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anti-static/grounding question...

 
 
gimp
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Posts: n/a
 
      01-04-2006
whoisthis wrote:
> Fact: A proper earth strap should have an integrated resistor built into
> it,


where can i buy those from - do the DSE ones have it...?
 
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Gavin Stephens
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      01-04-2006

"Stephen Worthington" <(E-Mail Removed)34.nz56.remove_numbers> wrote in
message news:(E-Mail Removed)...
> On Tue, 03 Jan 2006 20:42:04 +1300, gimp <(E-Mail Removed)> 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.

Gavni.


 
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Geoff@work
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Posts: n/a
 
      01-04-2006

"gimp" <(E-Mail Removed)> wrote in message
news:dpf3fb$ij0$(E-Mail Removed)...
> whoisthis wrote:
>> Fact: A proper earth strap should have an integrated resistor built into
>> it,

>
> where can i buy those from - do the DSE ones have it...?


Yes


 
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outofdate
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      01-04-2006

"gimp" <(E-Mail Removed)> wrote in message
news:dpd9mk$836$(E-Mail Removed)...
also does touching the PSU still work as a ground if the
> cable is unplugged..?
>


Nope. You are using the mains ground to your house when you are plugged in
at the wall.


 
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outofdate
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Posts: n/a
 
      01-04-2006

"outofdate" <outofdate@computer> wrote in message
news:(E-Mail Removed)...
>
> "gimp" <(E-Mail Removed)> wrote in message
> news:dpd9mk$836$(E-Mail Removed)...
> also does touching the PSU still work as a ground if the
> > cable is unplugged..?
> >

>
> Nope. You are using the mains ground to your house when you are plugged in
> at the wall.
>


Also you should not have the power turned on at the wall, the ground is
connected regardless of if power is on at the wall or not.


 
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Jerry
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      01-04-2006
Gavin Stephens wrote:
> "Stephen Worthington" <(E-Mail Removed)34.nz56.remove_numbers> wrote in
> message news:(E-Mail Removed)...
>
>>On Tue, 03 Jan 2006 20:42:04 +1300, gimp <(E-Mail Removed)> 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.....?
 
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Nicolaas Hawkins
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Posts: n/a
 
      01-04-2006
On Wed, 04 Jan 2006 13:08:05 +1300, gimp <(E-Mail Removed)> wrote in
<news:dpf3fb$ij0$(E-Mail Removed)>:

> whoisthis wrote:
>> Fact: A proper earth strap should have an integrated resistor built into
>> it,

>
> where can i buy those from - do the DSE ones have it...?


DSE; yes.

--
Nicolaas
 
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whoisthis
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Posts: n/a
 
      01-04-2006
In article <43bb6b91$(E-Mail Removed)>, Jerry <(E-Mail Removed)>
wrote:


> 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.....?


Almost right. It is only because the skin resistance is so high that it
wont kill you, however, if for example you were in an operating theatre
and 9v was placed across your heart... then yes, it would kill you.

Reason: Your internal body resistance is LOW, and the amount of current
directly across the heart for it to be put into fribulation is quite
low.
 
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Gavin Stephens
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Posts: n/a
 
      01-04-2006

"Jerry" <(E-Mail Removed)> wrote in message
news:43bb6b91$(E-Mail Removed)...
> Gavin Stephens wrote:
> > "Stephen Worthington" <(E-Mail Removed)34.nz56.remove_numbers> wrote in
> > message news:(E-Mail Removed)...
> >
> >>On Tue, 03 Jan 2006 20:42:04 +1300, gimp <(E-Mail Removed)> 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.



 
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Rob J
Guest
Posts: n/a
 
      01-04-2006
In article <(E-Mail Removed)>, http://www.velocityreviews.com/forums/(E-Mail Removed)
says...
> On Tue, 03 Jan 2006 10:25:09 GMT, Stephen Worthington
> <(E-Mail Removed)34.nz56.remove_numbers> wrote:
>
> >On Tue, 03 Jan 2006 20:42:04 +1300, gimp <(E-Mail Removed)> 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.

>
>
>
>
> YOU DO NOT NEED A EARTH CONNECTION WHEN WORKING ON A PC AND MOBO, YOU JUST
> NEED A ANTI STATIC STRAP.
>
>
> ITS THE POTENAL DIFFERENCE BETWEEN YOU AND THE COMPUTER CASE THAT MATTERS..


Yeah, I was working with a Unisys hardware tech today, and he didn't
have an earth on the case, just the antistatic strap.


 
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