Is it possible that the electrical supply to your facility is increasing during the other shifts because of the demand from other companies around your facility are drawing less power off the grid?

I mention this becasue we had a situation where we were going through transformers at a 100 yr old building.  These transformers supplied the entire property, not just out building.  After our own commercial electricians coulnd't figure out why some of our automated equiptment (drill line and saw) were burning out prematurely, they brought in the power supply people and sure enough it was something to do with the power being supplied to us was different at different parts of the day.  They took of off the same feed as some other companies around us and changed out some transformers and haven't had an issue since.  That was about 5-6 years ago.

I'n not an expert, just a suggestion
Chris

You can get the Electric Company to chart what is coming into to your plant for free. Though I ain't no electrician, I suspect that is the problem, but your welding power sources should be able to accomodate a variance in input voltage within at least + or - 10% I'd guess.
We had the same problem at a plant I used to work at when we'd fire up the big induction bender.
Whether or not you impose another WPS is  up to you but it seems to me you have to impose some kind of control mechanism.

Hello Keith, I would certainly agree with both of the other respondents to your thread. With the scenario that you are describing there could certainly be an adverse effect being imposed on your equipment due to different levels of use on your power grid at different times of the day. I experienced a similar situation while working for a company in Portland, Ore. We could start out with our fabrication and welding in the morning with the machines set to specific parameters and close to around noon everything would go to hell concerning weld bead appearance and operational characteristics. Did some checking on input voltages to the machines and noticed a considerable voltage change from the morning to the afternoon. Our machines were of an older vintage and didn't have power factor correction or some of the new innovations that current day machines possibly have so I believe that in our case we were seeing some serious volt/amp swings that detrimentally effected our welds. Might be worth looking into some in-line tattle tales to make comparisons in the weld output characteristics of your machine at these different times. I know this technology is available at a price, don't know if there are possibly companies out there that could be contracted to come in and do a survey for you, maybe some of the other more knowledgeable folks on the forum might chime in and give some suggestions. Can certainly understand that sort of frustration, best of luck to you and best regards, Allan

The voltage in Your plant is higher when there is less electrical load on the system. This could be an issue with the power in Your plant only, or on the entire regional power grid [low voltage at peak load].

Another variable that may affect Your plant voltage is power factor corectiion. Over corection at light load can increase line voltage.

At My home shop I notice low voltage in the late afternoon on hot days in the summer, about the time people come home from work, turn on the AC and cook dinner [everybody around here uses electric stoves].

I would compare line voltage on both shifts using a true RMS volt meter as well as a peak reading meeter if You could find one. Many modern power suplies do compensate for input voltage within a range, but You might be outside the range.