- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
You impress me too, I don't understand what happened.LarryFine said:
Ungrounded System
- Thread starter poweringtech
- Start date
- Status
- Location
- Massachusetts
georgestolz said:You impress me too, I don't understand what happened.
I am not following along either.
- Location
- Massachusetts
By the way.....what kind of electrician lands a neutral to ground bond hot?
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
Bob, I'd say that would be among the less controversial tasks I've seen done hot. Hopefully the guy that Larry was working with later came to more safe working habits.
People don't think about the consequences of a small mistake. The "are you chicken?" mentality persists today.
People don't think about the consequences of a small mistake. The "are you chicken?" mentality persists today.
- Location
- Massachusetts
georgestolz said:
It is dangerous.
If there is an existing ground fault down stream when you land it will flash.
It is possible that flash starts a flash over of the 480 feeders you have right in front of your face.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
georgestolz said:You impress me too, I don't understand what happened.
Allow me to explain:iwire said:
Because the neutral wasn't bonded, the pinched-by-the-housing ballast wire became the "ground reference" for the system, and instead of the neutral being at zero volts to ground and the ballast wire being (let's say) 600v to ground, the opposite happened:
The panel neutral, which is normally at (or near) zero volts to ground (whether bonded or capacitively coupled), rose to the ballast's 600v output voltage, because the neutral is part of the lamp circuit. This is similar to what happens when the first phase of an ungrounded Delta faults to ground.
When we bonded the neutral, the EGC (in this case, the fixture housing and EMT) effectively tied the ballast wire and circuit neutral together, shorting that one wire to ground and extinguishing the fluorescent lamp. This is similar to the second phase an an ungrounded Delta faulting to ground.
Not in our case, as the ballast current wasn't high enough to cause that much damage, or even trip a breaker.iwire said:
Make sense? If not, I'll make up a diagram.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
Sorta makes sense. Hit me with the diagram.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Okay, here goes:
1) A typical 2-tube, 8' fluorescent with circuit-interrupting lampholders. Note that we have not bonded the neutral yet, so everything is floating at this point.
2) Now, we short one of the ballast output wires to the fixture housing, and thus to the panel enclosure through the system EMT. There is a few hundred volts between the supply neutral and the ballast's blue wire, and thus between the neutral and the panel enclosure, aka ground.
The neutral bus in the panel exhibits this voltage to ground, but the light still works since there is no conductive pathway between the pinched ballast wire and the supply neutral.
3) Now, we bond the neutral as required, and we have suddenly interconnected the ballast's blue wire and the supply neutral, effectively shorting out at least part of the fizture. I don't recall whether it was the blue or red wire, but I'm sure either one would extinguish at least one lamp.
Please let me know if this doesn't do it. The next step would be a demonstration with a real fixture, but I'd rather not have to do that.
1) A typical 2-tube, 8' fluorescent with circuit-interrupting lampholders. Note that we have not bonded the neutral yet, so everything is floating at this point.
2) Now, we short one of the ballast output wires to the fixture housing, and thus to the panel enclosure through the system EMT. There is a few hundred volts between the supply neutral and the ballast's blue wire, and thus between the neutral and the panel enclosure, aka ground.
The neutral bus in the panel exhibits this voltage to ground, but the light still works since there is no conductive pathway between the pinched ballast wire and the supply neutral.
3) Now, we bond the neutral as required, and we have suddenly interconnected the ballast's blue wire and the supply neutral, effectively shorting out at least part of the fizture. I don't recall whether it was the blue or red wire, but I'm sure either one would extinguish at least one lamp.
Please let me know if this doesn't do it. The next step would be a demonstration with a real fixture, but I'd rather not have to do that.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
No need, I'm smelling what you're shoveling now.
- Location
- Massachusetts
LarryFine said:
Yes it now makes sense, and demonstrates why you should not bond live.
The person landing the bond was lucky not to have gotten knocked on their butt.
- Status