Strange voltages in electric panel

[Vonze]

Help! I responded to a service call about HVAC system controls not working and the problem seems to be MUCH BIGGER. I have attached a diagram of the service and a document detailing everything I found, but here is the problem in brief...

200A 240VAC standard residential service in the US.
L1-to-L2 read 247 VAC -- ok
L1-to-neutral bus on left read 123 VAC -- ok
L1-to-neutral bus on right read 247 VAChmy:
L2-to-neutral bus on left read 123 VAC -- ok
L2-to-neutral bus on right read 101 mVAC, i.e. essentially no potentialhmy:
Neutral left-to-neutral right read 123 VAChmy: As an odd test, I literally connected a drop light to the two neutral busses with alligator clip leads and the light went on! There is no continuity between the two neutral busses (O.L. for Ohm reading on multimeter).

Point 1) Yes I tried a completely different Fluke multimeter and got same results.
Point 2) The above anomaly (at least as far as the voltages go, not sure about resistance reading) disappeared when a breaker to a 100 A sub-panel was turned off.
Point 3) Moving to the sub-panel, there are 3 single pole breakers that cause the anomaly when turned on (with a load tying to energize) all of them fed from the L2 side.
Point 4) Everything including HVAC worked fine in this house for many years until recent problem became evident with HVAC controls. No physical changes were made to the electric power system nor any circuits that homeowner is aware of. I'm not saying everything in this house was electrically "correct", I'm just pointing out that everything worked.

I know there is a lot to read in the attachments, so I express gratitude in advance for anyone willing to wade through it all.:happyyes:

 

[K8MHZ]

Your drawing mentions no continuity between the two bars in the main panel.

That may be your problem. The two bars should be bonded together and also bonded to the metal enclosure at the main panel. Having the separate in the sub-panel is OK, in fact it's a requirement.

 

[growler]

Your drawing mentions no continuity between the two bars in the main panel.

That may be your problem. The two bars should be bonded together and also bonded to the metal enclosure at the main panel.

It sure sounds like an open neutral on one side. Wonder how that happened?

 

[K8MHZ]

It sure sounds like an open neutral on one side. Wonder how that happened?

I would like to see a picture of the panel.

 

[just the cowboy]

Double trouble

Double trouble

From what you describe it sounds like
1. the two neutral busses are not connected.
2. the L2 line is getting connected to the right neutral buss thru the sub panel .
3. Is the light bright or dim? If it is dimmer than l1 to neutral left then the right buss in panel is open and is getting backfeed thru the sub panel.
4 With subpanel turned off do you get anything to the right neutral?

 

[K8MHZ]

Point 4) Everything including HVAC worked fine in this house for many years until recent problem became evident with HVAC controls.

I am starting to think a lost bond between the two bars may be responsible for the HVAC problem, not the other way around. In most residential panels, a lost bond pretty hard to do. They are usually bonded with a strap or the like and bolted to the neutral terminal.

That's why I am curious about seeing a picture of the panel.

 

[ptonsparky]

I am starting to think a lost bond between the two bars may be responsible for the HVAC problem, not the other way around. In most residential panels, a lost bond pretty hard to do. They are usually bonded with a strap or the like and bolted to the neutral terminal.

That's why I am curious about seeing a picture of the panel.

I have seen this bond fail.

 

[K8MHZ]

I have seen this bond fail.

Any visible signs? Corrosion? Lightning? How did it fail?

 

[JFletcher]

Vonze, I read your notes and viewed the diagram. That field test report was very comprehensive documentation! I think Just The Cowboy is onto something. Just a SWAG From my very inexperienced (and thus probably very wrong) viewpoint, the right neutral buss is not connected to POCO neutral (or left neutral buss), and the "L1-to-neutral bus on right read 247 VAC" problem is the result of what JTC mentioned as point #2. That would also explain 0v L2-right neutral bus (continuity, not potential), as well as the problems disappearing when the 100A sub panel breaker was turned off.

Neutral L-R reading 123V would also not be weird if the L/R bars werent connected and there was a piece of equipment trying to start. Assume Neutral right in the main panel is now electrically isolated/grounded. Neutral Left is "true" neutral. Hook up a 120V piece of equipment that uses Neutral right and L2 as your point #3 mentioned "...all of them fed from the L2 side". The potential from L2 cannot return to source, so it stops at N2, giving you 247V L1->N2 (essentially L-L, just thru a piece of energizing equipment) and 0V L2->N2 (essentially continuity).

SDWAG is that this worked before because there are neutrals tied together elsewhere that connected NL and NR that functioned to keep them electrically tied together, and that has failed, resulting in the problems you have. Or the bonding jumper has come loose/corroded, resulting in the same problems.

 

[ptonsparky]

Any visible signs? Corrosion? Lightning? How did it fail?

It failed at the screw bonding two bars. IIRC, the green screw that would be used to bond the neutral also tied the second bar to the can. Short strap or jumper from neutral bar to green screw tied the two bars together. Screw had not been tightened correctly was my guess and eventually it burned itself free. It was a sub panel with the neutral bonded so we had a little extra work getting that fixed.

 

[johnmeto]

I might want to see a board's photo.

 

[K8MHZ]

It failed at the screw bonding two bars. IIRC, the green screw that would be used to bond the neutral also tied the second bar to the can. Short strap or jumper from neutral bar to green screw tied the two bars together. Screw had not been tightened correctly was my guess and eventually it burned itself free. It was a sub panel with the neutral bonded so we had a little extra work getting that fixed.

The green screw does not bond the two bars together, it bonds the two bars to the metal of the enclosure.

The two bars usually have a strap, like this:

What brand panel was it, do you remember? That type of failure is cause for me not to use them.

 

[ptonsparky]

The green screw does not bond the two bars together, it bonds the two bars to the metal of the enclosure.

The two bars usually have a strap, like this:

What brand panel was it, do you remember? That type of failure is cause for me not to use them.

I don't remember the brand. The call was for an obvius bad neutral and the loose screw was the why.

 

[Vonze]

Thank you all for input!

I would like to see a picture of the panel.

See pix attached (only had video of main, so had to do screenshot).

I'd been locked into mindset that nothing had changed physically (from the way it had been for years), but ptonsparky's statement (about what I think he's saying was a similar experience): "It failed at the screw bonding two bars." makes me wonder if that happened here. I'm afraid the picture doesn't reveal much, so I will have to go back to job to look more closely.

I can't understand how circuits are completed where the neutrals terminate on the right bus. I guess it could be through a ground tied to right bus that bonds to some metal somewhere that has continuity with metal that in turn grounds to the left bus. But in that case, I'd have seen some resistance value between right and left, not open line. We confirmed that single pole circuits from both L1 and L2 sides of main panel and have neutrals terminated on right do work. But how?

just the cowboy states:
From what you describe it sounds like
1. the two neutral busses are not connected. That is what the tests seem to indicate.
2. the L2 line is getting connected to the right neutral buss thru the sub panel. OK, but if I jumper the neutral busses together as a fix, won't I have a dead short somewhere? Remember, "jumpering" the two neutrals though a light bulb, turned on the light.
3. Is the light bright or dim? If it is dimmer than l1 to neutral left then the right buss in panel is open and is getting backfeed thru the sub panel. Hard to say about the light, might have been a little dim.
4 With subpanel turned off do you get anything to the right neutral? Not sure I understand question -- with sub-panel off, voltage anomaly goes away and readings between various points are as they should be. One thing I did not check is continuity (Ohm reading) between right and left neutral busses with sub-panel off. Will do this test.

 

[ActionDave]

4 With subpanel turned off do you get anything to the right neutral? [/I]Not sure I understand question -- with sub-panel off, voltage anomaly goes away and readings between various points are as they should be. One thing I did not check is continuity (Ohm reading) between right and left neutral busses with sub-panel off. Will do this test.[/COLOR]

Where are you checking voltages when the sub panel is off? At the main panel and sup panel? Both?

 

[ATSman]

Thank you all for input!



See pix attached (only had video of main, so had to do screenshot).

I'd been locked into mindset that nothing had changed physically (from the way it had been for years), but ptonsparky's statement (about what I think he's saying was a similar experience): "It failed at the screw bonding two bars." makes me wonder if that happened here. I'm afraid the picture doesn't reveal much, so I will have to go back to job to look more closely.

I can't understand how circuits are completed where the neutrals terminate on the right bus. I guess it could be through a ground tied to right bus that bonds to some metal somewhere that has continuity with metal that in turn grounds to the left bus. But in that case, I'd have seen some resistance value between right and left, not open line. We confirmed that single pole circuits from both L1 and L2 sides of main panel and have neutrals terminated on right do work. But how?

just the cowboy states:
From what you describe it sounds like
1. the two neutral busses are not connected. That is what the tests seem to indicate.
2. the L2 line is getting connected to the right neutral buss thru the sub panel. OK, but if I jumper the neutral busses together as a fix, won't I have a dead short somewhere? Remember, "jumpering" the two neutrals though a light bulb, turned on the light.
3. Is the light bright or dim? If it is dimmer than l1 to neutral left then the right buss in panel is open and is getting backfeed thru the sub panel. Hard to say about the light, might have been a little dim.
4 With subpanel turned off do you get anything to the right neutral? Not sure I understand question -- with sub-panel off, voltage anomaly goes away and readings between various points are as they should be. One thing I did not check is continuity (Ohm reading) between right and left neutral busses with sub-panel off. Will do this test.

If you are scared of connecting the two neutrals together then I would connect a fuse or breaker in series with a jumper, tie the two neutrals together and take your readings. I have a feeling this will correct your problem. Then bond them together properly.