I'm working in an industrial facility and we have an unintentional grounded system, on a 480V Y system. During our last shut down we were still unable to locate the unwanted short. Besides the unwanted short what other ramifications does this present. Stray voltage? Safety hazards? Any ideas are appreciated.
Unintentionally grounded system
- Thread starter binney
- Start date
- Status
If you know it's a WYE system then it should be grounded. I assume it's an ungrounded delta and accidental type. One of the advantages of the ungrounded system is allowing equipment to continue to operate when one phase goes to ground,. Once that "ground" has occurred you loose the advantage, set up a scenario where you have higher potential between equipment in the event of a fault.
- Occupation
- Licensed Electrician
I agree with Augie. I've never worked on an ungrounded system but I always thought they were delta. I do know that your first fault to ground turns your ungrounded system into a grounded system and if that is not cleared your second fault to ground is going to cause a spark or a blown fuse or breaker and shut something down.
robbietan
Senior Member
- Location
- Antipolo City
I have seen several ungrounded wye installations, all code violations BTW. One was having trouble with his PLCs; the PLCs would shut down even during a mild voltage sag. After installing proper grounding, the PLCs proved very tolerant of voltage sags
- Location
- Illinois
- Occupation
- retired electrician
Not all ungrounded wye systems would be a violation of the NEC.
- Location
- Illinois
- Occupation
- retired electrician
The biggest issue with a ground fault on an ungrounded system is what might happen if there is a second ground fault on a different phase. One or both of the ground faults may not be solid connections and result in serious arcing if there is a second ground fault.
robbietan
Senior Member
- Location
- Antipolo City
correct. however, most of the cases I was called in for were all violations
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
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- EE (Field - as little design as possible)
- Occupation
- Licensed Electrician
Interesting. On one hand with the present readings it sounds like an ungrounded wye, but on the other hand, with no faults, how would all three phases read 277V to ground on an ungrounded system? I would think the voltage to ground would swing around between phases just like when an XO does not get bonded properly on a solidly grounded system.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
In a truly ungrounded system equal stray capacitance to ground on all three phases (mostly in three phase loads like motors, rectifiers and heaters) will form a capacitive voltage divider to create a virtual neutral point at ground.
- Occupation
- Licensed Electrician
I'm listening but you are going to have to break this down a little more before I understand it.
Capacitance is a charge that can build up and is one of the problems with ungrounded electrical systems, yes? How do we get from there to a virtual neutral point?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Some people refer to this 'virtual neutral point' as phantom voltage. It is not stable, it does move a little based on loading, but most electricians seem to automatically 'round' the measured value to 277V when they discuss it.
Among other things, it is due to capacitive coupling between the normal current carrying conductors and the EGC.
The C phase is grounded. The voltage C-ground approx 7-10 volts is due to current flowing in C phase conductor; at grounded point, that voltage should read zero; so by tracking the voltage drop for minimum value along the C phase conductor for various ground points, the grounded point of the C phase may be located, IMO.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If you run equal resistors from each phase line to earth ground (or the EGC) you will have created a high resistance ground, electrically similar to having a high resistance ground at a real wye point.
If you use equal capacitances instead, you do the same thing but with the oossibilityvof a DC offset.
A small leakage resistance to ground anywhere in the system will take out the DC.
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
binney -
You really need to go find out if the system is un-grounded wye, or HGR. All the symptoms you describe are indicative of HRG. And HRG is quite normal for industrial.
Different techniques are used to troubleshoot - depending if un-grounded wye or HRG.
Really: First go figure out the system - then do the troubleshooting.
ice
- Status