We previously discussed doing a visual inspection of your distribution system to quickly identify systemic problems. However, this approach doesn't identify every likely problem or quantify before/after. For that, you need testing.
If your facility has a power monitor, great. But it's probably not monitoring at every load. To see what's going on at the loads, you need a power analyzer and a true RMS DMM.
Previously, we worked from the one-line down. Now we'll work in the other direction; start at one of the troubled loads.
First, visually inspect for proper separation between wiring of different systems. For example, you don't want 50mV signal wiring run alongside the 480V motor supply.
Next, measure the output of each DC power supply. The voltage should equal nameplate. If not, and the input voltage is correct, then replace the power supply.
Next, measure the power to the cabinet for RMS peak voltage and RMS nominal voltage. You should see about 1.414 times nominal voltage and nominal voltage, respectively.
If either voltage reading is low by 3% or more, go back to your drawings and calculate the conductor sizes for less voltage drop. Although most references say 5% is acceptable, that may be too low when you're trying to run electronics in a high harmonic environment. You're looking at a rewiring project to solve this problem.
In Part 6, we'll look at some other testing to perform.