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#49566 03/09/05 07:02 PM
Joined: Mar 2002
Posts: 86
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.


#49567 03/09/05 07:05 PM
Joined: Oct 2003
Posts: 259
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.

#49568 03/09/05 07:27 PM
Joined: Mar 2002
Posts: 86
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

#49569 03/09/05 08:13 PM
Joined: Mar 2002
Posts: 582
Ron Offline
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).

#49570 03/09/05 08:26 PM
Joined: Mar 2002
Posts: 86

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?


#49571 03/09/05 09:03 PM
Joined: Jul 2004
Posts: 9,881
Likes: 27
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.

Greg Fretwell
#49572 03/09/05 09:09 PM
Joined: May 2003
Posts: 2,876
e57 Offline
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: ) 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!

Mark Heller
"Well - I oughta....." -Jackie Gleason
#49573 03/09/05 09:41 PM
Joined: Feb 2005
Posts: 219
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.


#49574 03/10/05 06:50 AM
Joined: Jun 2004
Posts: 32
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.

#49575 03/10/05 11:43 AM
Joined: Mar 2005
Posts: 53
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

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