Meltdown. What happened?

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Also I just received an email from someone who was on site when the events happened. Note that at this facility, most of the lights, workstations, and things you would notice as being off, are on the generator and ATS. The circuit that burned up and made the smoke feeds that rigged transformer which feeds three AC compressors. Seems clear that the utility phase loss happened first:

I arrived on Sunday at 12:00pm, some of the lights downstairs were
either off or flickering, but to the best of my knowledge everything
else seemed fine.

At roughly 3:00PM the power went off completely; ~3:10PM the power came
back on at a reduced capacity, I presume the generators were running at
this time.

~4:30 - 5:00PM the fire alarm went off, and the smell of burning rubber
was becoming more and more evident.
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What I dont quite understand though is why the power appeared to be completely off for nearly 10 minutes. I dont know how the ATS handles a phase loss, but I would assume it would start the generator and transfer as soon as the generator source was accepted which should be about 10 seconds.
 
The problem with connecting the star point of an input wye to *anything* is that if there is a delta primary circulating currents will flow if there is *voltage* imbalance on the inputs even if there is no load at all. And those currents can be greater than the normal full load currents on any phase!
The utility feed may have a voltage imbalance but not the generator or vice versa. The ultimate voltage imbalance is, of course, a lost phase (shorted to ground, not open will be worst)
The circulating currents can blow one fuse out of three, causing a phase loss that escalates the problem IF there are running motor loads when it happens.
The power could appear to be lost because of generator protective circuits until the wye point connection finally burns out.
 
electrofelon said:
So it is possible for the accidentally bonded wye point to have a enough current from a phase loss to heat up the EMT to make the insulation fail?
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Yes.
Or there could be enough current in one or more of the phase lines to overheat it directly, possibly with the EMT raising the ambient temperature enough to allow the IR heating in the wire to exceed the insulation limit.
 
GoldDigger said:
Yes.
Or there could be enough current in one or more of the phase lines to overheat it directly, possibly with the EMT raising the ambient temperature enough to allow the IR heating in the wire to exceed the insulation limit.
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very interesting. So it appears I may be partially at fault for removing the neutral but not the bonding jumper, although we will never know if the neutral could have survived the incident. How does one calculate/estimate the wye current in a situation like this? Also how could the phase conductors have overheated, woulnt they be protected by the OCPD?
 
You can calculate the current if you know the voltage imbalance and the resistance of all windings. You should be able to estimate it knowing only the voltage imbalance and the %z of the transformer, but I am not familiar with that calculation, sorry.
And, yes, at some point the primary side OCPD should have opened if properly sized. But any secondary OCPD would not helped at all because the current on the delta side would be totally internal.
The bonding jumper may have been too small or there may have been excessive resistance in raceway EGC joints.
 
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