Use your brain when scenario testing Home backup with Pro Power

[CyclopsThere]

Electrician came by today to do the last bit of install, hooking up our small AC unit to the generac panel. We tested it out with success (no soft start needed, I guess because of the size of our AC), and he started putting covers on panels, etc. Once everything was back in place we did another test, but this time I had the bright idea to do a "hot switchover" with the AC unit running on utility power, over to the Pro Power onboard. It worked fine with the furnace going, so why not with AC?

POP, switched the power over. Instantly the AC stops. Our phones buzz twice in rapid succession:

1) Pro Power Onboard Turned Off: The maximum capacity of the bed outlets was exceeded...
2) Pro Power Onboard Service Required: Please contact an authorized dealer for service.

I couldn't turn back on the truckbed power after power cycling the truck, and it even kicked off another notification #2.

Did a bunch of searching where people said to push on the bed breakers (the white knobby thingys) just in case. I did, and even though I didn't feel any movement on those, on the next startup of the truck, I was able to re-power the bed outlets.

Had one of those "oh shit" moments for awhile in between. Goes without saying, there's no need to try a "hot test" unless you want to practice resetting your truck and probably taxing the entire system. I'm sure it wasn't too good on the AC unit either.

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[bmwhitetx]

No matter how fast you flipped the switch it was not a "hot" switchover. Everything lost power briefly. A/C compressors do not like to be instantly restarted. Pressures have built up and the compressor restarting will pull a lot of amps. Pressures need to equalize before re-starting and this could take several minutes.

Some A/C units or t-stats will have a compressor delay to keep the compressor from starting after a power failure or to prevent short cycling.

 

[Runaway Tractor]

Apparently his AC unit controller or thermostat didn't catch that fraction of second outage while switching over.

So yeah, don't do that

 

[chl]

Indeed.
Your electrician caused a momentary power surge (large current draw) by the switching while the A/C was running.

With out getting too technical, the A/C compressor is essentially a motor which is a reactive load, so it stores energy in its magnetic field and when starting or stopping there is a surge of current.

A reactive load resists sudden changes in current and voltage by trying to maintain them with the stored energy in the magnetic field in the case of the compressor. After start current surge, the magnetic field is established and the applied voltage is a gradually changing sine wave not a sudden change so the current stabilizes.

When power is cut, the compressor voltage and current does not drop immediately and there is a 'kick back current.' This is dissipated in resistive elements in the A/C.

With an A/C, however, the start surge current will be higher when the A/C has been running.
This is because their is a high-pressure differential in the refrigerant system until the A/C has been off for a few minutes and the pressure can equalize.
The high pressure differential means the compressor will draw much more current.

So the start current after a momentary outage is much higher when the A/C is already running due to the refrigerant pressure differential.

That kind of thing, sudden on/off/on can also damage the A/C, burn out the motor windings (wire melting).

That's why turning off your HVAC during an electrical storm is a good idea to avoid power fluctuations.

Brown outs can have similar ill effects.

BTW, refrigerators can have the same issue since they are compressor based machines.

Most automatic backup systems without integrated battery storage (hot storage) SHOULD have a time delay between loss of power and start of the generator to avoid damaging surge currents.

When doing it manually, waiting 5 minutes after the utility power is off before connecting the backup system would be good advice.

When testing, turn off the utility power for 5 minutes before connecting the backup power.

Generally, when I am using a backup generator, I turn on the load breakers one at a time to avoid big power demand surges on the generator.

I suppose I will do that as well when I hook up my Lightning as the backup power source.

 

[CyclopsThere]

Great info!! Thanks a bunch. The hot switchover was my call & doing, not my electricians. He gave me a weird look when I proposed it and did it in front of him

 

[tls]

It's wise to think of transfer switches as temperamental machines that work sometimes and should be babied unless an absolute emergency dictates otherwise.

In the late 1990s - just at the very end of the era when a major bank might have run a datacenter or, as some of us still called it then, a "machine room", in its US headquarters in midtown Manhattan (fortunately, I can even say it was Rockefeller Center without precisely identifying my client - who might object even after all these years) - one of my clients was doing biannual service on the transfer switch in their basement. This was I think, a full custom piece of equipment, built to switch between "A" and "B" 4000A services for the building, a bank of emergency generators, and a large battery bank with inverters. It had been specified to be able to switch between every input at full load - even if the battery bank was fully discharged. But this had, rumor had it, never been tested. That year the electrical engineer supervising the work insisted the batteries be run all the way down before the transfer switch test was performed.

When they switched from the "A" to the "B" service, they later speculated that the battery management system saw a brief loss of power, reset itself, and began charging at maximum rate with no delay. This perhaps - if it actually happened - exceeded the design maximum load for the system. Or maybe something else weird happened. Or maybe not, and the transfer switch was just balky.

Anyway: the transfer switch caught fire. This was long enough ago that we still had Halon systems in rooms like that, so as soon as enough metal burned away to kill the arc, no more fire. But, needless to say, no more transfer switch either. The utility had to basically jumper their service around the scorched cabinet, restoring power to the building, and then every single machine in that large international bank's main US datacenter had to be restarted. One by one by one. Many had never been restarted at all. Many didn't start without manual intervention. Some had circular dependencies so they couldn't be started at all unless some other system was restarted in a manually modified state first. Around 200 of us worked nonstop for the next 3 days to get it all back together.

My contract there was up and I wanted to take the summer off and climb. But I was a little bit tempted to stick around and see what happened when they did the entire exercise again to replace that melted transfer switch... I didn't, though, so I don't know how that went.

But I do know this: if there's some emergency condition your transfer switch is notionally rated to handle, but which in an actual emergency you could avoid - like turning off all loads before switching, or all non-critical loads if you actually must be able to switch hot, and prove it by testing hot - DO NOT test the switch in a way it's theoretically rated for but which you'd never do if really using it in an emergency. Document your procedures for emergency transfer (like, "turn off all breakers except #17, then switch, then turn on all breakers") and test only that. As we learned at ************ 30+ years ago, you are literally playing with fire.

 

[CyclopsThere]

holy crap that's a good story and advice, thank you. i'm learning so much

 

[Runaway Tractor]

I have two large rooms full of 6000 amp switchgear, two 34kv substations, megawatts of generators, battery systems, and rotary systems. When it all works, it's an amazing feat of moving a shitload of electrons all over the place automatically. When it doesn't work properly, you definitely do not want to be in the room! I remember one day after a lightning strike, one of the 7,000 amp substation breakers was tripped. We were all standing around wondering who's going to have to push the button to close it back in and none of wanted that job. They did it with a 10ft stick from the other side of a concrete wall in flame retardant PPE.

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