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Re: HV accident (fwd)
---------- Forwarded message ----------
Date: Sun, 05 Aug 2007 14:47:28 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: HV accident (fwd)
Hi Adam, All,
I could not resist responding to the "wet rag" discharging device!
Weird!. I'm sure it works to a point. Kind of like hosing down the coil
with water controlled to a specific area. I still wouldn't trust that
all was discharged however. No different than relying on a bleeder.
A mechanical means of discharge is a must! But, discharging a cap
directly across it's terminals with a screw driver or even a mechanical
driven switch is "bad" for the cap if it had remaining charge on a
continuous basis. In most of our TC cases, the transformer provides the
bleed, but not in all configurations. There are certain configurations
where the cap can retain a charge. In those cases, a bleeder resistance
is important for the health of the cap. As was mentioned in this post
earlier, bleeders are "not" human safety devices. They were never
intended to be human safety devices! Anyone thinking that bleeders were
intended as human safety discharge paths needs to rethink what they
think they know. Bleeders are used to bleed down a charge and "NO" they
are not only used in power supply filter circuits. They are a necessary
device in some high current and/or high voltage applications. It is true
that some engineers employed bleeders to prevent a human disaster, but
they were wrong for relying on bleeders.
There are high side switching applications (and other numerous
applications) where bleeders are "very" necessary for the health of the
components. They are NEVER the final discharge safety device and should
never be thought of in that way. If you have an application or
configuration where the cap can be isolated and remained charged,
mechanical discharge across the cap is very bad for the cap itself,
thus, bleed the voltage and then mechanically discharge. But don't rely
on the bleeder from a safety standpoint (that is IMPORTANT). Resistors
can certainly burn open. Thus, a mechanical discharge with a failed
bleeder would be bad for the cap, but a one time burst identifies a
failure of the bleeder and can then be replaced.
In a nutshell, bleeders are needed in some circuits from a component
safety and longevity standpoint. They are never to be assumed as the
discharge safety device. That is "STUPID" and that is what "KILLS"
people (the assumption that the energy has bled because the device
incorporated a bleeder resistance).
I've had one engineer nearly electrocute himself with a high
current/high voltage cap bank (far more energy than anything we use in
Tesla Coils). I managed engineers for a number of years and we dealt
with lethal high current and high voltage caps in high speed flash tube
technologies. The field engineers hired to maintain the equipment were
"not" trained for high voltage maintenance (the flash tube modules were
a later inclusion to the product). The engineers that designed the
equipment had no vision of who maintained the equipment. Luckily, I was
on the Tesla List a couple years at that point and realized the dangers
my engineers were to encounter. I was able to pass on some of my
acquired knowledge to them. One engineer simply made a mistake. He went
to the hospital that night. The next day, I ranted and raved to
management about high voltage training. Unfortunately, it sometimes
takes someone in the hospital before management realizes how important
the situation is.
With that said, in my particular TC circuits, I don't use bleeders. The
transformer provides the bleed. But, I always discharge mechanically
before anything is touched. I also clip on a wire across the cap during
all non-use situations. If ever there was a large discharge during my
mechanical discharge procedure, I would then know there was an open
circuit. It's not rocket science!
Take care,
Bart
Tesla list wrote:
>---------- Forwarded message ----------
>Date: Sun, 5 Aug 2007 03:57:07 -0700 (PDT)
>From: Yurtle Turtle <yurtle_t@xxxxxxxxx>
>To: Tesla list <tesla@xxxxxxxxxx>
>Subject: Re: HV accident (fwd)
>
>If you need to discharge a HV cap and don't have
>resistors that are up to the task, use a wet rag and
>throw it over the terminals. It may not be pretty, but
>it beats using a screwdriver. Just be sure to use the
>screwdriver after the rag has been there for a minute.
>And always store caps with a shorting wire or bus. It
>also doesn't hurt to use safety glasses when playing
>with HV.
>
>Adam
>
>--- Tesla list <tesla@xxxxxxxxxx> wrote:
>
>
>
>>---------- Forwarded message ----------
>>Date: Sat, 04 Aug 2007 05:36:41 +0000
>>From: nancylavoie@xxxxxxxxxxx
>>To: Tesla list <tesla@xxxxxxxxxx>
>>Subject: Re: HV accident (fwd)
>>
>>Is there really any truly "safe" way of doing what
>>we do? I mean come
>>on,every aspect of the hobby has a hazard of some
>>sort and I think that
>>The List does a pretty good job of pushing those
>>issues (sometimes to the
>>level of sheer paranoia-see archives for Thoriated
>>tungsten for
>>example).Perhaps it's time to review some basic
>>safety and maybe we should
>>all be looking for alternative methods for
>>discharging capacitors that
>>don't use the trusty screwdriver! I have had no
>>other choice in the past
>>and never was too crazy about doing it having seen
>>what even a small
>>photo-flash cap can do when discharged.Is it time to
>>step up to large
>>shorting relays or something? Wyatt
>>
>>
>>
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