Rubber beading on high voltage cable insulation

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Hey all,

Just curious what you think about this.  The photos at the link are  
showing large gauge wire used on a vintage 600 VDC rail vehicle.  We're  
talking 1930's electric streetcar technology.  Basically 600 VDC connected  
to a knife switch, then to a rotating manual controller which you  
partially see in the photo.  The rotation of it sets up the series and  
parallel stages to the traction motors.

We see this weird rubber beading occurring on the outside of the older  
wire insulation.  I'm not sure what that insulation is made of or how old  
it is.  This only seems to occur on the older wires.  The newer NFPA  
compliant wires don't have this symptom.

What is it and what causes it?  Any theories?  Thanks.

https://imgur.com/a/OqHQV8U


Re: Rubber beading on high voltage cable insulation
On 20/07/2022 07:53, Mike Mocha wrote:
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Bits of melted tar maybe...



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Re: Rubber beading on high voltage cable insulation
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Mike Mocha wrote:
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If it's anything like the HV wiring in 1940s/50s era locomotives that
I've worked on restoring, the insulation is essentially some kind of
cloth dipped in tar (pitch, whatever).


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Re: Rubber beading on high voltage cable insulation
On 20/07/2022 07:53, Mike Mocha wrote:
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Looks like heat damage to me!
Possibly caused by resistance heating in a tarnished/corroded  
connection?  I would disassemble, clean, grease and remake the  
connections.
Replace any damaged cable along the way:  If I am right, there will be  
some. If I am wrong, there may not be :-)

HTH, Chris

Re: Rubber beading on high voltage cable insulation
Quoted text here. Click to load it

Thanks for all the responses.  I guess that makes sense about it being tar  
over some kind of fiber wrap.  Maybe it's time for us to upgrade to  
something more NFPA 130 compliant!!

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