breaker trips when transferring power to generator

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Doug Sieckert

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Community hospital grand junction colo
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Master electrician
I perform a monthly generator test at community hospital. I have 6 transfer switches, I rotate starting the generator with a different one each month.
My question is we have a new CATH lab with a 150a square D breaker (No adjustments on the breaker) that feeds the UPS which supplies power to the CATH lab OR. This breaker is NOT on emergency power. Why would transferring the power to the generator cause the 150a breaker to trip? I have asked 2 electrical engineers and they have no idea.
 
Is the generatorTS 3 or 4 pole?
Does the Sq D have GFP?
How large is the ups? Do you hate monitoring software for the UPS?
I agree with Hbiss, it’s likely your ups, perhaps an in rush on startup.
 
Many power supplies (including UPS) convert to DC via rectifiers on the AC input. If the rectifiers connect more or less directly to the capacitor bank there can be a very large inrush when the DC bus is at a low voltage.
To prevent this they may use an inrush control circuit which inserts a resistance, slowly charging the bus and then switching it out of the circuit for normal operation. This may be a thermistor instead of a relay controlled resistance.
But if the transfer process takes long enough for the DC bus to drop significantly but not to reset the inrush control there can be a large surge when the generator picks up the load.
If there were directly driven motor loads, the first suggestion would be motor inrush due to phase mismatch when the transfer switch operates, but that is not applicable to the OP's situation.
Does the UPS switch the AC sources or is it the dual conversion design that always drives the load from the inverter?

Never mind, the OP says that the UPS is not powered from the generator, so none of the above applies.
Very strange.
 
Doug Sieckert said:
I perform a monthly generator test at community hospital. I have 6 transfer switches, I rotate starting the generator with a different one each month.
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Does the breaker trip no matter which transfer switch is activated, or is there one or more of them that don't cause a trip?
 
synchro said:
Does the breaker trip no matter which transfer switch is activated, or is there one or more of them that don't cause a trip?
Click to expand...
Its new cath lab 3 months, hospital is 6 years old. I have done 3 gen tests and this is the first time it has tripped. I am going to put a meter on the feeders to the panel and see if there is a spike in power next time I do a gen test. transfer sw 5 is the only one that has caused it to trip so far. I will let you know what happens the next time I do a test.
 
The circuit tracer I have is great for finding paralleled circuits, it draws current off the circuit to work, and when two different panels are tied together, whether it’s the hots or neutral, it splits the signal, and the tracer will indicate it with a weaker signal showing up in both panels. Usually I can narrow it down to which circuits it is, unless it’s multiple circuits.
 
Can you post a pic of the front of the 150A breaker or provide the complete cat or model #?
Since it is a fairly new installation and if the instantaneous trip units are adjustable, it's possible that the settings were lowered by the testing/ startup tech for easy testing. If the tech forgot to adjust the settings to max after testing then that could cause the nuisance tripping.
 
ATSman said:
Can you post a pic of the front of the 150A breaker or provide the complete cat or model #?
Since it is a fairly new installation and if the instantaneous trip units are adjustable, it's possible that the settings were lowered by the testing/ startup tech for easy testing. If the tech forgot to adjust the settings to max after testing then that could cause the nuisance tripping.
Click to expand...
The OP said that panel was not backed up by the generator, so there is something else going on like a backfeed to that panel from a generator fed panel.
 
If there is a transformer in between the ATS and UPS, one possibility is transformer inrush with an inadequate instantaneous setting.
 
Xptpcrewx said:
Since when is emergency power = generator power?
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For a long time usage in this area has been loose, even among electricians.
I think that the OPs intent was to state that the breaker was not fed from any transfer switch rather than to state that is was on an optional, legally required, or life safety circuit rather than an emergency circuit.
 
You should check the settings on the ats to how close to phases lining up before making the switch the farther out the phase angle the more inrush. Does it trip on the switch from normal to emergency or from emergency to normal. I’ve also seen new roof top units on the switch go to a reset cycle and it had 4 compressors in it that would all be calling for cooling in the hot summer and all them would come on at once causing massive inrush tripping the breaker. So to solve that I tightened up the phase angle and then I took a 2 wire from the ats elevator recall contacts to the plc In the hvac unit and had the Trane guy write a program saying hey we are testing and to stage the compressors back on one at a time vs all at once.
 
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