Ungrounded plant

[Smart $]

Not enough CURRENT to elevate VOLTAGE.

And now two steps back...

You know, I'll spend all the time it takes to read about this but I just can't follow some of this stuff.

As I thought about it- even if the chassis has a phase shorted to it from somewhere across the plant, the difference between the two leads of the xmfr is still 24v so it will continue to work.

That I can get on board with. But what you just said really spun me out. I'm still looking at something through a somewhat distorted lens.

Correct on the 24V. It is referenced to the equipment ground, even if that ground has a voltage above the zero reference at the system bonding jumper.

Grounded systems rely on grounded conductive pathways to carry enough fault current back to the source so as to cause the overcurrent protective device to trip. These pathways have resistance. Any time there is current through a resistance, there is a voltage differential between the points where current enters and leaves the pathways. Basic Ohm's Law.

 

[kwired]

Not enough CURRENT to elevate VOLTAGE.

And now two steps back...

You know, I'll spend all the time it takes to read about this but I just can't follow some of this stuff.

As I thought about it- even if the chassis has a phase shorted to it from somewhere across the plant, the difference between the two leads of the xmfr is still 24v so it will continue to work.

That I can get on board with. But what you just said really spun me out. I'm still looking at something through a somewhat distorted lens.

If a phase is shorted to a grounded object somewhere clear across the plant, you now have a grounded system. This doesn't mean current is flowing somewhere not desired it just means there is a reference to ground on that particular system conductor, and that voltage readings throughout the system will read line to line voltage to ground. You need a second ground fault (even if on the same phase) before you will have objectionable current.

 

[pintofstout]

Oh I'm not talking about tripping faults. I get why this voltage has no current, no way to return to its source. I see what you meant in that "current to elevate voltage" statement.

What I was referring to was simply the ability of 24v devices to continue to operate as intended but that, too, I have marinated in long enough to truly understand.

Thanks again. Really appreciate all the help.

 

[don_resqcapt19]

As far as the 24 volt control circuit having a connection to earth along with other systems, that is the way it is any time you have grounded systems. The fact that you have a accidental ground fault on an ungrounded system is not an issue for any of the other systems that are grounded.

It is likely that there are a number of systems in that plant that are grounded and have a connection to the earth. The utility supply, the 480/277Y systems or the 480 corner grounded systems, the 208/120Y systems, and your control power transformers.