[Amps] Arcing

[Jeffrey Madore]

Thanks, Rich & Marv,

Great explanation!  Fills in some missing holes in my memory and gives me
something to think on.

I once accidentally opened the secondary relaying circuit on a CT in a 4kv
system supplying load to a nuclear power station during refueling. The
instant blue flame burned an entry hole in the end of my finger and blew a
much larger hole out of the knuckle on the back of my hand where it was
touching the switchboard. My contact was a split second, as it blew me back
immediately. The CT was probably about 2ft in diameter and 10" thick, with a
ratio of 1200:5.  I was really lucky (blessed) that my knuckle on the same
hand was touching ground, as the voltage was tremendous and I don't think I
would have survived that potential across my chest.

I can't say I fully grasp the theory yet.

Repeating the situation in the lab on a smaller scale showed that the rms
secondary voltage was much higher than the source that fed the primary. The
secondary is meant to be shorted (connected to an ammeter) during normal
operation. The primary voltage is hard to determine as it is a single pass
of cable through the core. I can say that the core has a tremendous effect
on the primary if the secondary is open circuited. The impedance of the
primary (single pass of welding cable) is markedly increased without the
secondary's field bucking the CEMF caused in the primary.

This seems to hold true to transformer theory, as Z secondary is reflected
to Z primary, and I can picture how this happens electromagnetically.
However, in the case of the CT there is not enough primary Z to limit
current (thus flux) in the primary when the secondary is open-circiuted.
(desireable in a CT circuit). Thus the primary flux is affected, but little.

This is where I lose the picture as to explaining what determines the
open-circuit secondary potential of a CT.  I need the picture before I can
apply a formula. If anyone has any ideas I'd be interested.

73, Jeff - K1LE - CT ><>


 ----- Original Message -----
From: "rlm" <2 at vc.net>
To: "Radio WC6W" <wc6w at juno.com>; " AMPS" <amps at contesting.com>
Sent: Friday, March 07, 2003 5:04 AM
Subject: Re: [Amps] Arcing


> >   The turns ratio works just as expected in that application.   The
> >secondary voltage is equal to the primary voltage multiplied by the turns
> >ratio.
>
> Indeed, Marv.  I confirmed this with an oscilloscope.  The primary V
> spike is generated by the collapsing field when battery current flow
> stops in the primary -- i.e., the ignition contact opening, in days of
> yore.  Even though the battery is only 12v, when the flow of current in
> the primary stops, the collapsing magnetic field induces a few hundred
> volt spike in the primary.  The points' Capacitor (Typ 0.15uF) and
> L-primary resonate typically c. 100KHz in a '69 BMW.  The spike rings the
> L-C resonant circuit for a few cycles, and the typical 100 to 1 step-up
> iratio increases the few hundred V spike by a factor of 100.
> >
> >    Note:  the primary voltage is not always equal to the battery (or
> >system) voltage.   Do I have to explain the whole process here or... just
> >let this comment initiate a worldwide ringing?  :-)
> >
> >> Remove the capacitor in an old-style ignition, and watch the voltage
> >> drop!
> >
> >   Only because the points, which are not opened instantaneously, will
> >arc (gosh, there's that arcing again...) thereby stealing the stored
> >ignition coil energy out of the primary instead allowing it to exit the
> >secondary where it can help make smog.
> >
> >
> >73 & Good evening,
> >   Marv  WC6W
> >
> >
> >
> >
> >
> >
> >
> >*
> >
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>
>
> -  R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
> www.vcnet.com/measures.
> end
>
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