Strange thing going on

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g-and-h_electric

Senior Member
Location
northern illinois
Occupation
supervising electrician
Let me first apologize for this partially being an HVAC control question.

I am on call this weekend, and I get a no heat call from a commercial customer. He tells me that the feeder faulted, and toe POCO has him on a generator. This led me to think that a phase monitor in the rooftop tripped out due to incorrect rotation. No such luck. Found the control board which is fed from a 208 / 24 transformer throwing a "control fault" code (immediately after it powers up. Replaced said board, and the unit fires up, for about 45 seconds..... and i have inducer full flame etc. Unit goes out, and again throws me the same code as before.

Now here is the question, the generator is connected to the incoming service conductors ( they are still connected to the POCO transformer with the primary legs disconnected ( this is where the fault is). No one has changed any of the service grounding and bonding past the transformer (m rotation is correct too). Any ideas how merely adding the generator can create such an issue as I have? Usually the issue I have is caused by improper polarity or a bad ground. I appear to have none of these, just cant figure out how the gennie is causing me this much trouble......


Howard
 
I would swap 2 of the phases of the incoming service conductors (if practical) as you did not say what size (ampacity) service you have.
Either that or have the POCO swap it on their end.
 
ATSman said:
I would swap 2 of the phases of the incoming service conductors (if practical) as you did not say what size (ampacity) service you have.
Either that or have the POCO swap it on their end.
Click to expand...
Agree could be a phase sequence issue.
ABC CBA
The gen is required to be labeled. The gen needs hooked up to march utility.
May need more info to see if applicable.
 
Interesting issue. I mostly work with HVAC controls...

Is the HVAC a heat pump? Why would there be "rotation issues" if its convention gas heat? Is there a compressor involved?
 
So, lets start with rotation, I verified with the POCO that normal utility rotation is A B C clockwise, and they verified the generator is the same. The unit is a conventional gas furnace and not a heat pump. No compressor involved.

It is a Carrier unit, which was showing 8 blinks on the diagnostic LED when I got there, and doing nothing else. 8 blinks was control malfunction. I therefore replaced the circuit board, which got the unit to turn on the draft inducer and fire the burners for about 40 sec. The flame sensor current was about 4 micro amp after I cleaned the sensor. See why this is driving me crazy?????

POCO was there working a few hours ago, so MAYBE all will be fine after they are done?????


Howard
 
I don’t know anything about HVAC, but a couple things I know about generators is, depending on the unit, they can have high harmonic distortion, and they can have unstable frequency.

I have had electronics burn up from being powered by cheap generators.

What kind of generator is it? A standby, or a portable?

Also, I know that power management modules function by detecting the fluctuating frequency that is produced by a standby generator. Normally it’s not enough to affect control boards, but again, I don’t know much about them.

Frequency can be checked with many nicer multi meters.

Phase rotation can be checked with a phase rotation meter.

THD will require a power quality monitor, which are not in the tool box of most service electricians.
 
Rotation was checked and is correct. The generator is a towable BALDOR diesel machine furnished by the POCO. Never thought to check frequency, seeing as the circuit board has a 24 V transformer powering it and I am sure there is a rectifier and filter on the board converting AC to DC


Howard
 
Have you measured the neutral to ground voltage when it's powered by the generator to make sure this voltage is quite small. Also, measure the L-N and L-G voltages to make sure they are balanced.

The generator should not have a N-G bond. Otherwise, there might be current flowing through the equipment grounds due to the presence of more than one N-G bond. The flame sensor measures a very small DC current that flows between the metal probe and the burners that are connected to equipment ground. This DC current results from rectification of an applied AC voltage by the flame. Any currents present in the equipment grounds might possibly interfere with the flame detection process.
 
synchro said:
Any currents present in the equipment grounds might possibly interfere with the flame detection process.
Click to expand...
Plumbing electrodes are in parallel with service neutral, because of the main bonding point (MBJ).

Communication cables are in parallel with service neutral, because of the Demarcation point bonds.

So, isolating generator neutral from utility ground noise may be challenging.

250.6 (B) Alterations to Stop Objectionable Current. allow interrupting or moving some grounding bonds.
 
All is good now! The POCO finished repairing the faulted cables earlier today and the unit came back to life (I left the breaker off last night, and the client turned it back on).

I want to thank everyone and especially Synchro for all the guidance. I have to agree that most likely the problem was that the gennie had a neutral ground bond internally installed and this threw the Carrier unit controls into orbit! There were 2 Trane units that performed just fine.

The help and guidance I received was why I came here...... Never ran into a situation like this before, but now I know what to do if it arises again.


Howard
 
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