The company I work for got a $19,000 back charge for A/C Unit repair work at a job that I finished about two weeks ago.

They are claiming that the voltage surge from turning on the 600-amp main disconnect caused compressors to burn out and A/C Units to malfunction. They suggested that the proper way to turn on a distribution panel that is feeding 12 A/C Units is to shut down all disconnects in the distribution panel first. Then turn on the main 600-amp disconnect, then turn on each individual disonnect within the distribution panel. Therefore not creating a voltage surge.

What are your comments for I have a meeting with all of the upper management next week.

Thanks
Tev

Wouldn't the same thing happen when the power goes out and then comes back on?
I would have turned all the breakers of though and then fired up the main then one breaker at a time. I have no reason for doing it this way but have always done it like that.

Yes, Joey D,

That is a good comment that I can use. I was in a hurry and did'nt bother with the extra step. I know better now.

Thank You
Tev

I would assume you created a voltage sag and not a surge. The inrush of all the units simultaneously may have caused the line voltage to sag until they came to full load amperage.
As mentioned, during a power outage, this will occur too. It is poor design to not include a time delay, thus staggering the startup of the units a few at a time (or one at a time).

ron,

Where would the time delay be located in a situation like this?

Does each A/C unit have one within it's motor controls?

Maybe the A/C units do have time delays already. How would I find out?

Thanks
Tev

The time delay goes in the LV line to the compressor contactor circuit. They are typically a little black box about twice the size of a Zippo lighter.
I have a few in my garage if someone wants to see a picture. I use them in my lighting controller to delay the turn on for my motion detectors. It keeps them from latching "on" on a short power hit.

Stand firm and confident.... Tell 'em to take it to court. They have no grounds on the arguement they gave you. Thier Engineer should have set the system up with delayed start, and it would've happened at power failure the same way. And most motors will withstand in-rush and voltage sag with no real problems, it would have to be extreamley sever.

However, if the units were started empty, or out of rotation,(if 3 phase) they might have you, if you were the one who turned them on initially. Or did a service/panel change and changed rotation... As a rule, I never start new AC units, for that reason. (A leaky system, or out of rotaion unit will "cause' compressors to burn out and A/C Units to malfunction.") I check rotation at the disco (with a meter, not the unit. I have the old mechanical type simular to this one:http://www.hotektech.com/StST109D01.htm ) install the fuses, and leave it off for them to turn on. 99% of the time rotation is good for the unit, sometimes the unit is wired wrong, and they call me back, and they get charged for the trip, only to find that one motor goes one way, and another the other. And, if they start it, and it leaks (burns the compressor), it's thier problem!

I'm having a hard time seeing all the units firing off at once. But the A/C guy may have a valid point.When I am doing new construction I will energize to the unit but leave it up to the A/C tech to do the final throw at his unit. It is our job to supply power to the equipment provided by others but not to start it.

1. Never energize a new circuit under load.
2. Check voltage and rotation before energizing any equipment.
3. Why were the disconects at the units turned on when you energized the service?

This is what is called "Hard Start". Its totally common, not good and the gear should be designed to handle it. I do service for a major grocery chain and when PG&E goes down they immediatley call so I can go out and bring them up soft. This eliminates the problem. But not all business's do this and they don't seem to be hurt by a hard start. But it does cause harm to certain equipment.

JMHO
Rob

Straightedge,
Not your problem! e57 nailed it, power monitoring relays are an option for almost all AC equipment but are almost always not specified because of the added cost. I work for a large MEP contractor and see it all the time. You should not pay the charge and should get a CO to install relays on the equipment.
KB

who closed the A/C disconnects? even on a residental a/c I wire up the a/c (because inspector says I have to) but leave the fused dis. pulled/open for the a/c guy to energize

I might add that most of my exsperience is in power plants or substation's (Elec. utility co.) and even at that level we energize equiptment one at a time and let it cook for a while (especially on new install) and even on a complete un-exspected outage of a substation it's heated back up one at a time, but!!!! the feeder leaving to all us customer's when the power comes back on it comes back on, enough said so if you dont want a inrush best to have your own protection installed, for instance at's of god (lighting) recloser trips and come back on, you got a surge and then a inrush of current.
lol blame the electric utility

Straightedge,
It's not on you or your company, unless the disconnects were on, thermostats were installed and left on, prior to phase checking, AND the A/C guys notified you of this, AND you signed a waiver accepting responsiblity for it.

ALL trades acccept responsibility for their work and equipment until final inspection and acceptance by the GC or final customer.

If the power company hooked you up the wrong voltage, and you didn't check it prior to powering up, and you smoked all your panels, lights and ect, are you going to blame the power company?

The A/C contractor should of had his guys check incoming power, double check rotation, and installation prior to enabling the equipment.

He don't have a leg to stand on. He is trying to see if you'll pay for his mistake.

Keep in mind the power outage senario. If they burn up due to a sag, that's on the engineering outfit. You just ran what they spec'd out.

This was mentioned already but it is very unusual for commercial air conditioning equpment to not have time delays on the compressors. These are used because a compressor has a very difficult time starting under pressure and a unit needs time for pressures to equalize before a restart.

Line monitors are not as common but if the equipment was sensitive to the inrush currents associated with power outages, the HVAC company should have had the foresight to install those too.

Someone playing with the thermostat or a momentary power outage would cause a condition where a compressor would stop and try to start again under pressure. this "short-cycling" can burn up compressors.

And if you did cause a momentary (fraction of a second) high voltage condition due to in-rush currents, the heavy windings in the compressors would have handled it fine. It's the microwaves, fire alarm cirucits, computers, PA system, and other solid state devices that would have failed first.

If you want lots and lots of answers from a very knowledgable AC crowd, ask your question here: http://www.hvac-talk.com

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