Re: Short circuit/Control Panel

Subject:  Re: Short circuit/Control Panel
  Date:  Fri, 6 Jun 1997 13:28:14 -0400
  From:  "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:  "Tesla List" <tesla-at-pupman-dot-com>

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: Short circuit
> Date: Thursday, June 05, 1997 11:44 PM
> Subject:  Short circuit
>   Date:  Thu, 5 Jun 1997 18:09:23 -0700 (PDT)
>   From:  gweaver <gweaver-at-earthlink-dot-net>
>     To: Tesla-at-pupman-dot-com
> While running my 1350 watt Tesla Coil today something very strange
> happened.
> The ON/OFF switch shorted itself closed and I could not turn the
> Coil
> off. I tried the switch several times and it refused to turn off. 
> then
> walked a few steps to unplug the cord from the wall when it
> arced
> on the side of the cord about 2" from the plug.  The arc turned
into a
> hissing ball of fire.  For several seconds the Tesla Coil was still
> making
> sparks and the extension cord was arcing also.  I unplugged the
cord and
> found the copper wires were melted and burned in half on the cord. 
> know
> high voltage can do strange things but never had this happen
before.  If
> only the switch had gone bad or only the cord had fried I would
guess it
> died of old age.  But 2 things fried within seconds of each other
> me
> think it was caused by something.  Has anyone ever had something
> this
> happen?   
> Gary Weaver

We will assume that the 1350 watts is *actually* 1350 watts (but my
own experience is that the plate rating of a transformer is not
necessarily what it will deliver in an operating TC circuit. For
*that* you will have to actively measure it, not just assume it is
the plate rating).

While 1350 watts is only about 11 or 12 amps at 120 volts, it has
been my experience that inrush currents can easily be double this and
more. You will *not* experience the effects of inrush current if you
the switch on when the variac is at its minimum setting. But if you
turn the switch on and the variac is near its maximum setting, then
the inrush currents can be significant. The exact inrush current is a
function of angle of the AC current at turn on. The WORST is when you
just happen to hit the switch at the PEAK of the AC cycle. The high
inrush current will then often fuse the switch contacts together. 

That is the reason why I always use a contactor to provide the power
to my TC circuits. A contactor is a sort of glorified relay. The
"power switch" actually just powers the relay, and the relay contacts
then power the TC circuit. I usually use contactors rated at 100 amps
on the contacts and 110 volts on the relay coil. You can get these
things from electrical supply houses, Home Depot, etc. I usually
scrounge mine off of old electrical equipment. Be aware that most of
the contactors out there are using 220 volt relay coils. If that is
all you can find (and you need a 110 type), then get a 110 to 220
volt transformer to power the coil. The contacts can be used for
either 110 or 220.

My guess is that the inrush current fused your switch. The failure in
the cord was just coincidental. By the way, if you yank the cords out
instead of gently removing them, then this strains the wire at the
plug and will eventually cause it to fail.

Of course it is also possible that you have an overload condition in
your TC somewhere. This could be caused by a failed transformer, for
instance. This would cause increased current load and would explain
both the fused switch and the burning line cord.

Is your TC metered? If the current jumps up significantly above
normal operating currents, then you have some kind of circuit fault.
My own way of handling potential overloads is to first run my unit
and determine its maximum normal operating current. I then fuse it
for 125% of this value. Actually, I should say I use slow blow
circuit breakers (not fuses). My main control panel has a bank of
circuit breakers of several different values. I just "activate"
several in parallel to reflect the maximum current I want to allow.
For example, if I want 40 amps, then I activate two 20 amp breakers.
Just because my electrical service can supply me with 100 amps is no
reason to use a 100 amp breaker! Four 20 amp breakers and one 10 amp
breaker and two 5 amp breakers allow me to adjust from 5 to 100
amps in 5 amp increments.

Separate 10 amp breakers are used to control things such as blowers,
gap motors, etc.  My power to the TC power transformer(s) is separate
so that I can exercise more control over the main power. A master
switch controls the delivery of voltage to all relay coils (yes, I
use separate contactors for transformers and blowers/gap motors).
This master switch is in the form of a latching relay, so if power
goes off or I hit the PANIC button, then everything is instantly
switched off. I don't want any surprises of the wrong type! I even
have a provision for a "remote" panic button so that I can shut the
system down from a remote position. This remote panic button is
mounted in a box and attached via a cable to the control panel. The
panic button is "portable" in the sense that I can move it anywhere I

PANIC buttons should be a LARGE pushbutton, and preferably
illuminated so that you can't miss it in the dark! Place it where you
can slam it quickly in case of an emergency.

By the way, ONE control panel can be used to control *all* your
projects. Ideally it should be fully metered. Meters can be
"calibrated" in units other than volts. For example, you might have a
meter calibrated in approximate BPS for a spark gap rotor.

Fr. Tom McGahee