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Voltage sag #219689 10/03/18 09:14 PM
Joined: Mar 2004
Posts: 782
BigB Offline OP
I have a client renting 4 spaces in a small retail strip building. They are running a pharmacy and over the years have expanded into 4 of the 5 suites in the strip. They have a large processing area and a retail area as well as 20 tons of AC. They use heat sealing packaging equipment as well, and they have numerous (over 20) PC work stations. The 5 spaces are all fed from one 75KVA pole mount single phase transformer. The 5th space is a juice bar and I am guessing they have 5 tons of AC. along with their refrigerators and juice machines and dish washers.
Problem is they are complaining of regular voltage sagging issues, many times per day they report the PCs flicker and they hear the multiple UPS devices clicking on. They have lost several PCs this year, and the two UPS machines running the servers have burned/melted the plastic off the battery leads.

I am no expert on power quality but was asked what to do. We checked all the panels and wall receps and found nearly perfect 240/120 everywhere. No loose connections. Here is what my plan is
1. Request power monitoring at the incoming service by the POCO
2. Have the IT guy log all the UPS reports to see how often and how long the sags occur.
3. Have Soft Starts installed on all the AC units after we get the results of the power monitoring
4. Perform a load calculation/study

My gut feeling is that the AC units are causing it. The 75KVA transformer is a good 100 feet away from the first 3 suites and I don't know what the wire size is but the 4th suite has it's own drop at 150 feet with 1/0AL, this is the suite with the servers, the processing lab and 5 tons of AC. The Soft Starts may solve the issue but I want the POCO to monitor first to see if they will up the transformer/wire size.

What else should I be looking at, besides calling in a power quality expert which I will do if we cannot figure it out quickly?

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Re: Voltage sag [Re: BigB] #219690 10/03/18 10:23 PM
Joined: Jul 2004
Posts: 9,623
gfretwell Online Content
If you were any closer I would loan you my Dranitz but that is the answer Rent or borrow a logger and track it yourself. Once you get a pattern, you may be able to track it down. If you are just willing to user a meter, an analog one is better than a digital, even a pretty cheap analog. You are just looking for the change, not an absolute value. If you watch it you might develop a pattern. That is what a logger can do.
If you are just looking in real time, hook your clamp to the HVAC compressor circuit and watch the voltage on the bus with your cheap analog.

Greg Fretwell
Re: Voltage sag [Re: BigB] #219691 10/04/18 10:17 PM
Joined: Mar 2004
Posts: 782
BigB Offline OP
Thanks, I may have to ask around for some equipment. I did consider spending some time there and I do have an old Fluke 27 with a MIN/MAX record setting, along with several analog meters

Re: Voltage sag [Re: BigB] #219705 10/12/18 06:59 PM
Joined: Jul 2002
Posts: 8,397
Trumpy Offline
I agree with what Greg is saying, there are companies that rent out data loggers, the best device I've seen yet (whose name escapes me at the moment) is one that you can monitor via Bluetooth to a laptop and not only record say a week of data, but see it in real-time in graph form of fluctuations.
What I would do for a start is try and get the data logger as close to the incoming mains as you can and then move "downstream" to rectify where the problem actually lies.


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