Unbalanced voltage on 120/240 system

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tadavidson

tadavidson

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Location
Georgia
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Electrical Contractor
Hi all, It's been several years since I've been on this forum. Now I have a perplexing situation. I have a sub panel located 15 feet from the main 200 amp panel. The sub panel is 100A. It is fed with SE cable and has a driven rod with #6 bare to the driven rod. Today I was called out because of unbalance voltage on the system during the use of a pump motor. The pump motor is fed from a sup panel off the First sub. When the pump motor starts the voltage across the Main breaker in sub panel 1 goes to 180 volts to ground on A phase. I know I have integrity on the grounding electrode that is connected to the "neutral" bar. What am I missing?
 
How about this? Would a "lost" neutral in the feeder from sub 1 to sub 2 cause the voltage on the main in sub 1 to read the 180 to ground on A phase?
 
I didn't open Sub 2. I felt that as long as I had the solid "ground" in Sub 1 that sub 1 would read normal.
 
so If a "lost" neutral between Sub 1 and Sub 2 (which actually feeds that motor) can cause this in Sub 1, I assume that A phase in all other sub panels would be affected. Correct?
 
I know that a "lost" neutral is all that can cause this situation but was hung up because of the effective ground in Sub 1. I quess I was tripping over my education.
 
tadavidson said:
I didn't open Sub 2. I felt that as long as I had the solid "ground" in Sub 1 that sub 1 would read normal.
Click to expand...

Grounding electrodes are very seldom a "solid" path when it comes to resistance of the path. They are not installed to take the place of a circuit conductor, they are there to establish a reference to earth and are a place to bleed transient voltages such as lighting or transients from load switching (typically surges coming from POCO side).
 
tadavidson said:
Hi all, It's been several years since I've been on this forum. Now I have a perplexing situation. I have a sub panel located 15 feet from the main 200 amp panel. The sub panel is 100A. It is fed with SE cable and has a driven rod with #6 bare to the driven rod.
Click to expand...

Besides the lost neutral situation, are the main and subpanels in the same building? Are you saying the neutral in the sub panel is connected to a
GEC and bonded to the subpanel?

Roger
 
kwired said:
Grounding electrodes are very seldom a "solid" path when it comes to resistance of the path. They are not installed to take the place of a circuit conductor, they are there to establish a reference to earth and are a place to bleed transient voltages such as lighting or transients from load switching (typically surges coming from POCO side).
Click to expand...


But isn't the "circuit conductor" you speak of a vessel to the earth ground? If so then the GEC at the connection point of the meter and then from the "Grounding" bar in the panel located within 2' of meter (3R) is less resistance to earth to provide the ground for the system. Yes or no?
 
roger said:
Besides the lost neutral situation, are the main and subpanels in the same building? Are you saying the neutral in the sub panel is connected to a
GEC and bonded to the subpanel?

Roger
Click to expand...

The service drop, meter and Main are pole mounted outside. The Sub panel #1 was located in a greenhouse 24' of wire away. It was originally served with Type SE cable and had a driven rod. There was a bonding jumper installed to the "neutral" bar. There was a #4 bare EGC 4' to a driven rod.

I pulled the SE today and several strands of the bare ALU was broken. I repulled 4 wire and broke the bond. No problems so far.
 
Teach me please.

Teach me please.

All things previously stated here's another question. Is it NECESSARY for a 120/240 system (or any other for that matter) to have a conductor from the "neutral" bar of the main and subsequent panels back to the xfmr tap to avoid the "lost" neutral situation? IF so why? Why doesn't the grounding electrode via the EGC accomplish this?
 
tadavidson said:
All things previously stated here's another question. Is it NECESSARY for a 120/240 system (or any other for that matter) to have a conductor from the "neutral" bar of the main and subsequent panels back to the xfmr tap to avoid the "lost" neutral situation? IF so why? Why doesn't the grounding electrode via the EGC accomplish this?
Click to expand...
...because the EGC and the subsequent grounding electrodes are not connected to the neutral once they leave the Service Equipment, unless you are doing it wrong, incorrectly, or just plain screwed up.
 
ptonsparky said:
...because the EGC and the subsequent grounding electrodes are not connected to the neutral once they leave the Service Equipment, unless you are doing it wrong, incorrectly, or just plain screwed up.
Click to expand...

How kind of you to notice.
 
tadavidson said:
All things previously stated here's another question. Is it NECESSARY for a 120/240 system (or any other for that matter) to have a conductor from the "neutral" bar of the main and subsequent panels back to the xfmr tap to avoid the "lost" neutral situation? IF so why? Why doesn't the grounding electrode via the EGC accomplish this?
Click to expand...

I meant GEC instead of EGC. O'Lord, I'm just a soul whose intentions are good. O' Lord, please don't let me be misunderstood.
 
The neutral of a 120/240 single phase three wire service will normally carry some load current. The NEC for valid safety reasons does not allow the EGC to carry normal current.
For this reason the EGC cannot be in parallel with the neutral to provide "backup" for a lost neutral.
If the POCO service neutral is the one lost the alternate path through the GEC and electrode through the earth to the ground rod of the POCO secondary neutral will have too high a resistance to substitute for lost neutral.
 
tadavidson said:
All things previously stated here's another question. Is it NECESSARY for a 120/240 system (or any other for that matter) to have a conductor from the "neutral" bar of the main and subsequent panels back to the xfmr tap to avoid the "lost" neutral situation? IF so why? Why doesn't the grounding electrode via the EGC accomplish this?
Click to expand...

You are not fully understanding the reasons for having a EGC or a GEC.

The grounded conductor (usually also a neutral conductor but some cases it is not technically a neutral) is nothing more then a current carrying conductor for circuits that utilize that conductor.

It becomes "grounded" when we intentionally make a connection to earth. We don't make this connection for the purpose of extending functional parts of the system as the resistance encountered through electrodes is usually too high for successful results of using it as a circuit conductor.

As I mentioned before we ground the system to create stability in voltage to earth and to help facilitate bleeding transient currents on the system to earth. When we do have a malfunction in the system or inadvertent grounding beyond service equipment it does allow current to flow in undesired paths and can create hazards. Past decisions on how to utilize equipment and the choice to use grounded conductors as normal current carrying conductors has some drawbacks, but it is hard to suddenly say we will no longer use some of that equipment as there is a lot of it out there, especially 120 volt equipment that usually is operating on a circuit with a grounded conductor.

Equipment grounding conductors are there to provide a low resistance path back to the source should an ungrounded conductor develop a fault to objects bonded to the EGC. This in turn will allow a high amount of current to flow and will help allow overcurrent devices to respond quickly because of the rapid rise in current flow when such an event happens, which ultimately results in very short duration of time when dangerous voltage levels may exist on exposed metal components. They are not intended to carry current of the grounded conductor, and if you connect them parallel to the grounded conductor you didn't really gain anything other then some decrease in initial resistance but more potential paths for stray currents to flow when something does fail someday.
 
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