3 circuits (red, blue, black, neutral white; approx. 100ft in metal conduit [ed: alongside 2 other similar 4-conductor MWBC]) feeding several outlets in a couple rooms. Voltage tests 110V(ish) on all three, neutral-to-earth voltage is around 9V. Plug tester says everything's fine. (Y'know, that consumer plug with 2 yellow LEDs & one red LED.)
[ed: light industrial +office location, all work professionally done ~10 years ago, main is 250A 480V/3𝚽 𝚫 but this is off a 208/120 3𝚽 wye transformer, 12AWG solid copper wires, 20A breakers, boring office loads (few computers, router, 802.11 switch, a small printer, alarm, a space heater, micro-fridge)]
However, I plug something in, and get no juice. It's a verrrry localized brownout. Other circuits on the panel are fine. Does this sound like something obvious?
The first symptom we had was a short between blue and neutral, giving 210V on red and black outlets. (Yikes.) After dis- & re-assembling all the outlets and conduit junctions, the short disappeared, but (as above) the circuits are still not usable.
This seems like the most appropriate forum, but if this question goes somewhere else please let me know.