"Jerry" <> wrote in message
news:43bcbce4$...
> Gavin Stephens wrote:
> > "Jerry" <> wrote in message
> > news:43bb6b91$...
> >
> >>Gavin Stephens wrote:
> >>
> >>>"Stephen Worthington" <_numbers> wrote
in
> >>>message news:...
> >>>
> >>>
> >>>>On Tue, 03 Jan 2006 20:42:04 +1300, gimp <> wrote:
> >>>>
> >>>>
> >>>>
> >>>>>every motherboard manual i've read says when doing a system build the
> >>>>>power cable should be unplugged to avoid electric shock.... but
> >>>>>seriously if its switched off at the wall there's no danger - is that
> >>>>>correct...? also does touching the PSU still work as a ground if the
> >>>>>cable is unplugged..?
> >>>>
> >>>>I always work on my PC with the mains cable in, but the wall switch
> >>>>off, in order to retain an earth connection. If it is not plugged in,
> >>>>then there will be no earth. I always use a DSE earthing strap - it
> >>>>is much safer than trying to remember to ground yourself at crucial
> >>>>times. I consider it pretty safe to work like this as long as I am
> >>>>not trying to work on the inside of the power supply - when you open
> >>>>the power supply case, you are exposing high voltage wiring and
> >>>>*really* need the assurance of unplugging from the wall.
> >>>>
> >>>>The voltages on the outside of the power supply are not at all
> >>>>dangerous to a human, as long as the power supply has not suffered a
> >>>>catastrophic fault that allows the high voltages through it into the
> >>>>PC, so your real danger if someone turns the power on is firstly that
> >>>>you will damage your PC and secondly that the fans will catch a bit of
> >>>>you or your clothing.
> >>>>
> >>>>If you are leaving the plug in, make sure that where you are working
> >>>>on the PC has the plug in your field of view, so no-one can turn it on
> >>>>again without you noticing. If you can not do that, put parcel tape
> >>>>over the switch and label it with a marker pen: "DO NOT TURN ON!" and
> >>>>your name.
> >>>
> >>>
> >>>On occasion I've seen other earthed devices (such as audio or video
> >
> > leads)
> >
> >>>connected to a PC powered from a different 230v outlet. The potential
> >>>between a PC case and any other wire that becomes disconnected that
runs
> >
> > off
> >
> >>>a different mains socket can kill, especially if it's on a different
> >
> > phase.
> >
> >>>Most of the time it's just a tingle, but if you have even a small skin
> >>>injury where you are earthing yourself to, you only need 5-20mA of
> >
> > current
> >
> >>>through your body to kill you full stop (even a multimeter can kill - I
> >>>heard of one guy who was stupid enough to see what his body resistance
> >
> > was
> >
> >>>and touched a probe with a wounded hand, the resistance of his body was
> >
> > the
> >
> >>>last thing he read). Make sure you disconnect every other connector
from
> >
> > the
> >
> >>>PC case before diving inside and before connecting the earth strap. And
> >>>don't plug or touch their end leads until you've finished the work in
> >
> > the PC
> >
> >>>case and taken off the earth strap. Then when you're left with the
mains
> >>>lead connected to the power supply, make sure the power supply mains
> >
> > switch
> >
> >>>is off, and the wall socket is off.
> >>>
> >>>Or some more peace of mind, have a computer lead that has the neutral
> >
> > and
> >
> >>>phase pins chopped off with just the earth pin left plugged in to the
> >
> > power
> >
> >>>supply if you pull them appart regularly. Again, make sure all other
> >
> > devices
> >
> >>>and plugs are disconnected from the case though because they could be
> >
> > faulty
> >
> >>>or the potential difference could kill.
> >>>
> >>
> >>A multimeter cannot kill you, you can't get enough current out of its
> >>battery. As I recall it is 100 - 200 ma that is the killing range of
> >>current. The multimeter story sounds like an urban legend. Did you
> >>hear about the guy that gave his girlfriend spanish fly, then left here
> >>in his Volkswagen while he went to the toilet.....?
> >
> >
> > Electrocution is sensed at 5ma, 10 becomes painful, and 15-20mA AC will
> > screw up the heart.
> > True that DC requires more, however fo the average 70KG male you can not
let
> > go from a DC current abov 75mA.
> >
> This is a long way from your initial statement that a multimeter is
> deadly. the chart at
> http://www.allaboutcircuits.com/vol_1/chpt_3/4.html agrees with your
> unable to let go at 76mA for men, but heart fibrillation is 500mA DC.
I think I read somewhere when investigating transformer designs and
frequency limits once that 50Hz is worse for electrical shock than 60Hz
aswell? An even lower current rating to stop the heart. That one I'm not
sure on. But getting back to my orginal statement, I could have mentioned AC
curent to be extremely clear, however I was orginally talking about mains
grounding to do with static electricity in which I was referring to AC
current, although happened to mention even a multimeter can kill (not
definately will but can) but gave no direct details on DC current.
Gavin.
. Yes I do know the difference. But urban ledgend 
