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From: "ChrisND@privacy.net" <chrisnd@privacy.net>
Newsgroups: sci.electronics.design,sci.electronics.misc,sci.electronics.repair
Subject: Re: Rubber beading on high voltage cable insulation
Date: Wed, 20 Jul 2022 15:40:37 +0100
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On 20/07/2022 07:53, Mike Mocha wrote:
> 
> 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
> 
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