Got a question about the power company, brown out...

[lakedude]

We just had an hour long brown out which was interesting. Incandesent bulbs were dim, most LED lamps were pretty bright but the most expensive dimable LED sputtered and flashed like a strobe light. The computer would not come on but the DSL router worked just fine. I measured 57 VAC at the power outlet. Some of our emergency lights came on but some did not.

The building maintence guy @ work said that a phase got dropped and I'm sure he is correct, however that does not add up to me. It seems like if a phase got dropped roughly half the stuff in the house would work just fine on 120VAC and the other half (on the bad phase) would be completely off. Maybe he was talking about losing a phase farther up the line than I'm thing of? Half the town had this issue (like 10k people).

Any guesses as to what happened? Is there anyway to get more details from the power company? They sent an email saying power was back on but no details.

 

[PowerEngineer]

Well, my guess is that your building maintenance man may be right. Homes are normally served by a single phase from the three-phase power system. Voltages on all three phases stay balanced (i.e. equal) provided that the loadings on each phase are roughly the same. What may have happened is that one of the three phases somewhere nearby on a transmission line opened up (rather than faulted) and there are a lot of unbalanced currents (and ground currents) that are causing the voltage on phase feeding your home to be way off.

 

[lakedude]

Yeah I was being a bit stupid and thinking about a local drop of one 120 VAC leg of the 240 VAC at the house. Clearly the leg was not local to our house or only our house would have been affected. Obviously the bad leg was up stream because over 10k households had power issues. I'm only qualified on up to 600 VAC. All my work is on the secondary side of the last transformer so I got tunnel vision and was not thinking so much about farther up stream.

 

[John Connor]

This is why I have the power company programed into my scanner so I can hear what is going on.

 

[Mark R]

Could be one of the phases primary size shorting to earth.

With a delta primary side, a dropped primary phase, with earth continuity could result in an undervolt condition on 2 Y secondary phases. (depending on whether the delta circuit is high impedance grounded or not)