Short circuit test

I chose to install a GFCI breaker on the line powering my machines. Also, when I rewired my machines I grounded the motor housings, mill/lathe main castings and the metal cabinets they were mounted onto.

I did radio and TV repair when I was young and I was always getting zapped by the cheap tube radios that were designed with hot chassis. Over the years I have become a bit of a grounding zealot.
 
Since this lathe motor is simply single phase 120V, you could provide yourself a good safety factor by just plugging it into a GFCI wall receptacle or put one on that extension cord you're using.

Ted
 
Since this lathe motor is simply single phase 120V, you could provide yourself a good safety factor by just plugging it into a GFCI wall receptacle or put one on that extension cord you're using.

Ted
I thought large motors were never supposed to be run on a GFI circuit? Wrong?
R
 
OK, looks like I found the problem. The extension cord I plug into has 3 outlets and one of them has a loose ground. I tested this by connecting my ohm meter from the lathe to a known good ground, tested all 3 outlets and one intermittently shows no continuity to ground. To check if the hot leg was correct, old plug was not polarized, I measured AC voltage between the lathe and the known good ground. I measured 99 VAC so I reversed them but strangely, I still get 12VAC ???

Thanks for all the help!

You checked voltage between each leg and a known good ground and got 99VAC on one and 12VAC on the other? That is a problem. One leg should be reading around 120V and the other 0V (the neutral). Either the ground you used to test this wasn't good after all, or there is something going on with your neutral bond in your panel somewhere. That looks like the neutral at that receptacle is 'floating'. Obviously an extension cord with a broken ground is a problem, but you may have more than one issue here...
 
You may be thinking of arc fault circuit interrupters (AFCIs). AFCIs have a reputation for not getting along with motors. As far as I know, ground fault circuit interrupters (GFCIs) are compatible with large motors.

Combination circuit breakers that include both AFCI and GFCI functions have become popular. But I would not recommend them for our purposes. I installed a circuit breaker that has just the GFCI function. They are actually more costly than the combination breakers - about $50 versus $40 at the big box stores.
 
I have two motors which occasionally trip a GFIC circuit on startup. One is a synchronous motor on a bathroom fan and the other is brush type universal motor on a can opener. I am not sure that I would want a GFIC circuit on my machinery. I know that I wouldn't want it on the CNC.
 
I thought large motors were never supposed to be run on a GFI circuit? Wrong?R

I am not aware of any reason to not run a motor on a GFCI. As a matter of fact, most construction sites have plug-in modules with multiple receptacles which are GFCI protected and that is what the workers plug power tools into. I have been running my drill press, pedestal grinders, stationary belt sander, etc. up to three HP on GFCI receptacles for years. A GFCI simply monitors the current balance between the line and neutral conductors and if there is an imbalance greater than 5mA, it instantaneously opens the circuit. The only thing I would not recommend running on a GFCI receptacle is your refrigerator and freezer or life support equipment simply because some other device on that circuit could cause the GFCI to trip and you could lose your food or your life.

Ted
 
AFCIs will also trip on ground faults. Instead of the 5ma threshold they are around 15ma. A lot of the issues with AFCI's tripping in the home are really caused by ground faults. It is amazing the amount of household equipment that leaks small currents to ground. Some of the wall switch timers, wall switch occupancy/motion detectors are notorious for this...especially the 'CFL' compatible ones from 5-10 years ago that didn't require a neutral. They actually used the ground as the 'neutral' to operate and as long as they didn't pull more then a certain amount of current UL would pass them.

The old school versions that worked with incandescent bulbs didn't have that problem. They worked by actually passing a small current through the light when the light was switched off...a very small amount. It wasn't enough to light the bulb, but it passed enough current for the electronics in the motion detector/timer to work. That didn't fly with CFLs and LED bulbs.

This is why the electrical code basically outlawed switch loops a few cycles ago. You now have to pull neutrals to the wall switch box even if you aren't going to use it. There are some exceptions to this if the back of the wall remains accessible, or if you run conduit and therefore could pull a neutral later, but the general rule is a neutral is required now.
 
Very informative discussion. Should I consider putting a 240 vac GFI on my lathe circuit? That voltage scares me more than the 120. (which I get shocked by at least once a year somehow!)
R
 
Very informative discussion. Should I consider putting a 240 vac GFI on my lathe circuit? That voltage scares me more than the 120. (which I get shocked by at least once a year somehow!)
R

As long as your gear doesn't have any issues that cause nuisance trips, a GFCI is never a bad idea.

I don't know where it sits currently with the code panel, but last I heard they were actually looking at extending the GFCI requirement to all 240V 15A and 20A receptacles as well in wet areas, garages, basements etc. It is very possible that it will be a requirement in the 2017 code anyways. As of the current 2014 code it is only required on 120V 15A and 20A circuits.

As for danger, A 240V circuit (assuming standard single split phase residential service) isn't any more dangerous than a 120V one. You are never more than 120V with respect to ground. The only way you can get hurt more is if you somehow manage to touch both legs at the same time...but that doesn't usually happen unless you are really trying.
 
Back
Top