What's That Sound?

     There's an intermittent humming from the far side of the street -- sounds like it's in the next neighborhood over, and it sounds like downed power line.

     There's a lot of juice behind the wires; even a drop to a single house from a stepdown transformer is deadly.  In our neighborhood, the power company gets "a lot of lights from a single match," and four or five homes might share a transformer.  It's protected on the primary side by a big, old-fashioned fuse, often a length of heavy fuse wire clamped inside a ceramic tube.  They're set to pop free of the clamps when they go, making a blown fuse easier to find, and they often let go with a bang.  But it's a big fuse.  It takes a lot of current to blow.

     Downstream, your house has breakers or fuses to protect the wiring.  Upstream, power companies use "reclosers" on the high-voltage wires: they open up on excessive current, wait a bit and try again.  The most common problem on a high-voltage line is a branch or an unfortunate creature, and those problems will usually clear.*

     I'm not sure what's going on across the street -- there's no smoke rising and the sound echoes and carries, but I know one thing: stay away!  Between that big fuse on the stepdown and the reclosers, a downed power line is often live.  It may arc, fall quiet, and then start arcing again, and it may or may not be "live" when it isn't visibly sparking.  There's a voltage potential in the ground under your feet, highest where the current goes to ground and diminishing over distance.  Get too close, take a long step and you're in danger before anything is obvious.

     The sound gnaws at me.  I have been too close to power problems a few times over the course of my work and the more you see what happens when it goes wrong, the less safe you feel.
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* There is a tradeoff between excessive "nuisance trips" and protecting the system.  Reclosers are one of the compromises, and one that works well more often than not.  But they're also why you can never assume hat downed power line is safe.