Loss of neutral-ground continuity, but still have neutral load.

[Cberg]

I have a newly completed large commercial building, 2500A 480/277v 3 phase service with 400kw 480/277v 3 phase diesel backup generator.

My issue started with a call from AV department complaining of AV equipment, laptops/their power supplies, thinkpad batteries, and some network equipment being “burned up”.

First visit we check receps/panels for the area and find no issue. Good voltage L-N L-G and no voltage N-G. Can’t find any issue that would cause their problem so we let it go thinking maybe it was a weird fluke.

Get another call couple weeks later multiple computer monitors have had their hdmi ports fried. Monitors all still work fine with dvi connection just not hdmi. Again we have all good voltages nothing out of the ordinary voltage wise. But while amp clamping taking readings in a panel I find we have almost 5 amps on the panels ground and no continuity neutral to ground.

Ok so we have a lost feeder neutral I think right? However I still have normal voltages L-N and still have a load on the panels main neutral. I go to the panel feeding this panel and neutral ground continuity is good, but I still have a load on ground. I go to the transformer feeding this panel to check that it’s properly bonded and it is, but I still have a load going back on GEC to the ground system.

Everything is properly bonded at MDP, generator and transformers, building steel, cold water all of that. The building has a ton of grounding, large ground busses in all electrical rooms everything is tied together ground wise, multiple ground rings all that. Majority of building is hard piped with some mc.

We’ve checked neutral connections on everything feeding to the problem panel that has lost continuity.

Is it possible to have a lost neutral but still have a load on neutral, normal L-N voltage no N-G voltage and loss of N-G continuity? And could this cause the load on our egc or is that just normal current it’s picking up from any list of things?

 

[retirede]

I presume the equipment “burning up” is 120V-powered. You don’t mention the source of that power, only 480/277.
Guessing there’s a transformer outputting 240(208)/120?

It sounds like you may have an intermittent neutral connection somewhere.

Edit - all my laptops have power supplies that are rated 100-250V. Really odd that those would be having problems!

 

[Cberg]

I presume the equipment “burning up” is 120V-powered. You don’t mention the source of that power, only 480/277.
Guessing there’s a transformer outputting 240(208)/120?

It sounds like you may have an intermittent neutral connection somewhere.

Edit - all my laptops have power supplies that are rated 100-250V. Really odd that those would be having problems!

Yes sorry the panel with lost continuity is 120/208 3 phase life safety power on generator backup. Transformer feeds panel 1, panel 1 feeds panel 2 that has loss of neutral ground continuity.

 

[Tulsa Electrician]

Can you post a riser diagram for review.
Does these issues arise when the Generator runs or exercises.
Do you have any lighting using a relay sharing em and normal circuits.
I take it the netural is not switched in the ATS.

What is the GFI status at the MDP.

 

[jaggedben]

Losing a neutral by itself wouldn't cause current on the EGC.

The current on the EGC most likely indicates that ground and neutral are connected somewhere on the load side of your measurement. It does not explain why you have no N-G continuity, unless you do actually have continuity and the current is messing up the reading.

If you lost the neutral completely, you wouldn't have any current on it.

 

[LarryFine]

Yes sorry the panel with lost continuity is 120/208 3 phase life safety power on generator backup. Transformer feeds panel 1, panel 1 feeds panel 2 that has loss of neutral ground continuity.

It sounds like the secondary neutral was never bonded. I've seen this overlooked more than once.

 

[jim dungar]

I would expect there to be some amount of N-G voltage. This voltage is basically the drop caused by current flowing on the neutral.

Fried ports, on devices fed from different receptacles tells me there is current flowing on the ground conductor inside the HDMI cable.

Do you have current on the ground when the transformer is turned off? This would indicate some other circuit is using your ground conductors as a return path instead of their own neutral.

 

[Cberg]

Can you post a riser diagram for review.
Does these issues arise when the Generator runs or exercises.
Do you have any lighting using a relay sharing em and normal circuits.
I take it the netural is not switched in the ATS.

What is the GFI status at the MDP.

Don’t have access to prints at the moment I’ll try to post it tomorrow when I get back over there.

No correlation to generator running or exercise times.

I don’t believe lighting shares relays but it could.

Neutral is not switched at ats. Ats feeds straight to transformer with no neutral. Normal power and generator power neutrals are both in ats but aren’t needed feeding the transformer.

I’ll have to check mdp to see

 

[Cberg]

It sounds like the secondary neutral was never bonded. I've seen this overlooked more than once.

Transformer has good secondary bond.

 

[Cberg]

I would expect there to be some amount of N-G voltage. This voltage is basically the drop caused by current flowing on the neutral.

Fried ports, on devices fed from different receptacles tells me there is current flowing on the ground conductor inside the HDMI cable.

Do you have current on the ground when the transformer is turned off? This would indicate some other circuit is using your ground conductors as a return path instead of their own neutral.

Building is occupied and they really don’t want to shut down for security reasons. We’re trying to exhaust all options of trouble shooting that don’t require shutting anything down before we go that route.

 

[jim dungar]

Building is occupied and they really don’t want to shut down for security reasons. We’re trying to exhaust all options of trouble shooting that don’t require shutting anything down before we go that route.

You don't need a total or lengthy outage. If this life safety panel is not on a UPS then they can tolerate a planned outage. After all, an unplanned outage of several seconds occurs every time they loss the utility and have to wait for the generator. I have found many facility people think of all outages as being lengthy total blackouts rather than localized and lasting the few minutes it takes to make measurements.

 

[Cberg]

You don't need a total or lengthy outage. If this life safety panel is not on a UPS then they can tolerate a planned outage. After all, an unplanned outage of several seconds occurs every time they loss the utility and have to wait for the generator. I have found many facility people think of all outages as being lengthy total blackouts rather than localized and lasting the few minutes it takes to make measurements.

You’re right and I agree but explaining it to them is another thing. There’s a lot of valuable art work housed and stored in the building so they’re very strict on it. It’s probably going to be the only option if they want this figured out.

 

[zbang]

I have some basic concerns about what "burned-up".... lots of laptop power supplies are ungrounded (and rated for 100-240 volts); monitors are often the same; twisted-pair Ethernet is transformer isolated at each end (using itty bitty transformers).

I'd be looking for at least one outlet that has hot and ground reversed (been there....).

 

[jim dungar]

You’re right and I agree but explaining it to them is another thing. There’s a lot of valuable art work housed and stored in the building so they’re very strict on it. It’s probably going to be the only option if they want this figured out.

It is their choice. Loose power or continue to loose equipment.
I had a situation where I needed a 2 minute outage at an airline's baggage handling data center's UPS. The maintenance people said no way, but once I got to speak to the operation manager and explained how limited the actual outage would be, it was scheduled within days in the middle of the night.

 

[retirede]

Fried ports, on devices fed from different receptacles tells me there is current flowing on the ground conductor inside the HDMI cable.

.

This is a key point. Most of these devices have switching power supplies that can handle large voltage excursions. These issues can’t hardly be caused simply by over voltage from a faulty neutral.

 

[synchro]

... But while amp clamping taking readings in a panel I find we have almost 5 amps on the panels ground and no continuity neutral to ground.

A suggestion is to put a clamp meter around all of the conductors (both line and neutral) feeding this panel. If the current reading is not close to zero, then it's possible that the indicated current is being injected into the EGC somehow. This could be due to a N-G connection, a load tied to ground or a ground fault either inside of the panel, or in one or more of its circuits.

If this is happening, you might be able to isolate it to a particular circuit by putting a clamp meter around all of the conductors in each circuit to see if you get a non zero reading. But this could be tedious if there are a lot of circuits.
Circuits that include a neutral (e.g., L & N of a 2-wire circuit; L1,L2, & N of a 3-wire MWBC, etc.) could be checked to help isolate possible N-G connections that would cause ground currents.

 

[Cberg]

Was able to shut down the problem Pannel this morning to check it.

All 3 hot legs, neutral, and ground feed are good. We still had a load of 6 amps on neutral with the panel off. Narrowed it down to a couple lighting circuits that have neutrals crossed with regular power lighting circuits.

Leaning towards the ground amperage just being a ghost, we have no voltage on ground.

Still have to go farther into it to find where neutrals are crossed which is going to be a very tedious long task.

Not sure if this would cause any of the issues with the AV equipment or has any relation as the AV booth is farther down stream. Or if that is another issue all together that may or may not have anything to do with electrical problems.

 

[retirede]

Was able to shut down the problem Pannel this morning to check it.

All 3 hot legs, neutral, and ground feed are good. We still had a load of 6 amps on neutral with the panel off. Narrowed it down to a couple lighting circuits that have neutrals crossed with regular power lighting circuits.

Leaning towards the ground amperage just being a ghost, we have no voltage on ground.

Still have to go farther into it to find where neutrals are crossed which is going to be a very tedious long task.

Not sure if this would cause any of the issues with the AV equipment or has any relation as the AV booth is farther down stream. Or if that is another issue all together that may or may not have anything to do with electrical problems.

I’m beginning to wonder if there aren’t some shenanigans here. All of your checks you’ve carried out so far don’t point to anything that could cause these types of failures. It may be time to have a private conversation with the higher ups about installing some discreet surveillance cameras.

 

[Seven-Delta-FortyOne]

Is there a second N-G bond at the generator?

 

[jim dungar]

If you have crossed neutrals from the secondary of transformers then it is probable the grounded conductor has current flowing on it.

For example, if the Normal panel circuit NH1 hot wire is paired with the Critical panel neutral circuit CN2, the current path will be:
Transformer Normal-> NH1->Load->CN2->Transformer Critical neutral to ground bond->All Ground paths->Transformer Normal ground to neutral bond->Transformer Normal.