Re: HV Cable

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 05:53 PM 12/2/2002 -0700, you wrote:
>Original poster: "Daniel Hess by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><dhess1-at-us.ibm-dot-com>
>
>
>Matt;
>
>The General Motors HEI ignition systems reportedly can achieve 45kv so
>silicone jacketed HEI rated spark plug wire should be more than adequate on
>an MOT system. I think most varieties should be able to withstand 20kv at
>least.
>
>BTW, Walmart sells a high performance, silicone spark plug wire kit; About
>30' bright orange 8 mm silicone cable. About 14 gauge copper conductor.
>Last time I bought one of these kits I paid $8.00 American. The wire is
>super flexible, solders well, looks good and the spark plug boots make nice
>covers for where the cable attaches to the NST HV bushings.
>
>Daniel
>
>"Tesla list" <tesla-at-pupman-dot-com> on 11/30/2002 05:25:55 PM

having spent quite some time dealing with the actual performance of HV 
cabling versus it's labelled rating, I can offer the following "insights":

1) Just because a HEI ignition can potentially put out 45kV doesn't mean 
that's the normal voltage it works at (under load), which is probably more 
on the order of 20 kV.

2) Automotive components, in general, are designed to be just good 
enough.  It's a huge volume market, and saving a few pennies on each item 
adds up when you make millions. If the cable would work fine at 20 kV, and 
wouldn't fail catastrophically if someone leaves a sparkplug wire off (so 
that it really did go up to 45kV), that would meet the requirements for the 
auto manufacturer.

3) Spark plug wires can actually have significant "leakage" (pinholes, 
corona, etc.) without adversely affecting the engine performance.  This is 
why the wires are supported on looms away from grounded metal engine 
components (like valve covers, etc.).  A pinhole from a transient wouldn't 
affect normal performance, until it got really, really bad.

4) DC ratings and AC ratings are very different for most insulated wires. 
In the AC case, the field is always changing which is both good and 
bad.  In any case, the way in which a tiny defect turns into a failure is 
quite different.

5) It pays to carefully look at the test conditions for a given 
rating.  It's very different to test a piece of wire lying on a metal 
plate, as opposed to, for instance, immersed in a conductive liquid.

6) Ignition coils put out a waveform that doesn't really look much like a 
60 Hz output from a NST, or the ringing signal in a TC primary (closer to 
the latter than the former, though). Waveform does make a difference in 
insulation breakdown.


All this said, I think sparkplug wire is actually pretty nice stuff to work 
with. It's fairly flexible (compared to, say, RG-213), the copper cored 
kind is solderable or crimpable, and, on sale, it's fairly cheap.  On the 
other hand, it doesn't have any rating of any kind or any specs on 
consistency of manufacture, so don't believe in the insulation properties.

By the way, based on some lab testing, and if you have a lot of money to 
burn, you can try getting Kynar insulated HV wire.