[RFI] Christmas light display - SOLVED

Fri Jan 11 08:10:26 EST 2019

There are several different designs for distribution line arresters.  Early ones were a simple spark gap in a tube arranged so the arc blew itself out the tube to stop the conduction, I doubt if there are many of those left in service.  There are also SiC (Silicon Carbide) arresters, those are an early style that required either an external or internal spark gap to turn off since SiC doesn't stop conducting completely by itself.  More recent are the MOV types, these normally stop conducting when the voltage drops back into it's normal range, however they do wear out... after so many surges the MOV discs degrade and they no longer stop conducting by themselves.  To stop them from conducting at that point they can either use a gap like the SiC ones did or an exploding disconnect... essentially they have a .22lr blank in a carbon resistor in series with the discs, if current continues for too long the blank cooks off and blows one connection off the arrester to remove it from the circuit, this of course makes it easy to find failed ones but can be hazardous if you are nearby when it blows.

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373

-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Jim Brown
Sent: Friday, January 11, 2019 07:27
To: rfi at contesting.com
Subject: Re: [RFI] Christmas light display - SOLVED

Great report, Jeff.  Many thanks.

Can you please take a good photo of one of these MOVs on a pole so that we know what they look like? Dave, NK7Z and I can post it on our websites.

MOVs can be pretty nasty devices. They are a train wreck on branch circuits in surge suppressors, where they are likely to CAUSE destructive failures when equipment plugged into protectors at different locations are connected together. And as they absorb charges over time, they degrade, but it takes a while before they fail destructively. I suspect, but do not know, that in that intermediate state is when they may be causing the noise you've documented.

73, Jim K9YC

On 1/10/2019 10:12 PM, Jeff Blaine wrote:
> About 10am the noise went silent.  I called the guy and he told me 
> they checked out the pole with the caps on it first, and then went 
> over and pulled the MOVs on the NW pole.  I guess the lesson of the 
> MOVs as culprit was not quite learned - but after the 2nd pole I think 
> the fellow was a believer.
>
> Much greatness was expressed.  And I've enjoyed the quiet conditions 
> since!  Many thanks to the local that helped me out on the 
> identification and to the power company who had no problem fixing the 
> problems once they were convinced the proper pole was identified.
>
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
>
> On 23-Dec-18 8:15 PM, Jeff Blaine wrote:
>> Turns out this was not a Christmas light display, but apparently is a 
>> MOV on a pole in front of the guy's house.
>>
>> Diagnosis and tracking involved driving around looking at signal 
>> levels on a PL660 SWL.   Then looking at higher and higher 
>> frequencies to try to localize.  I still could not identify a 
>> particular pole though.
>>
>> Next step was to use a MFJ VHF noise detector which allowed me to 
>> localize down to a 4 pole location over a 200' area.  The local power 
>> company was not motivated to go out and check into it with this level 
>> of diagnosis.
>>
>> So a buddy K3PA brought over his industrial stuff today and we got it 
>> nailed down to a single pole which has a pair of MOV. Tools used in 
>> that were a 440 yagi with a Yaesu FT817, then a confirmation with a 
>> Radar Engineers Ultrasonic/UHF detector. The guy had an ultrasonic 
>> dish but we did not need to use it.
>>
>> The power company was updated with my single pole diagnosis and today 
>> promised they would contact me for repair details in 48 hours.  That 
>> would be great and just in time for the SP next weekend!
>>
>> 73/jeff/ac0c
>> alpha-charlie-zero-charlie
>> www.ac0c.com
>>
>> On 08-Dec-18 12:15 PM, Jeff Blaine wrote:
>>> I'm going to go see a neighbor today who always has a great looking 
>>> display about this time each year. This go around, he's expanded to 
>>> even more and better looking stuff.
>>>
>>> The problem is when it fires up the noise floor from 160 up through 
>>> 10m bumps up a few dB to a few tens of dB.  And the guy is about 1/2 
>>> mile away!
>>>
>>>
>>> Looks like the primary energy point is about 6 Mhz which was a 
>>> surprise.  Very wide banded.  And amazingly strong.
>>>
>>> 73/jeff/ac0c
>>> alpha-charlie-zero-charlie
>>> www.ac0c.com
>>> _______________________________________________
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