Silicone rubber high voltage insulation failure

[jamarno]

How does insulation break down when it seems to be perfect?

I have a silicone rubber spark plug boot, 4mm thick, that emitted a
0.5" arc to ground, yet it didn't seem to have any tears, cuts, voids,
or carbon tracks (arc occurred 0.75" from end). According to:

www.dowcorning.com/content/rubber/rubberprop/elec_dielectric.asp

the dielectric strength should have been roughly 48,000V, and a 0.5"
arc through air isn't that much outside this range.

 

[CFoley1064]

Subject: Silicone rubber high voltage insulation failure

From: [email protected] (jamarno)
Date: 5/6/2004 5:28 AM Central Standard Time
Message-id: <[email protected]>

How does insulation break down when it seems to be perfect?

I have a silicone rubber spark plug boot, 4mm thick, that emitted a
0.5" arc to ground, yet it didn't seem to have any tears, cuts, voids,
or carbon tracks (arc occurred 0.75" from end). According to:

www.dowcorning.com/content/rubber/rubberprop/elec_dielectric.asp

the dielectric strength should have been roughly 48,000V, and a 0.5"
arc through air isn't that much outside this range.

Insulation breakdowns can be caused by many things, including the effect of
weather, engine heat and vibration over a long period of time. You don't
necessarily see the fault, in part because the path of your arc would be inside
the body of the boot. If your spark plug cables are new or almost new, it's
probably a manufacturing defect (fairly common with the knock-off generic
substitutes from the auto parts superstores). If you've still got the receipt,
get a refund, and buy a reputable brand, even if it costs more. Quality in
spark plug cables does make a difference, especially after a couple of years.

Good luck
Chris

 

[Jamie]

carbon tracks are hard to find, its very possible there some foreign
bodies in the compound it self causing a slight conductivity.
pin holes are also hard to find, some people spray protective
coating on their wires. in some cases this coating after a couple of
years may break down the silicone ..
i work for a company where we make Power Cables, Electronic wire ,
high voltage wire etc... i would say that you have some silicone that is
going pass its cure point due to the corona that builds up on cheap wire
and over time starts to break down the silicone. this is evident many
times when seeing white powder build up.

 

[Bob Myers]

How does insulation break down when it seems to be perfect?

It doesn't take much of a flaw to cause a breakdown. I've
had numerous situations in which CRT displays with silicone-
insulated anode connectors (pretty common) were arcing from
this point SEVERELY, and yet no damage was evident to a
casual inspection. We had to section the connector and look
for evidence very, very carefully to find the discharge paths,
which generally were pinhole defects or damage through the
silicone cap.

If you actually manage to breakdown through intact insulation,
it's generally a good deal more obvious. So in your case I would
bet either on a tiny, nearly-invisible flaw or damage, or a conductive
path being present due to foreign materials/contaminants.

Bob M.