Replies

1. 
   jako17 | Oct 13, 2008 01:52am | #1

   Test for a neutral to ground short in the motor or plug

   
   

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   Edited 10/12/2008 6:53 pm ET by jako17

1. 
   tooljunkie12 | Oct 13, 2008 02:02am | #2

   Thanks, I've checked continuity of the ground terminal of the plug to the bed of the jointer & it has 0.3 ohms. Neither of the other terminals have any continuity. Does a tool with a three prong grounded plug used in a dry garage really need a GFI outlet?

1. 
   knuts | Oct 13, 2008 03:11am | #3

   I'm a firm believer in GFCI. Years ago a woodworker acquaintance was electrocuted using his bandsaw.

1. 
   forestgirl | Oct 13, 2008 05:21am | #5

   Have you any details on what went wrong??forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-) 

1. 
   roc | Oct 13, 2008 07:45am | #7

   One way to have a problem (get your fanny zapped) with a power tool like this is to wear non rubber soled shoes, mocassins, and have the neutral and live plug wires reversed. For example when some bright boy breaks off the ground plug and the plug happens to have the old same size prongs that you can plug in either way or the black and white wires are reversed when they wired up the machine. In this situation you can get current flowing from the case of the machine through you, through your mocassins and into the concrete floor which has some dampness and conducts to the earth.Sounds like the bandsaw was 220 if it was enough to kill the man.With the above sinerio and rubber soled shoes one could still get zapped if you have your hand on another grounded machine or electrical conduit on the wall etc. and this would be worse for two reasons. You don't have the resistance of the damp sock, leather and concrete instead you got grounded metal to bare skin on your hand (better conductor) and secondly the current is now going directly arm to arm across your heart instead of down one side of your body.Sure makes your hand tools look attrractive doesn't it ?

2. 
   brucet9 | Oct 14, 2008 03:46am | #12

   "...and have the neutral and live plug wires reversed. For example when some bright boy breaks off the ground plug and the plug happens to have the old same size prongs that you can plug in either way or the black and white wires are reversed when they wired up the machine. In this situation you can get current flowing from the case of the machine through you, through your mocassins and into the concrete floor..."Why would this happen? The neutral wire is never wired to the case of a motor or the machine, nor is it connected to ground. This scenario is no more likely with reversed polarity than it would be with hot/neutral connected normally.BruceT

3. 
   roc | Oct 15, 2008 09:20am | #16

   >Why would this happen?Good question; I been pondering that passively in the back of my head today. And yet I have experienced this very thing. A chinese grinding machine with an extension cord as I described. I was dressing the wheel and every time I put the metal handle of the dressing bar on the tool rest it felt like some one hit my funny bone and my arm would get numb. I fixed the extension cord plug and never happened again. I think. Or perhaps it was a neutral and ground were reversed at the receptacle outlet. (don't see that being the case) Has been too long now and I do not remember. It was an old house. There was this long wire suspended in the air from the patio wall socket on the house to the shed. When you used the shed you plugged in the wire at the house.Sorry. I am going to try a new thing for me; just read the forum and not write for a whole week. Wish me luck. It will be hard. I got to let my spell check cool down any way; good thing I got one that is water cooled ! But hopefully I will also get more shop time that way.Edited 10/15/2008 2:30 am by roc

   Edited 10/15/2008 2:31 am by roc

4. 
   brucet9 | Oct 15, 2008 11:48pm | #20

   Sounds like minor voltage leak to the motor frame that, with a properly grounded extension cord, was safely conducted to ground; with a defective line/extension cord was not.
   BruceT

5. 
   knuts | Oct 13, 2008 08:31pm | #10

   No, but the insurance company did take the saw according to his wife.

2. 
   Tinkerer3 | Oct 13, 2008 04:31am | #4

   Your problem really taxes the mind because it blows the GFCI after the motor is shut off.  That should eliminate the motor and the wiring as problems.  The only thing I can think of is if there is a capacitor or inductor that builds up enough voltage so it jumps from one electrode to another when it is shut off.  A new GFCI may solve the problem.  A GFCI trips when one conductors is carrying more than the other signifying a leak of current from the circuit.

   If nothing were conductive in the room,  (bed room for instance with dry wooden floor and no grounded appliances, you would have no need of a GFCI.  This isn't the case in a shop where all your tools are grounded.  Conductivity of the ground or other conductors should have nothing to do with it.  Perhaps an electrician or electrical engineer will step in here.

3. 
   HowardAcheson | Oct 14, 2008 06:36pm | #13

   Most codes require GFI receptacles in garages because of the possible involvement with water. And, I have found--and been told by many electricians--that GFI devices are subject to tripping when used with many higher horsepower motors.I have removed all the GFI devices from my shop circuits.Howie.........

1. 
   dougmeyer | Oct 14, 2008 11:02pm | #14

   You would be amazed at what gets wired wrong in houses. My sister had two circuits going into a box under the sink, One was switched and the other was not, but they were on opposite sides of the panel (thus you could get 220 from them) and somehow the garbage disposal was wired up to get 220 when switched on. It would take off like a jet engine for about 30 seconds then its thermal protection would kick out and that was that. Figured it out by plugging a long extension cord in and it worked. Had and electrician look at it and sure enough it was 220. Oops.  So even the pros can have issues.  Looking back on it, it was kind of funny.

   Doug M.

1. 
   Tinkerer3 | Oct 15, 2008 01:53am | #15

   It would seem that someone was a little careless with the code.  The hot wires should be black or red and the neutral white.  Sounds like the mistake was further back than the box under the sink.  With that unprofessional wiring, it may pay to have your whole house wiring checked.

2. 
   roc | Oct 15, 2008 09:27am | #17

   garbage disposal . . . would take off like a jet engine for about 30 seconds then its thermal protection would kick out and that was that.ha, ha, ha, ha, ha, ha, ha, : ))

4. 
   paulsomlo | Oct 15, 2008 10:04am | #18

   The answer is no.  The NEC allows for fixed machinery, meaning apparatus which will be stationary and always plugged in to the same outlet, to be powered from non-GFI circuits.  Many of us have large machines running on 240V, 30A dedicated circuits, non-GFI.  If not GFI, the receptacle which powers the jointer needs to be a single, not a duplex, thus eliminating the possibility of ungrounded metal cased appliances being plugged in.  In my own shop, none of the dedicated circuits are GFI, but all the general ones are.

1. 
   larinUP | Oct 15, 2008 07:11pm | #19

   I'v read "nuisance" trips of GFIs protecting motor (inductive) circuits, at shutoff, is due to the motor's collapsing magnetic field. I have both GFI and non-GFI outlets in my shop. No GFIs on dedicated circuits. Improper grounding is likely much more of a hazard for dedicated power tools. Larry

2. 
   roc | Oct 13, 2008 07:21am | #6

   I have experienced this once in a while with a small tape player boom box in my garge shop. Happens when I turn on once or twice a year. The player is more than twenty years old and has always been in the shop environment but only had this shop about eight years. It works perfectly. Sony brand. I do not sand in this area and it is not very dusty. I no longer use the player as of this year 'cause I am all iTunes and WiFi now!

3. 
   davcefai | Oct 13, 2008 07:53am | #8

   Since the fault occurs after you switch off and the motor is winding down it could be that the current it is generating (a spinning motor is a generator) is leaking to earth.

   You need a megger to test for these faults. It could be that the insulation on the windings is failing. I suggest that you borrow a megger to test the motor or get an electrician to test it. At this moment you are heading towards an alectrocution or fire risk.

   (Transatlantic translation in case it is necessary: Megger = High voltage insulation tester).

1. 
   dgreen | Oct 13, 2008 08:19am | #9

   Why would the failing insulation only trip the gfi on power down? 

    

   ................................................

   Men will never be free until the last king is strangled with the entrails of the last priest.~ Denis Diderot

4. 
   WillGeorge | Oct 13, 2008 09:04pm | #11

   I worked with Electronics for a very long time. However I will NOT post anything that may get you hurt in any. 

   Just a suggestion...

   I worked with electric problems all my life in service.. If I had a problem I 'knew' I could not solve... Or I did not understand.. I would call in a Expert!

   Beter safe than sorry