Nuisance Tripping Main after new VAVs

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Sean.Day72

Member
Location
Florida
Have a VAV replacement project going in a school environment. The new VAV have electric heating elements in them. We are experiencing an issue where the main breaker (3000Amp insulated case breaker with a Siemens ETU776 trip unit) is tripping at random.

The electrical distribution is pretty straight forward, Siemens Distribution Board feeds smaller branch panel around the facility. They only adjustable trip unit is the Main Breaker. All other breaker are just inverse time thermal breakers.

Is it possible that a faulty heater inside VAV can trip the main? All VAVs are fed from 20Amp /277V circuits. Heaters are 3-5KW. This is the first cold day of the year, so I'm guessing this is the issue. Is there a quick way to check each VAV? We have 30-40 new ones so it would take some time.

The only other thing I can think of is it is coming in from the Utility side. If you guys can think of any other reasons for nuisance tripping a 3000A main I'm all ears.
 
Yes, a faulty VAV could cause it to trip. The ground fault settings on the main are probably set at the factory lowest default setting. If the main has a box CT, it’s also possible not all of the conductors are ran through it, had that problem at a new install where a couple were too short, and the electrician ran them outside the ct to reach the lugs. Every time the air compressor started, it would trip the main. Most likely though, it is a heating element in one of the VAV’s is shorting to ground.
 
The main distribution board is 25 years old. No previous issues that I know of. Put new VAVs in and first cold day the teachers showed up to cold classrooms turned the heat and started getting random tripping, by tripping I mean the whole school losses power obviously a big problem. The replacements are 1 for 1. The heating loads are unchanged from old vs new. Could the trip unit itself be the issue? Just looking for ways to help pinpoint the issue. If I put a multi-meter between the output of the branch breaker feeding the VAV and the equipment ground conductor and see a voltage would this be a way to detect a faulty heater putting current the ground?
 
No, the easiest way is force the heat on at each VAV, put an amp clamp around the hot and neutral. You should read zero amps with the heating element on. Anything else means the current is traveling back on the ground.
 
First figure out what the main is tripping on. Most likely the main is tripping on a ground fault detection, but ideally the main has a way of informing you why it tripped. If you can be certain that it is tripping on overload, then you know what to chase.

The below is about ground fault monitoring, but I'm editing after @hillbilly1 's post. Go one at a time, and force heat on for each VAV while all the others are off, and use an amp clamp on both the hot and neutral at the same time. Any current going out on the hot should match the current returning on the neutral, to give a net zero reading on the amp clamp. So if you measure any significant current flow you've found a fault.

With resistance heat you can have a fault to ground via the heating element. The heating element limits current flow so an ordinary overcurrent breaker doesn't see a problem and doesn't trip, but the ground fault sensor sees the unbalanced current for an extended period of time and trips on that.
The other benefit of forcing 1 on at a time is that if you turn one on and the main trips, you have a clear indication of the problem.

More about ground fault monitoring:

If you can add external ground fault monitoring and logging to the main, then you can get an idea of the magnitude of the ground fault current that you are dealing with.

The way ground fault detection works is by measuring 'residual current', measuring the net current of all circuit conductors at once. You can do this with a clamp meter: just put the clamp around all the circuit conductors (including the neutral if used, but not the ground). If you measure 'net current' on all the conductors for a load, that means some current is going someplace unexpected.

Since you have a bunch of 20A circuits, you might be able to use cheap energy monitoring hardware to measure residual current on every circuit and hunt down the faulted circuit(s).
 
Been there, done that.

99.99% sure your main breaker is tripping due to a ground fault caused by the VAVs.
Last time I was called on this project the issue was traced to the Duct Heaters. Evidently they had some type of internal connection to ground. When only a few were on the system ran fine but when more heaters came on line, as the temperature dropped, the cumulative current was too much and the building main would trip. It didn't really help that the building main GF protection was set at minimum values (the installing contractor didn't want to pay for an engineering study to determine the settings).

I always recommend multiple levels of GF so that the area of the building affected by an equipment failure is reduced. I also try to coordinate my feeder GF with a typical 30A branch breaker, this allows the building main GF to not trip due to someone rewiring an energized light fixture.

Edit: Corrected equipment type.
 
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Sean.Day72 said:
Heaters are 3-5KW. This is the first cold day of the year, so I'm guessing this is the issue. Is there a quick way to check each VAV? We have 30-40 new ones so it would take some time.
Click to expand...
Sean.Day72 said:
The replacements are 1 for 1.
Click to expand...
I worked on a school project that had the exact same problem well not the main but a feeder.
HVAC guy was pulling his hair out and cursing me out.
They only supplied me with 5kW units on the pallet, while the prints showed replacing 'like for like', we had a tight deadline only used 5kW units, and most we took out were 3KW, so we had a simple overload on the feeder breaker when 20 units all called for heat and each unit was 2kW more.
I think one or two were also defective.
 
Did some more investigation . It appears a lot of the heaters are connected to Phase A , and there is possible imbalance with Phase B and Phase C . Can this trip the GF protector ?. Say there is 200 amps on the neutral because of imbalanced load and the GFP is set for say 200amp at .4 seconds (guessing don’t know the settings yet) I’m assuming this could possibly cause an issue ? We are still going to try and track down a possible faulty heater


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Sean.Day72 said:
Did some more investigation . It appears a lot of the heaters are connected to Phase A , and there is possible imbalance with Phase B and Phase C . Can this trip the GF protector ?. Say there is 200 amps on the neutral because of imbalanced load and the GFP is set for say 200amp at .4 seconds (guessing don’t know the settings yet) I’m assuming this could possibly cause an issue ? We are still going to try and track down a possible faulty heater


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Click to expand...
No, the GFP does not care about imbalance...it is only looking for current that is flowing outside the normal path,
 
Got it . It adds P1 + P2 + P3 + Neutral currents . If it does not equal 0 then there is a fault . So neutral currents caused by an imbalanced load don’t matter


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I could see situations where the phase imbalance of the heaters could trip the GFP, but they all involve some other problem.

For example if the GFP was not including the neutral current ( a defect in GFP installation) or some sort of N-G fault that doesn't carry much current unless there is significant imbalance causing significant current on the EGC.

GFP for a large 480/277V system isn't very sensitive, and if something isn't connected properly then it might require hitting a threshold before it trips, rather than an instant trip on a single problem.

-Jonathan
 
winnie said:
GFP for a large 480/277V system isn't very sensitive, and if something isn't connected properly then it might require hitting a threshold before it trips, rather than an instant trip on a single problem.
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I have seen a single 277 volt ballast failure trip a 1600 amp main breaker....the breakers are typically shipped with the GFP set at the lowest setting. Sometimes, as in my case, the trip setting was never adjusted when the building was commissioned.
 
Hello all, finally signed up here for this one.
Did the service have any line to neutral loads prior to the VAV’s? If not, or if only a few there may be a neutral CT for the ground fault system installed with reversed polarity. The CT if installed backwards will instead of canceling its current reading out with the phase currents trigger a trip if the neutral current exceeds the ground fault trip setting. If there were few to no 277 loads previously the neutral current could have been below the trip setting.

I see now reading through again they are electrically like for like replacements so the scenario I described is unlikely.
 
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don_resqcapt19 said:
I have seen a single 277 volt ballast failure trip a 1600 amp main breaker....the breakers are typically shipped with the GFP set at the lowest setting. Sometimes, as in my case, the trip setting was never adjusted when the building was commissioned.
Click to expand...

Agreed, but in this case you are talking about a device failure with fault current flowing. It might be a single ballast on a 20A circuit, but you could easily see several hundred amps flowing to ground.

In the case of the OP, a very plausible scenario is a 277V heaters with some sort of connection to ground. The ground fault current is limited to the 10's of amps or less. Consider this scenario:

Main ground fault threshold 100A.
30 heaters, 5kW 277V each, all miswired at the factory so they are connected hot to ground.

How many heaters need to be on before the main trips? Minimum? Maximum?

IMHO it is much more likely that everything is wired correctly but there is a fault or 2 somewhere. But if the fault current is limited by heater resistance then it might take several concurrent faults before the ground fault threshold gets hit.
 
Update after overnight investigation. Clamps were put on the incoming main. Phase A - 493A, Phase B - 248A, Phase C- 221A.
Moved some of the new heaters over to Phase B and Phase C to help balance the load.

Siemens rep came out, performed injection testing, and stated there is a "faulty trip unit" . They actually could not get the main to trip when testing it. Awaiting this report. From this thread and what I read, an imbalance should not trip GFP unless the originally installation was performed incorrectly.
 
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