Shop equipment tripping GFIC

[Lee Schierer]

In the recent 120 versus 240 volt lathe thread,it was mentioned that some lathes will trip GFCI's. Are there reasons why lathes do this. And do other pieces of shop equipment have the same problem. I don't have GFCI's in my shop as the wiring predates their use.

[Ken Fitzgerald]

Lee,

I don't know what the problem is. I wired my entire shop. Every 120 vac outlet circuit is on a GFCI and I have not had a single trip.

I didn't put my 120 vac light circuits (2) on a GFCI.

At one time I had a Jet VS Mini lathe and it ran on 120 vac without tripping any of the GFCIs.

[John K Jordan]

I have GFCIs on all 110v circuits in my shop except for those that run the Jet 1642EVS lathes. These each have a Delta inverter which drives a 3-phase motor. I don't know why but these always trip a GFCI. There must be a few milliamps of current leaking to ground, perhaps through some protection circuitry, or maybe something is going on due to induction.

[Mike Cutler]

Lee

Just a gut feel, but any solid state devices acting as a speed controller, or a truer VFD, might cause an issue. In these circuits the neutral reference may be momentarilay lost, or just "off" enough to cause the problem when control and power circuits are initially energized. For residential GFCI's the spec is 5 ma delta, and 20-25 msec's. It would only take two or three complete cycles to trip it.
They make more robust GFCI's, higher current delta, and longer time response, for industrial applications, but I don't know if the're coded for residential installation.

[David L Morse]

Straight from a drive manufacturer: https://library.e.abb.com/public/bbe...EOTN14U-EN.pdf

[Rollie Meyers]

If the tool has a VFD, hardwiring it with a appropriate disconnect removes the requirement for GFCI protection.

[Allan Speers]

I have GFCIs on all 110v circuits in my shop except for those that run the Jet 1642EVS lathes. These each have a Delta inverter which drives a 3-phase motor. I don't know why but these always trip a GFCI. There must be a few milliamps of current leaking to ground, perhaps through some protection circuitry, or maybe something is going on due to induction.

I've never heard of this. However, I'm thinking maybe it has to do with the concrete slab floor? (as in, the shop is a garage, and the machine in question has no mobile base.) Concrete is know to be rather conductive. If a machine's metal chassis is in contact with the floor, maybe that changes the ground potential enough to screw with a GFI?

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EDIT:

Nevermind, David has it for sure, below.

[Allan Speers]

If the tool has a VFD, hardwiring it with a appropriate disconnect removes the requirement for GFCI protection.

Say, are all power tools suppose to have GFI outlets? I didn't know this.

[Rod Sheridan]

Say, are all power tools suppose to have GFI outlets? I didn't know this.

No.

The issue isn't one of what's plugged into the outlet, it's an issue of where the outlet is located.

Check your local electrical codes, in general they're required where the floor is conductive, garages, concrete floors in basements, outdoors.

They're meant for portable tools, the sort of stuff you hold in your hand, machinery isn't generally Ground Fault protected..........Regards, Rod.

[Alan Trout]

In the 2014 NEC all outlets in a residential structure pretty much have to be AFCI or GFCI protected unless hard wired. Lots of crazy issues with this. They are now putting refrigerators on GFCI protected circuits and AFCI protected circuits are not compatible with many electronics and appliances. It has gotten a bit crazy. I know I am an inspector. Not all municipalities operate under the latest code so check.

[Julie Moriarty]

When GFI circuits first made their way into local building ordinances, "wet locations" was the primary criteria in determining if a GFI circuit was required. At first, most municipalities only required them for bathroom receptacles and exterior receptacles. (GFI breakers were far more reliable than GFI receptacles back then and were often the go-to choice.) Soon, kitchen receptacles within 6' of the sink required GFI devices and after that I began seeing them being required in garages, and later, for every bathroom device. All of the codes included the word "wet" in them when requiring GFI circuits (later referred to as GFCI).

The reason garages were included is because the floors can become wet, like when pulling a car in from the rain. Concrete, when dry, has about the same conductivity as a rock. It's not a recognized conductor. But water laying on it is. It's the water, not the concrete, that determines if a GFCI circuit is required.

[Rod Sheridan]

The reason garages were included is because the floors can become wet, like when pulling a car in from the rain. Concrete, when dry, has about the same conductivity as a rock. It's not a recognized conductor. But water laying on it is. It's the water, not the concrete, that determines if a GFCI circuit is required.

Julie, the issue with concrete conductivity is salt and water content of the concrete slab.

There doesn't have to be any surface water on the concrete, a slab on grade that appears dry can have low resistance due to contamination by salts or water content.

Regards, Rod.

P.S. Julie, you got me wondering what the concrete floors in the power building at work are like, so I measured them.

Resistance readings to ground, 500VDC test

- point source ( insulation resistance meter probe pressed against concrete floor) 4,000,000 ohms

- four inch square cover plate pressed against floor 17,000 ohms

- quantity 2 four inch square cover plates pressed against floor 8,000 ohms.

Now I would expect that if I were in bare feet I could possibly have the same ground area as 3 or 4 four inch square cover plates which would result in a low enough impedance to ground for me to trip a GFCI, or receive an electric shock that could result in fibrillation.

The concrete I was testing has never had water or salt contamination aside from salts in the concrete itself.

I would be interested in seeing your test results for concrete where you are.

Thanks, Rod.