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Does anyone use GFCI breakers on their CNC machines?

sfriedberg

Diamond
Joined
Oct 14, 2010
Location
Oregon, USA
SeymoreDumore, a GFCI intended for multi-phase supply is a bit more complicated than one intended for single-phase, but the principle is the same. The currents on all the supply conductors (3 hots for Delta, 3 hots and a neutral for Wye) are summed. If the sum isn't (almost exactly) zero, the breaker pops.

As I described earlier, a GFCI is really comparing incoming and outgoing currents. A ground fault will produce an imbalance, but so will lots of other conditions! In a multi-phase system, as long as all the outgoing current returns on the same set of 3 or 4 wires, things are OK. So imbalances between phases are not considered a fault. But if the currents on all the wires don't sum to (almost exactly) zero, that's a fault.
 

M. Moore

Titanium
Joined
Jun 8, 2007
Location
Vancouver Island, B.C. Canada
National code mandates, hmmm, sounds like the lobby group did its job.
I would like to know how many people die every year from electrical faults compared to auto crashes in Europe? I believe in building codes and there are many good rules but some things just make you shake your head. What about the millions of buildings that don't have new wiring? If you die from a fault in one of those buildings you will be fined?
 

SeymourDumore

Diamond
Joined
Aug 2, 2005
Location
CT
A ground fault will produce an imbalance,

That is exactly what I do not understand!
How can a 3ph Delta supply detect imbalance with a one leg ground fault?
2 faults? Yupp, obviously pop the breaker.
Only one leg though ... should still pop the breaker just the same.

From what I thought, all the GFCI circuit did is to monitor and react when even the Neutral conductor had a short in addition to the phase leg.
Without a Neutral though, I don't see what can short out that is not seen by the overcurrent circuit breaker or fuse.
 

steve-l

Titanium
Joined
Mar 2, 2012
Location
Geilenkirchen, Germany
That is exactly what I do not understand!
How can a 3ph Delta supply detect imbalance with a one leg ground fault?
2 faults? Yupp, obviously pop the breaker.
Only one leg though ... should still pop the breaker just the same.

From what I thought, all the GFCI circuit did is to monitor and react when even the Neutral conductor had a short in addition to the phase leg.
Without a Neutral though, I don't see what can short out that is not seen by the overcurrent circuit breaker or fuse.

You are not thinking clearly. Current flows between 2 points. In poly phase circuits you just have more than one circuit. Remember this is AC not DC. Current flows both ways. Both sides of this 2 point circuit is a return path half the time/ Summing currents works just fine.
 

gustafson

Diamond
Joined
Sep 4, 2002
Location
People's Republic
That is exactly what I do not understand!
How can a 3ph Delta supply detect imbalance with a one leg ground fault?

Same way it does on a 240 volt breaker, only with another leg. No neutral involved in 240

I could be wrong, but I do not believe there is a code requirement for GFCI for installed equipment. Only outlets
 

Pete Deal

Stainless
Joined
Apr 10, 2007
Location
Morgantown, WV
DavidScott I think the bigger thing you may run into is too high of voltage. My voltage runs in the high 240’s. I had to install a 3 phase step down transformer on my Brothers.

It's been a while since I measured these currents but I'm sure the 6 amp draw is at idle. So I guess not correct to say 6 amp most of the time. Probably more like 12-20 average while running. I have a peak measuring current meter and did measure the 120 amp peaks.

I'm pretty sure the way all ground fault breakers do their thing is with all conductors running through the same current sense transformer. Current transformer output should under all normal circumstances be zero. Not zero = fault.
 

kustomizer

Titanium
Joined
Aug 17, 2007
Location
North Fork Idaho
Same way it does on a 240 volt breaker, only with another leg. No neutral involved in 240

I could be wrong, but I do not believe there is a code requirement for GFCI for installed equipment. Only outlets

If they are hardwired they don't need gfic, if you plug them in you do as I understand.
 

steve-l

Titanium
Joined
Mar 2, 2012
Location
Geilenkirchen, Germany
If they are hardwired they don't need gfic, if you plug them in you do as I understand.

That is unsafe. Is it against local code? I don't know, buy it is stupid. I had a 400 volt 3 phase 6000 watt heat treating oven and it started tripping my GFCI breaker. It was hard wired. The fault was caused by metallic splatter which caused a leak path through some insulating furnace brick. It was not enough to trip the over current breaker, but the leakage was enough to trip the GFCI. Now, had the oven frame not been correctly grounded, the oven frame would have risen to lethal voltage and still not created an over current trip. GFCI breakers have been designed to eliminate that risk.
 

sfriedberg

Diamond
Joined
Oct 14, 2010
Location
Oregon, USA
FWIW, the US National Electric Code is updated every three years. They don't change the basic stuff, but they do add stuff based on new technology, which might be things like solar panel local generation, or better types of wire insulation, or safety-related stuff like GFCI and AFCI breakers. I had never even heard of AFCI (arc fault) breakers until they got added to the code. Now they are a requirement for certain circuits in residential construction.
 

Pete Deal

Stainless
Joined
Apr 10, 2007
Location
Morgantown, WV
Ever since they changed the code to require that receptacles be installed upside down I concluded it was over run by busy bodies with nothing meaningful to occupy their lives. I know their "reason" but it's stupid!
 

gustafson

Diamond
Joined
Sep 4, 2002
Location
People's Republic
That is unsafe. Is it against local code? I don't know, buy it is stupid. I had a 400 volt 3 phase 6000 watt heat treating oven and it started tripping my GFCI breaker. It was hard wired. The fault was caused by metallic splatter which caused a leak path through some insulating furnace brick. It was not enough to trip the over current breaker, but the leakage was enough to trip the GFCI. Now, had the oven frame not been correctly grounded, the oven frame would have risen to lethal voltage and still not created an over current trip. GFCI breakers have been designed to eliminate that risk.


Again, not necessarily up on the latest code, but hardwired single purpose circuits have never been GFCI protected

You stove? You know right next to the sink, right under the counter from the GFCI outlet? Not GFCI

The fridge? Same deal

Dishwasher, full of water, not GFCI


WE have all worked in shops for many decades and never seen a GFCI on a hardwired machine tool, and nary a zap.

Now, those 120v drops, I could see them being GFCI

Because gawd knows what might be plugged into them, unlike a single purpose outlet


None of this is unsafe or stupid.



Again, you pretty much cannot GFCI a welder, EDM guys may chime in on those.

Losing power randomly to industrial machinery can be dangerous, destructive and expensive.


Please note that in your example the ground system worked exactly as it was intended.

GFCI are intended for times when the electrons are not contained in a big grounded metal box
 

mhajicek

Titanium
Joined
May 11, 2017
Location
Minneapolis, MN, USA
That is unsafe. Is it against local code? I don't know, buy it is stupid. I had a 400 volt 3 phase 6000 watt heat treating oven and it started tripping my GFCI breaker. It was hard wired. The fault was caused by metallic splatter which caused a leak path through some insulating furnace brick. It was not enough to trip the over current breaker, but the leakage was enough to trip the GFCI. Now, had the oven frame not been correctly grounded, the oven frame would have risen to lethal voltage and still not created an over current trip. GFCI breakers have been designed to eliminate that risk.

There's a big difference between a heat treating furnace (fancy toaster) and a CNC machine. Ever look in the electrical box on your CNC?
 

Bobw

Diamond
Joined
Feb 8, 2005
Location
Hatch, NM Chile capital of the WORLD
There's a big difference between a heat treating furnace (fancy toaster) and a CNC machine. Ever look in the electrical box on your CNC?

And occasionally I warm up my burritos in the heat treat oven..

The powder coat/film lube oven... Not a chance, nope, smells too much
like chemicals.

And one of the best, using my foundry furnace to cook burgers. It'll melt 40lbs of aluminum
in about a half hour.. Even though its not wood or charcoal, that wicked crazy stupid high heat,
makes one heck of a burger.. Put a grate over the top, and a few 1" blocks to hold
the cover up. One of these days I need to try it on a steak.

No Grounding Needed.

7071477695_8dff8784b4_c.jpg
 

SeymourDumore

Diamond
Joined
Aug 2, 2005
Location
CT
OK, I have still not found a single GFCI breaker that is 240V/3Ph!
There are plenty under 50 A ( max required by NEC ), but they are all for 208V systems, which automatically means 208/120 Wye with Neutral.

Does anyone have any links to a true GFCI device for Delta services?

Steve ...
From what I saw, at this time NEC only requires GFCI for :
L-Ground voltages below 150V
Current of 50A or less for single phase, 100A or less for 3 phase
Receptacle connections only ( hard wired connections do not require GFCI )
 








 
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