Monday, May 13, 2019

A question for electrical line-men

Daily Timewaster wrote a post on Pacific Gas and Electric's plan to intentionally black out regions when high winds are predicted.

PG&E was quoted as saying "... a transmission line that snapped in windy weather probably started last year’s Camp Fire..."

I have seen various electrical failure modes in my life. They are usually of short duration as limit switches and fuses kicked in.

The power signature of a snapped transmission line would not be an overload but a sudden under-load.  The thermal profile would be metal arcing high overhead where there is little risk of it igniting a fire, then the hot wire falling to the ground and shorting. The most likely breaking point would be where the wire hangs from the insulator on the pole.

Do power transmission transformers kick-out when there is a sudden loss of current or gross imbalance in current between the two output sides of the transformer?

The other question I have is, do you see more snapped power poles or snapped transmission lines?


  1. Poles. Big windstorm here knocked down a long (half mile, mile?) stretch of poles, snapped them four feet up. (I think I have a picture somewhere if you'd like to see it - hit me up).

    Lines were fine (more or less).

  2. (not a lineman, just a driver)

  3. When the line hits the ground, (I'm talking middle voltage here, like local lines, not cross country) there is a fair bit of heating at the point where the line touches the ground.

    Generally, the system tries 3 times to clear the fault (short) over about a one second period. .

    And California is dry in a way that you, living in Michigan, cannot fathom....those few seconds of heat are enough to start a brush fire.
    Lots of sparks and such in that one second.

  4. When I worked in Kazakhstan, we learned to watch out for the highlines - the poles were never a problem. The Russians had designed the oilfield there such that highline power was everywhere to supply the drilling rigs (instead of using diesel generators). They were charged at about 700 volts and made from aluminum, and would snap from time to time. Usually a brush fire would start when one would hit the ground, if it was still live. We never knew which lines were still live. One day on my travels I noticed a dead cow, with all four legs sticking straight up in the air. Rather than get out an check myself, I sent the electrical crew and sure enough, a live, downed highline was right there in the weeds, but not visible. Just the fact of a breakage would not necessarily be enough to trip a safety feature or blow a transformer. I would imagine it's the same here, maybe depending upon how the local grids are laid out.