AC power line question

John Teller jsteller at
Sat Jun 30 13:59:06 CDT 2012

Those are most probably some sort of potential indicator.  It's kind of 
a weird circuit in that one end is physically connected to the MV line 
while the other end is capacitively coupled to ground.  At those 
voltages you don't need much current to flash an LED - which would 
indicate that the circuit is live.

It's odd that the power company would put up something that flashes when 
everything is powered normally, as this generally prompts calls from all 
and sundry who normally associate flashing lights with problems.  I was 
under the impression that power companies tended to use mechanical 
devices that displayed a red or green flag and were driven by magnetic 
flux developed around an MV line that was carrying current.

Here's a link to an image of an install of some current sensors 
installed in Australia:

In this picture you can see the mechanical indicators - they are the 
three devices attached near the dampers just beyond the disk insulators 
(the line post insulators on top are actually the current sensors).  
That's a three-phase delta system, which is why the transformer is 
connected to two lines.


On 06/29/2012 11:52 PM, wb5mmb wrote:
> We just had a line of storms blow through Northern Virginia with many 
> hard power hits but power never went all the way down. On the power 
> pole in front of the house there are cylinders about 3" in diameter 
> and 3" long hanging from each of the 3 high voltage lines about 1' 
> from the insulators. Each cylinder has a blinking red led on it.
>  Two questions, what does it mean when the LEDs are blinking and how 
> do you power an led on a box hanging on a 13KV line that looks like it 
> could handle several hundred Amps.
>      Sandy
>      WB5MMB
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