Medium Voltage Grounding
Just curious I recently saw the installation of a 4800V medium voltage motor (mechanical equipment) installed there was an across the line starter, but the feeder was from a meduim voltage switchgear setup 4800V there was a ground run from the switchgear to the starter and to the motor. I was under the impression that (and I think confirmed) 4800V is a delta service, so why the 600V ground conductor? what does it do? It's basically an ungrounded system? Correct? I questioned why not put in a resistance grounding system to help control arcing ground faults but am confused what the purpose of the ground conductor is??
The same as if the system were 480V ungrounded.
Even ungrounded systems must have the enclosures grounded.
Why not use an HRG ? Cost or system capacitance is too great.
Not sure I am pretty the system capacitance is not an issue and to me the cost of the resistance grounded system is well worth it the first time you burn up one of those motors. Ok now if it is for the equipment enclosure and the enclosure became energized what would trip?
Nothing, on the first fault. It would become an inadvertantly grounded system at that point. On a second fault you would operate the OCP.
Otherwise, with one fault of an ungrounded conductor to a 'floating' non-current carrying metal enclosure, it would become energized. If a different phase faulted to ground or the adjacent machinery, there would be a potential of 4800 volts between the two.
Last edited by Volta; 03-31-10 at 12:07 AM.
Ok I understand (i think trying to draw the picture in my head) that the ungrounded system (on paper a triangle with the ground runing thru the cente) when one phase faults it becomes the reference point (the faulted phase of the triangle is now at the ground potential). I guess what I cannot seem to figure out is if you have an equipment ground routed with the phase conductors (for example in the motor terminal box) you would have the three phase conductors and the equipment ground connect to the "box" if one phase were to fault to the terminal box what would happen the current would flow thru the enclosure to thru the equipment ground back to the sw.gear correct??
No siginificent current flowing yet, just a little capacitive charging current. Your ungrounded delta system just became an inadvertantly corner grounded delta system.
The facility should be aware, as a ground detection system should be in place.