Re: Capacitor safety questions

Robin,

Thank you, I worded that poorly. The cap needs to be at the 3 time
constants level within the TIME set by the safety org. (This is from
a UL spec, and the time was 30 seconds.)

BUT... Safety orgs DO sometimes use the rules to limit the overall
voltage as you outline. So having a rule which keeps the cap voltage
below some "considered safe" value-- like the 1000V you mention is
NOT unheard of...

Ballendo

P.S. My group related point in this is that very few, it seems, have
ever studied, or even looked at the appropriate safety standards for
the class of machine they are making. Reading and understanding these
can answer a lot of questions, and ultimately make for a safer shop
to work in...

(It's been awhile since I read the UL spec I'm "remembering" this
from. I'll look into it again.)


--- In CAD_CAM_EDM_DRO@yahoogroups.com, Robin Szemeti <list@r...>
wrote:

> ****Moderator Note: You guys are starting to wander out in the

weeds with this one. Please post some links to useful info or provide
useful calculations for people building CNC supplies. -L

>
> On Saturday 27 December 2003 15:15, ballendo wrote:
> > Safety organisations like UL set requirements for the voltage

level

> > remaining after 3 time constants. (That is, it must be to a "safe"
> > value within this time.) Bleeder resistor calcs should bear this

in

> > mind.
>
> Thats just plain wrong.
>
> after 1 time constant [1] the voltage will be 1/e, after 2 it will

be 1/e^2

> and after 3 it will be 1/e^3 or roughly 1/20th ... this will

always be the

> case regardless of what value of R or C you choose, after 3 tc has

elapsed

> the voltage will be 1/20 of its initial value
>
> so if safe voltage is, for example, 50V according to your version

of the

> rules that means that no capacitor can ever have more than 1000V on

it ...

> because it will always be at more than 50V after 3 lots of time

constants

> have elapsed...
>
> I suspect that the time to safe voltage is specified in terms of

seconds, and

> probably takes account of how long from power off it takes to get

the covers

> off too.
>
> --
> Redpoint Consulting Limited
>
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