Ground Fault Problem
Hey guys,
I am a part time BSEE student while I work my full time job as an industrial electrician. I ran into a problem that is stumping me that I am hoping to see if any of you have ran into the same issue.
A building that houses manufacturing equipment was commissioned about one year ago, so everything under the roof is new. Twice now, I have run into two ground faults months apart on two seperate pieces of equipment. One was a 480V 3 phase exhaust fan that had a shorted phase to ground, and the other was a heating rod (277V) that also shorted to ground. My problem is that even though each piece has overloads, fuses, and breakers for circuit protection, the short never tripped any of these protections. When the problems occurred, it affected other equipment attached to the same MCC distribution panel. That's when I noticed that when I checked voltages to ground, instead of reading 277 on each phase, I would read 480V on two legs and 0V on a third leg.
I checked the earth ground to make sure everything was properly grounded and found the case of the MCC had 0 Ohms to the grounding rod, as well as the building structure and the equipment. Not sure what would cause the circuit protections to be bypassed like this.
Thanks for any help you can give.
I am a part time BSEE student while I work my full time job as an industrial electrician. I ran into a problem that is stumping me that I am hoping to see if any of you have ran into the same issue.
A building that houses manufacturing equipment was commissioned about one year ago, so everything under the roof is new. Twice now, I have run into two ground faults months apart on two seperate pieces of equipment. One was a 480V 3 phase exhaust fan that had a shorted phase to ground, and the other was a heating rod (277V) that also shorted to ground. My problem is that even though each piece has overloads, fuses, and breakers for circuit protection, the short never tripped any of these protections. When the problems occurred, it affected other equipment attached to the same MCC distribution panel. That's when I noticed that when I checked voltages to ground, instead of reading 277 on each phase, I would read 480V on two legs and 0V on a third leg.
I checked the earth ground to make sure everything was properly grounded and found the case of the MCC had 0 Ohms to the grounding rod, as well as the building structure and the equipment. Not sure what would cause the circuit protections to be bypassed like this.
Thanks for any help you can give.
Replies
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Jay KostaI'm not an EE (or electrician) .....
Were the 2 previous GFs to the system ground wire that is part of the GF protection, or direct to some 'earth ground' that maybe wouldn't be detected by the GF protection? If the fault was direct to earth, then the system ground wire might NOT have been carrying current.
About the overload breakers not tripping -
maybe the GFs were not carrying enough power (watts) to trip the protection. For those 2 GFs that you found, did they actually carry enough current, or might they act like a 'heating element' and have low resistance when cold and then when hot have enough resistance to remain a GF but not trip the breakers? Perhaps something like an 'arc fault'.
Jay Kosta
Endwell NY USA -
Droopy
As far as I can tell, it is to an earth ground. The fan motor is a 10hp 3 phase motor that starts in the MCC with a 100A breaker feeding a 480V distribution panel. The 30A breaker in the distribution panel feeds the motor. The heater is a band heater that is wrapped tight to the barrel of an extruder. This the power starts in the same MCC, different bucket feeding a panel dedicated to the control and distribution of the heaters on the machine. In this panel is a set of relays for heat control and a 15A panel breaker.Jay KostaI'm not an EE (or electrician) .....
Were the 2 previous GFs to the system ground wire that is part of the GF protection, or direct to some 'earth ground' that maybe wouldn't be detected by the GF protection? If the fault was direct to earth, then the system ground wire might NOT have been carrying current.
About the overload breakers not tripping -
maybe the GFs were not carrying enough power (watts) to trip the protection. For those 2 GFs that you found, did they actually carry enough current, or might they act like a 'heating element' and have low resistance when cold and then when hot have enough resistance to remain a GF but not trip the breakers? Perhaps something like an 'arc fault'.
Jay Kosta
Endwell NY USA
At the time I didn't think about taking amperage readings, but in hindsight that would have made sense to check. I don't think it would be considered an Arc Fault, as an Arc Fault is usually a fault in the insulation between wires or phases and the faults I've had happened within components from phase to ground.
Still, every other building on site will trip a breaker if there is a ground fault... not sure where to start to fix this problem.
Thanks for the reply -
Jay KostaI'd start by rigging a test circuit with a load of about 1kW (space heater/hair dryer/etc.) to verify that the GF mechanism is working.
Test 1) Improperly wired receptacle with Neutral going to circuit Ground.
Test 2) Neutral going direct to good earth ground via some sort of 'jumper cable'.
I don't know how GF detection works ...
Does it detect the fault because the current load is DIFFERENT between Hot and Neutral, or by detecting a current load on the circuit Ground?
Sorry that I can't provide any real 'help', but I am interested to learn what you find and how to fix it.
Jay Kosta
Endwell NY USA -
Droopy
Hey Jay,Jay KostaI'd start by rigging a test circuit with a load of about 1kW (space heater/hair dryer/etc.) to verify that the GF mechanism is working.
Test 1) Improperly wired receptacle with Neutral going to circuit Ground.
Test 2) Neutral going direct to good earth ground via some sort of 'jumper cable'.
I don't know how GF detection works ...
Does it detect the fault because the current load is DIFFERENT between Hot and Neutral, or by detecting a current load on the circuit Ground?
Sorry that I can't provide any real 'help', but I am interested to learn what you find and how to fix it.
Jay Kosta
Endwell NY USA
Thanks for the reply. Keep in mind that the equipment is direct wire, no receptacles in the circuit. Also in all industrial distribution the neutral is always bonded to ground. Your reply gave me a few ideas about keyword searches and I came up with this article on EC&M (#-Link-Snipped-#) It gives me a few ideas to check, one being to check the contractors connections in the transformer feeding the MCC.
Thanks again! -
Jay KostaThe 'test circuit' I mentioned was a way that I thought the GF function could be tested.
My limited knowledge about industrial GF devices is from a 1982 vintage text book - "Industiral Electricity (revised)" by Rex Miller. In that book there are several diagrams showing a single GFI device that is connected directly to the Service Panel, and feeds from the GFI device going to a Distribution Panel - and a note says to NOT ground the neutral in the Distribution Panel. But of course the neutral IS grounded in the Service Panel (which is 'upstream' from the GFI device). Also, the diagrams do not show any of the neutrals AFTER the GFI device being grounded to earth. And another note says "Do not ground the neutral conductor on the load side of the Circuit Guard" (after the GFI).
If you have time, I'm interested in learning how your problem gets resolved.
Jay Kosta
Endwell NY USA -
lal
That looks like the ground acts as a third phase! And third phase/line and ground being in phase in this case (as third phase and ground are touching) shows a zero potential difference. That gives me a feeling that the ground connection there is open or of huge impedance.DroopyThat's when I noticed that when I checked voltages to ground, instead of reading 277 on each phase, I would read 480V on two legs and 0V on a third leg.
What about the machine ground to distribution panel ground?DroopyMCC had 0 Ohms to the grounding rod
You are reading an archived discussion.
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