[Prev][Next][Index][Thread]

Re: Protection from NST zap, discharging strings of caps

Tesla List wrote:
> 
> Original Poster: Nteslamoot-at-aol-dot-com
> 
> Yes you can wear rubber gloves, but make sure the voltage rating on the
gloves
> exeed the max voltage of the NST.
 Also make sure the gloves have no holes in
> them(no matter how small!!!) if yiu have a hole DONT use them!!!!! Test them
> by closing off the open end by rolling it toward the fingers as you do the
> gloves will expaned due to the air in the gloves after you have pressure
> listen, feel or even submerge them in a soapy water bath to check for
bubbles.
> Pat


I might add that "real" HV gloves, as used by live line workers are
quite pricey (>$100 for the low voltage ones) and have a layer of
colored plastic embedded within them so that you can visually see if the
surface has become abraded.

I wouldn't even contemplate trusting "dishwashing" type gloves, or,
perish the thought (or perish you), latex exam gloves.

Work barehanded and make sure the equipment is off and grounded before
you start.  I cannot conceive of any good reason why you might need to
make physical adjustments on live HV equipment by putting your hands on
a live part. This is what insulated shafts on controls are for.  

The lineman works on a live circuit because he has to, for some other
reason. You don't.

While we speak of grounding, etc., I think that we should recall that
shorting a series combination of capacitors does NOT necesarily
discharge the components of the string.  If you have a series string of
caps (with no bleeders), touching each junction sequentially with your
grounding rod doesn't discharge the string.  You have to ground them all
at the same time (or jumper the caps).



Assume you had 4 caps in series that need to be safely grounded. Call
the terminals A, B, C, D, E, where B is the junction between cap 1 and
cap 2, C is between cap2 and cap 3, etc. 

A reasonably safe procedure would be something like:

1) Hook the grounding rod (aka hot stick) to one of the ends of the
string: terminal A

2) Leaving the grounding rod attached, connect a suitable cliplead from
the terminal to ground.

3) Remove the grounding rod. Leave the clip lead.

4) Hook the grounding rod to the next terminal in the string (Terminal
B).

5) Connect a cliplead from B to A (or B to ground.. It doesn't matter).

6) remove the grounding rod. Leave the clip lead (now you've got two)

7) Connect the grounding rod to terminal C, connect a cliplead from C to
B, remove the grounding rod

8) Rod to D, cliplead from D to C, Rod off

9) Rod to E, cliplead from E to D, Rod off

Now all the caps are discharged, individually shorted, and grounded. 
Leave the clip leads on until you are done, then reverse the process.

HV caps are notorious for developing a voltage after the shorting wire
is removed.  This can occur even if the cap has been shorted for a
number of seconds.

There are some other concerns: A safety grounding rod is probably not
the best thing to discharge a cap with because the low impedance will
discharge the cap quickly resulting in 1) loud bang (hard on the ears),
2) brilliant flash (hard on the eyes), 3) bits of molten metal (hard on
the equipment and screw threads), 4) damage to the cap from the voltage
reversal (hard on the wallet).  I actually use two different rods when
working with multi kiloJoule stored energies. One has a big HV resistor
of a few K in it to discharge caps with (limiting the current to 5 amps
or so) and another which I use for safety grounds (which has a series
resistance to ground of a few milliohms (#8 stranded wire, etc.).  The
discharge stick is easily made with plastic pipe and salt water ( I use
copper sulfate, but almost anything works).  You don't really care what
the exact value of the resistor is, and the water provides a good
thermal sink for the stored energy.

If your stored energy is only a few joules, you probably don't have to
worry about using a discharge stick.