This one is for any electrical wizards / gurus out there. Read carefully...there might be a quiz at the end.
Note: this really happened, and I witnessed it with my own eyes. Anyone who can explain it (correctly) will get a
along with a
Okay, here we go...
A newly installed 200kVA Uninterruptible Power Supply (UPS) in a major mission critical data center alarmed on a fault and switched to emergency
bypass (normal utility power). The error on the UPS was "Low voltage - Input - Phase B", "Major fault - Critical" and "Switched to Bypass". The UPS
was fed from a 600A 480VAC 3 phase electrical panel. Engineers (myself and others) and Electricians were dispatched immediately to investigate what
was going on. I was the lead engineer, and this was serious business!
The electricians (now in flash suits) checked the input voltages and here's what we found...
Phase A to ground = 277 volts
Phase B to ground = 277 volts
Phase C to ground = 277 volts
Okay, so that all looks good, now let's check phase to phase voltages...
Phase A to B = 480 volts
Phase B to C = 277 volts (wait...what???)
Phase C to A = 480 volts
No opens, nothing. Voltage on every phase.
So what happened?
Now, before you say this is impossible, remember I actually witnessed this with my own eyes. We were all standing around scratching our heads. This
shouldn't be possible, but there it was...double, triple and quadruple checked with (3) different meters. I didn't solve it at first, but one of the
electricians said there's only one possible scenario where this could happen (and it had a 1 in a bazillion chance of happening). What was it?
A couple key points to remember here:
1. There were no opens on any meter reading (i.e. zero voltage)
2. Continuity checked across all three phases
edit - I've intentionally left off one (more) test which we did to confirm the problem once we had established a theory of what was wrong.
on 1/12/2018 by Flyingclaydisk because: (no reason given)