Re: What has everyone got against PCB?

["Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

> > Just
> > wanna make sure:  what exactly is PCB for the slightly ignorant (like
me)?
> 
> This is how it was explained to me by a PSCO lineman:
> 
> PCB's (a family of chemicals, not just one) are (if my memory serves me
> correctly) Poly-Chlorinated Biphenyls, chemicals added to mundane oils to
> make them flameproof for use in transformers.
> 
>  (someone correct me- do they also have any benefit toward dielectric
> strength?)

Actually, PCB''s (aka Askarels) were used as a total replacement.
> 
>  In the bad old days, when pole transformers overheated, melted down, and
> blew up on a hot summer night, they tended to rain burning transformer
oil
> down on all and sundry below, ruining your day if it happened to be you,
> your house, or your car that got immolated.

A bigger problem is substation accidents, and circuit breakers catching on
fire. High power circuit breakers (unlike the ones in your house) need
something to specifically quench the arc that forms when the contacts
separate.  Lots of techniques are possible: magnetic fields, air blasts,
physical high speed separation, and, relevant to this discussion, the blast
of hydrogen gas formed from the decomposition of the oil by the heat. 
Since the container is mostly sealed, there isn't any oxygen to react with
the hot hydrogen, and the arc gets blown out very nicely. HOWEVER,
containers aren't perfect, the oil level might be a bit low, etc.  Now
you've got an arc, hot oil, hydrogen, and a fire.  THen the circuitbreaker
next to the one that caught fire catches fire, etc,etc.etc....  

In Europe, where the packing density of civilization is much higher, this
was a real problem, partcularly when bombs are falling... Electrical
installations being a good target for bombing, and having them full of
flammable oil just makes them more attractive.. Air blast CBs have the
problem of the supersonic gas flow being quite loud, even with mufflers.
Gas Insulated Switchgear (GIS) using SF6 has largely replaced oil
switchgear in dense areas.

> 
>  So mandates for flame-proof oils were enacted.  PCB's were a quick fix. 
I
> don't know what is done to modern transformer oils to make them
> fire-retardant.

Nothing... modern transformer oil burns quite nicely, just like bacon
grease or cooking oil... A  way to check if an unknown liquid in a
component is PCB or oil is to try igniting some.  PCB's don't burn, oil
does.
> 
>  PCB's are powerful carcinogens, it turns out.  They are not death on
> greased wheels (as compared to say, nerve gas), but they are to be
avoided.

Again, the PCB (like Askarel) isn't the problem, it is the contaminants and
decomposition products.  You CAN swim in Askarel without much problem, IFF
is it clean.


askarels are typically 20-25 kV/mm breakdown (viz oil at 15 kV/mm), are
hydrophobic, and have a dielectric constant (epsilon, permittivity) of 4.8
(viz oil at 2.2-2.3).  Oddly, though, askarels have worse dielectric
strength than oil for impulses and high frequencies.