Ford:
That continuous arcing after the bolt of lightning is known as power
follow-through. The lightning induces a voltage spike into the power
network and arcs over the surfaces of insulators weakened by dirt, cracks,
etc. However, once the lightning bolt has dissipated, the path to ground
across these insulators still exists because it's being fed by the power
grid (power follow-through.)
What usually has to happen is a protective device on the power system
has to open - this may take a second or two, especially if the device is an
oil circuit recloser (OCR). These things are intended to open under fault
conditions, then reclose and test for further faults. This trip, open,
close, test cycle is repeated a few times before the device locks open and a
guy in a truck has to motor on out to the spot to repair the fault and reset
the device.
The size of the insulators tells me that the line is more likely a 23
kV or thereabouts line; 69 kV insulators are around three feet tall. (If
the insulators had less than a half-dozen skirts, then the voltage is closer
to 23 kV.) That being the case, I would suspect you saw an OCR operate; a
69 kV line would have tripped open permanently. More things get damaged at
the higher voltages when you keep closing in on a fault, so the protective
devices usually operate once to clear the fault. The System Operator then
has to manually close the breaker once the fault has been identified and
cleared.
Aren't you glad you asked?
73 de
Gene Smar AD3F
-----Original Message-----
From: Ford Peterson <ford@cmgate.com>
To: towertalk@contesting.com <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Sunday, July 22, 2001 11:51 PM
Subject: [TowerTalk] Lightning Observation
>
>I was doing tower work on Saturday and decided to head for the shack
because
>of cloud-to-cloud lightning. As I put everything away and was walking to
>the house, a bolt of lightning shot overhead east to west. It was
perfectly
>still and quiet in the yard. Simultaneous to the flash (no thunder was
>heard), every power line insulator within 1/4 mile (also running east-west)
>started to arc over. It sounded like 15 arc welders going off at once.
>
>What surprised me even more was the length of time after the flash that the
>arcing continued. It must have been well over a second before the power
>lines quieted down. Mother nature can be scary at times.
>
>These power lines are the 69kV 3 phase type with 15" (or there abouts)
>insulators. They run east and west. My 80 meter dipole runs north/south.
>No damage to anything, either here or in the neighborhood.
>
>Is it possible that the three lines were temporarily DC biased by the
>lightning? How do you explain the > 1 second to discharge the lines?
>
>Sounds bizarre to me....
>
>Ford-N0FP
>ford@cmgate.com
>
>
>
>
>
>
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