Re: Capacitor safety questions OT

Mariss:

I always enjoy your engineering posts. I'd like to add another
couple of safety tips concerning the power feed to your tool:

The first time you turn on a newly installed disconnect switch or
circuit breaker stand to the side and wear googles (or at least turn
your eyes away). Test rotation before you turn on your motor. Make
sure everything is PROPERLY grounded. NEVER work hot unless
absolutely necessary. If you KNOW what you are doing - be extra
careful and use appropriately insulated mats, gloves etc. made for
the purpose when forced to work hot. If you don't know what your
doing with power DON'T fool with it. Its better to hire an
electrician than end up in the hospital with burns (or worse).

Yes I know that they charge what brain surgeons wish they could
make, but they know how to safely work with something that can be
hazardous to your health or property.

Jeff

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@y...> wrote:

> 1) Voltage does't kill, current does.
> 2) You are just a large resistor.
>
> The following is taken from
> http://www.physics.udel.edu/~watson/scen103/colloq2000/safety.html
> Note that the current has to pass thru the chest.
> -------------------------------------------------------
> Current effects on human body, current through chest
> (A, amps)
>
> <0.01 tingling or imperceptible
> 0.02 painful, cannot let go
> 0.03 breathing disturbed
> 0.07 breathing very difficult
> 0.10 death due to fibrillation
> >0.20 no fibrillation, but severe burning, no breathing
> -------------------------------------------------------
>
> Dry skin has a resistance of 500K Ohms, so it would take 50,000V.
>
> I set my multimeter to "mA", connected it to a 60VDC supply, then
> washed my hands but didn't dry them. I grabbed my multimeter

probes

> in each hand and measured 1.2mA of current.
>
> I then sprinkled table salt on my still wet hands and tried again.
> The best I could get was 9.8mA and a barely perceptable tingling.
>
> The internal resistance of a human body is about 500 Ohms. If your
> skin were removed, it would then take about 50 volts to be lethal.
>
> High voltages (115VAC, 230VAC) can kill by burning thru the skin

at

> the point of contact and then connecting to the much lower

internal

> body resistance. This takes time though, about 15 to 30 seconds at
> 230VAC. The heat generated is about 20W at the point of contact

(200V

> times 100mA) and takes that long to burn thru the skin.
>
> The 9.8mA at 60V I tried generated a bit more than 1/2W (588mW)

and

> could do that; the body is simply too much of a heat-sink at that
> power level.
>
> As another reference said: "Electrocution is very difficult at
> voltages below 100V, you must really work at it to succeed".
>
> Safety suggestions:
>
> 1) Use only 1 hand when working with high voltages (115VAC or

more).

> Keep the other hand behind your back; never rest it on anything.
>
> 2) Don't stand barefoot on a concrete floor or don't stand on a

wet

> concrete floor with shoes on when working with high voltages.
>
> 3) When touching a high voltage, do it with the back of your hand

or

> back of your finger first. Should there be a shock, muscle
> contraction will pull your hand away from instead of onto the
> conductor.
>
> 4) Keep your hands dry. If they get sweaty, dry them often.
>
> 5) Obviously, have the power off. Off means the plug is pulled

from

> the wall. Always double check you see the pulled plug before you

get

> into the equipment if you walked away from it for even a minute.
> Someone could have plugged it back in during your absence.
>
>
> Mariss
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "volitan712003"
> <volitan@o...> wrote:
> > Hello,
> >
> > I just ordered a 12,000uF x 100V capacitor for my power supply.

I

> know
> > capacitors can be deadly but I have no concept of what a micro
> farad
> > is. If this cap is fully charged, is this thing big enough to

kill?

> >
> > Another question is how can I check it so I know it's

discharged.

> I'm
> > guessing it would have too much power to hook my volt meter up
> > directly.
> >
> > Thanks!
> > David