What is the ground rod for?

[coulter]

...The high Voltage to Ground (from Capacitive Coupling) on an Ungrounded AC Power System, stresses the Insulation of Conductors to the point that leakages may be produced - in the form of Arcing.
...

Scott -
Are we discussing re-striking, arcing ground faults? If so, I think we just made a jump to industrial grade systems. I don't know too much about this particular phenomena other than reading IEEE Red Book and IEEE std 142. I've worked on two different ungrounded 480V systems: Navy ships, and feeds to VFDs. Neither had that problem. However, for 480 systems, other than the special cases I mentioned, my preference is resistance grounded Wye.

...Voltages to Ground on Ungrounded Systems may range from as low as 10 Volts, to as high as 1000 Volts. ...

What system voltage are we discussing? 120/240, 240D, 480D/Y, 575, ... 13.8kV. Are we still discussing arcing, re-striking, GF? Without some limitations, the statement is not something I can evaluate.

carl

 

[RayS]

As I understand that's true. But what does that have to do with the above 600V shock enigma-2 says we can get from an un-grounded system as opposed to a grounded system? My question refers speciffically to posts 41, 43, 44, 49.
carl

I was referring to the hypothetical crossed lines scenario.
To answer your question, I believe that with an ungrounded system, phase voltage to ground could float anywhere between 0 and phase voltage, 240 in this case. I suppose in some freak situations where there was a lot of static buildup and extremely low leakage, it could rise above phase voltage, but not "real world"

You may have to draw me a picture on this one. Through what parallel path is any current diverted from the hot wire to to the grounding system? I'm not seeing the science on your explanation.

carl

hmm, dont have the time or knowledge to draw and post a pix, but just imagine an overvoltage on an ungrounded system, with no-where to go except thru the insulation somewhere. Add a solid ground to the system, and the surge has to pass thru at most 1 phase winding to get to ground, and may not breach the insulation.

I have posted before that earth grounding, to me, doesn't seem to do as much for the user as it does for the utility.

 

[tom baker]

On ungrouneded delta 480 v systems, a fault to ground can result in ringing and capacitance charge up to 2300 volts. Mike Holt has a graphic on this in his GvB material.
It was found early on (1940's) that this ground fault would damage the windings in motors, the weakest link in an electrical system.

 

[coulter]

On ungrouneded delta 480 v systems, a fault to ground can result in ringing and capacitance charge up to 2300 volts. ...

Yes, that would be the arcing, re-striking ground fault I described in post #61. Are you suggesting this would explain the over 600V shock delivered by an un-grounded 240V system?

carl

 

[coulter]

...but just imagine an overvoltage on an ungrounded system, with no-where to go except thru the insulation somewhere. Add a solid ground to the system, and the surge has to pass thru at most 1 phase winding to get to ground, and may not breach the insulation. ...

Yeah, that makes sense. I can see where the ground on my side of the transformer would mitigate a crossed line also on my side of the transformer. I'm not seeing that the ground on my side of the transformer would mitigate a crossed line on the utility side of the transformer. I'm also translating this is roughly your opinion - yes?

... I have posted before that earth grounding, to me, doesn't seem to do as much for the user as it does for the utility.

I would agree with that. However, along with an ungrounded system, I am also a fan of surge supressors, and lightning arrestors on my side of the transformer.

carl

 

[RayS]

Yeah, that makes sense. I can see where the ground on my side of the transformer would mitigate a crossed line also on my side of the transformer. I'm not seeing that the ground on my side of the transformer would mitigate a crossed line on the utility side of the transformer. I'm also translating this is roughly your opinion - yes?
carl

agreed- the earth return on the secondary wouldn't help the primary fault.

I would agree with that. However, along with an ungrounded system, I am also a fan of surge supressors, and lightning arrestors on my side of the transformer.
carl

Yeah, I think with the renewed interest in arc flash hazard, an ungrounded delta system may become more common.