting lumber.

How do I troubleshoot this?

Is the building where you use this separate from the house/main building where the electrical service enters and the breaker panel is located? Also, does the saw connect to a sub-panel? If a sub-panel is in a separate building, it's required that a bonding wire be run with the other power wires, with its own grounding stakes.

If the tester is just a light, it's testing for voltage- use a meter that can measure up to 600VAC (a typical limit).

The fact that the shock only happens after turning the sa won tells me that it has a magnetic switch- can you post a photo of the saw and power switch? I looked at a photo of this model and it shows a magnetic switch- if the neutral was open, you wouldn't be able to use the saw- do you have 120VAC in the same area, fed from the same service?

Extremely dry soil can cause the grounding rod(s) to lose good electrical contact and that can cause many types of problems.

Do you know any electricians who might do a low-cost quick check of the service and grounding?

It's possible that the motor's field is shorting to the frame- turn the saw on and measure voltage from the metal to a known good ground on a different junction box or even better, the breaker panel.

 

[Gary Beasley]

Another remote possibilty that happened to me a few years ago. Squirrels chewed through the neutral cable coming from the pole and caused current through any available ground source. It was going through the cable ground and was melting the connections. Never got any shocks from anything luckily. Power company came out and had it spliced pretty quickly.

 

[Leo G]

What is your meter reading? 294 what? Ohms? Volts? Especially with the 2 leads where they are on the switch.

 

[weepee]

What is your meter reading? 294 what? Ohms? Volts? Especially with the 2 leads where they are on the switch.

BW and MB. Probably reading ohms of the motor windings

 

[weepee]

BW and MB. Probably reading ohms of the motor windings

Oops MW and MBk

 

[Bazoo]

I didn't see this mentioned... are you using a dust collector with the saw? Dust collection units, depending on the conditions and the piping, can cause static electricity to build up in the saw.

We had some pvc pipe as part of our dust collection system at work. Occasionally you'd touch the saw and it'd light you up, I mean it felt like 220. The saw was sitting on linolium. I don't remember what I did to ground it out, but we grounded it and the problem was solved.

 

[woodnthings]

Post 8 above:

It's a weird issue. I doubt it's static shock, but might be?

I didn't see this mentioned... are you using a dust collector with the saw? Dust collection units, depending on the conditions and the piping, can cause static electricity to build up in the saw.

It could be that simple, but it should be checked out to make certain.
I used a bare copper wire on my DC from the separator to the motor. No more shocks!

 

[Karthik]

Thank you all.
Here's the latest, and hopefully the last update. To recap, I ruled out the damaged cable by doing continuity tests on the power chord.

Since the weekend, I did further investigation (aka ChatGPT) - and narrowed down it to three possible root causes.
(a) The Switch (to be specific, the Noise/EM filter capacitor inside it)
(b) Motor
(c) It was just a static shock. Non Contact testers can dish out false positives, and I didn't trust my multimeter or my skills enough to measure the voltage between the metal cabinet and ground. I couldn't get a 120 V reading on my multimeter when I tested it against the socket. (More on this later).

With the help of ChatGPT, I ruled out (b) - did continuity tests for line (black) & neutral (white) wires of the motor with the metal cabinet. I replaced the switch.. but I still got 2 bars on the non-contact tester. Unwilling to touch the cabinet for safety reasons, I got hold of another electrician to diagnose and confirm things were okay. He performed all the diagnostics and declared that the machine was safe to operate.

At this point, it could be (a) or (c). And, he verified it by measuring the voltage between the metal cabinet and the motor housing.. and touched the metal cabinet while I was cutting a piece of lumber. Also, he told me that they don't use multimeter to measure voltage out of the sockets because the contact points are not uniform.. and he uses a plug-in type meter to measure voltage.

So, in summary, it could either be a static zap or a faulty switch. And, the switch replacement should resolve it if it was the latter.

I sincerely appreciate all of you chipping in with your thoughts & troubleshooting steps to guide me - despite being very new to the site!
For those interested in the ChatGPT troubleshooting - here are the chat threads.
ChatGPT - Table saw shock diagnosis
ChatGPT - Test DWE7491 capacitor

 

[Karthik]

Post 8 above:


It could be that simple, but it should be checked out to make certain.
I used a bare copper wire on my DC from the separator to the motor. No more shocks!

I am using a shopvac based dust collector (the dust cyclone w/ 5 gallon bucket) connected to the table saw via ... PVC pipes! This could be it!

 

[jgrickett51]

Good Morning!
My electrician is ghosting me.. and hasn't yet turned up. Didn't want to let the weekend slip away doing nothing.

Just to recap: American electrical system; the outlet has GFCI - tested it using receptacle tester, like the one jgrickett51 (comment #18) mentioned. Line and neutral are properly wired; tested the overload button on the outlet, and the socket tester. Both tripped the line - as expected.

Disassembled the switch board on table saw. Tested the terminals. Here's the interesting part.. the continuity tester shows a reading of 294 when I place the tester leads on the Motor's White (MW) and Black (MB) terminals on the switch - meaning there's a mild short. There was no sound, just a non-1 reading.

(MB - Motor Black, MW - Motor White, LB - Line Black, LW - Line White)

All other tests on the terminals shows 1 i.e. open ckt with the switch OFF.. and loud beep between MB to LB & MW to LW when the switch is turned ON.

Photo 1 is the reference diagram on the switch.
Photo 2 & 3 shows the continuity test reading with the leads placed on MW and MB on the switch turned OFF.

Does this show there's a mild leak between line and neutral? Is this the smoking gun? Is this why there's mild shock only when I turn the line on.. because that's when current flows from the line to the motor leads?

Again, thanks for patiently engaging me.. much appreciated.

My thought would be that there should be NO leakage between the line and neutral. If there is, that could certainly be your problem, though I'm a bit puzzled as to how current is getting through those plastic buttons. I'm also puzzled because that's the sort of thing your GFCI is supposed to detect. I'm not familiar with that meter. Is it set to give you an audible tone if it detects continuity? Asking this because back when the world was young, I thought I'd found a bad component because I saw, very briefly current flowing when it shouldn't have. After lots of messing about, I realized that I had my meter on a setting that was actually measuring the capacitance of the circuit. A sloppy analogy would be thinking i had a leak in a plumbing system because the the first water I dumped into it vanished...until I'd added enough water to fill the pipe.

 

[woodnthings]

I am using a shopvac based dust collector (the dust cyclone w/ 5 gallon bucket) connected to the table saw via ... PVC pipes! This could be it!

I'm no electrician, but I don't think a static shock will trip a GFI?
I think it only monitors the current draw between the AC lines. But I could be mistaken.
In my case, upstairs on a wood floor there are no GFIs, so when I got those powerful, stinging static shocks, nothing else happened.
I didn't know if a shop, vac will generate enough static, but apparently it does!

 

[Karthik]

One of the crucial learnings in this saga was that GFCI doesnt protect you from Line-Neutral leaks when you wear shoes and are not grounded. As you said, GFCI measure current flowing out of line and in via neutral and trips when it detects a delta - assuming that there's a faulty ground somewhere in between. If you are barefoot, the current flows to the ground through you, and that trips the GFCI.

If you wear shoes, and you get an electric shock, you are merely completing the line-neutral-you-neutral circuit - GFCI doesn't detect a difference in current, and hence doesn't trip.

I was wearing shoes!

 

[BobVilaDIY]

If you wear shoes, and you get an electric shock, you are merely completing the line-neutral-you-neutral circuit

Electric shock in this context is a difference in potential. That's why birds can sit on high power lines with no problems.

If you think you were not grounded at all, where do you think current was entering your body and where was it exiting? Were you using 2 hands to grab different parts of the saw to get shocked?

 

[jgrickett51]

I'm no electrician, but I don't think a static shock will trip a GFI?
I think it only monitors the current draw between the AC lines. But I could be mistaken.
In my case, upstairs on a wood floor there are no GFIs, so when I got those powerful, stinging static shocks, nothing else happened.
I didn't know if a shop, vac will generate enough static, but apparently it does!

So this was a static shock? That's a completely different animal from a line current leak. And yes, a shop vac or dust collector will generate a LOT of static electricity. That's why it's important to include a ground wire in non-metallic dust collection runs and conductive jumpers where non-conductive fittings are used in a metallic system. That's not for electrical safety; it's because you don't want static sparks in a dusty environment where they can start a fire. It's high voltage with nearly no amperage, so it's not normally dangerous, although it can be definitely painful.

 

[Karthik]

Yep. Left and right hands were both on the table.

 

[BobVilaDIY]

Yep. Left and right hands were both on the table.

The table is a solid piece though. It can't have a difference of potential across the table. If you were touching the power button with one hand, and the table with the other hand, and only then getting shocked, that would make more sense.

 

[Biotec]

A ground fault circuit interrupter, called a GFCI or GFI,

from the above link
A ground fault circuit interrupter, called a GFCI or GFI, is an inexpensive electrical device that can either be installed in your electrical system or built into a power cord to protect you from severe electrical shocks. GFCls have played a key role in reducing electrocutions. Greater use of GFCls could further reduce electrocutions and mitigate thousands of electrical burn and shock injuries still occurring in and around the home each year. Ground fault protection is integrated into GFCI receptacles and GFCI circuit breakers for installation into your electrical system, especially for circuit outlets in particularly vulnerable areas such as where electrical equipment is near water. Portable GFCls are also available to provide on-the-spot ground fault protection even if a GFCI is not installed on the circuit. The GFCI is designed to protect people from severe or fatal electric shocks but because a GFCI detects ground faults, it can also prevent some electrical fires and reduce the severity of other fires by interrupting the flow of electric current.

This saw does not use a three wire plug that is not frame grounded to Earth in any way.

If I were testing this saw myself I would go to the outlet where the ground of the GFI is located having the saw nearby where my meter can reach between the ground Earth pin and the table top of the saw should be zero voltage or near there since it is a floating surface there could be some static electricity build up from the spinning saw blade but you should not notice it.

If the original poster had a qualified electrician to come out and check the gfci and it tested correctly then you're down to either a pinch cable touching the table top or static electricity.

 

[BobVilaDIY]

This saw does not use a three wire plug that is not frame grounded to Earth in any way.

If I were testing this saw myself I would go to the outlet where the ground of the GFI is located having the saw nearby where my meter can reach between the ground Earth pin and the table top of the saw should be zero voltage or near there since it is a floating surface there could be some static electricity build up from the spinning saw blade but you should not notice it.

If the original poster had a qualified electrician to come out and check the gfci and it tested correctly then you're down to either a pinch cable touching the table top or static electricity.

A qualified electrician would say that a three wire plug is not necessary at all for GFC interrupter to work.

 

[FrankC]

Basically it comes down to the fact that it is nearly impossible to diagnose the problem without a qualified electrician there in person.