[sdiy] scsi shock

[Tim Ressel]

The answer, in a word, is leakage. All transformers
have a certain amount of leakage. Because of the high
impedance bewteen the primary and secondary, this
leakage can build to quite a large fraction of the
input voltage. Usually the secondary side is
referenced to ground in some meaningfull way, and this
tends to mitigate the problem. 

This can be a wierd one to solve. Sometimes when I am
hooking up stereo and/or video gear, I'll get a hum
from a ground loop or leakage. I'll solve it by
identifying any plugs that are not polarized (2-prong
US plugs), and turn them around one at a time unti the
hum goes away. It's strange, but it works.

In your case, i think what happened is this: Leakage
from one or more pieces of gear is causing a large
voltage differential. this is a high impedance thing,
so the minute you attach the cable, the voltage gets
shorted out. You probably got across the circuit while
it was open and charged.

One solution is to get a power strip with a switch (if
they have such things where you live), and plug it all
into the power strip. One flick of the switch and no
more shocks.

--TR


--- Dave Krooshof <synthos at xs4all.nl> wrote:
> Hi list,
> 
> I email the list as we had some nice discussions on
> grounding and shocks.
> Here's another story to share, and questions to
> ask...
> 
> I'm typing with a sore hand, as I just got shocked
> by my
> image scanner (apple color onescanner) while
> patching it.
> The SCSI chain is: Mac G3 -> CD Toaster -> Scanner.
> All worked fine this very evening. Nothing got moved
> a lot.
> 
> I just patched the cd toaster back into the chain,
> with both the computer and the scanner switched on.
> (yes.)
> I was holding the 'outgoing' SCSI cable from the
> toaster
> and touched the connector on the scanner with that
> same hand.
> 	> Bzzzap! <
> Took a voltmeter, found 126 volts AC.
> The wall socket is 239 volts at the moment (good
> weather today).
> It's a zero/phase/ground wall socket. All equipment
> shares 1 socket.
> The scanner doesn't seem to have any short circuits,
> as far
> as I can see now.
> 
> questions are:
> - Why didn't my computer crash, or smoked the SCSI
> controller
> or simply die?
> - Why didn't the safety ground switch flip? (It does
> work normally)
> - If a short circuit is there, why isn't it 5, 12
> dc, or 230 volts AC?
> (can't test the voltage on the lamp, as it can't be
> switched on while
> not connected to the mac. No trace of 126 volts
> anywhere inside).
> 
> Any hints on finding the error are welcome.
> 
> I'll give my hand a rest, now.
> 
> Thanks,
> 
> Dave
> -- 
> ____________________________________________________
> And I will make thy windows of agates, and thy gates
> of carbuncles,
> and all thy borders of pleasant stones. (Isaiah
> 54:12)
> 
>         <Please> no html-mail! html does not belong
> in email. </please>
> 
> http://www.xs4all.nl/~krooshof


__________________________________________________
Do You Yahoo!?
Yahoo! Health - your guide to health and wellness
http://health.yahoo.com