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anti-static/grounding question...

 
 
gimp
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      01-03-2006
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..?


 
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Shane
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      01-03-2006
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...?


as long as someone else doesnt switch it on behind you
Theres a big debate on this issue, on the one hand some people say the
power cable provides an excellent earth
OTOH people say its too easy for a faulty switch to be on, or someone else
switch it live without your knowledge to make it safe
Its your risk..

> also does touching

the PSU still work as a ground if the cable is
> unplugged..?




--
An empty cab drove up and Sarah Bernhardt got out. -Arthur Baer,
American comic and columnist

 
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Rob J
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      01-03-2006
In article <dpd9mk$836$(E-Mail Removed)>, http://www.velocityreviews.com/forums/(E-Mail Removed) says...
> 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..?


Any electrician will tell you not to rely on the mains earth as being a
true earth and totally safe. A wiring fault in a building could result
in the earth voltage being jacked up well above zero.

Having said that - everyone regards the mains earth as safe when working
on PCs.

Touching the metal case does not work as a ground if there is no
physical connection from the case to ground.

The power supply itself is working with mains voltages within, you
should not open the case of the power supply with the mains connected.
Also be aware that most modern boards are still powered even when the PC
is "off" as they are able to be woken up by system events. Mains power
should be switched off at the wall when working on these PCs.

 
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XP
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      01-03-2006
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




Yes there is as it can be switched back on again.

With the Plug out its 100 % Safe.

I worked on Power distribution boards in the UK, when we wished to disable a
circuit we removed the Fuses and placed them in our overalls, so no one could
plug them back in..


>- is that
>correct...? also does touching the PSU still work as a ground if the
>cable is unplugged..?
>




Get a earth strap from DSE, then you do not forget..




 
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Stephen Worthington
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      01-03-2006
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.
 
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EMB
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      01-03-2006
Stephen Worthington wrote:

> 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.


Make up a short extension lead with only an earth conenction and plug
the PC in via that - then there's no chance of a faulty switch leaving
power on the PC.

--
EMB
 
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Rob J
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      01-03-2006
In article <(E-Mail Removed)>,
http://www.velocityreviews.com/forums/(E-Mail Removed)34.nz56.remove_numbers says...
> 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.


I made a special mains cable with only the earth pins connected both
ends.

 
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Bruce Knox
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      01-03-2006
On Tue, 3 Jan 2006 21:22:19 +1300, Rob J <(E-Mail Removed)> wrote:

>In article <dpd9mk$836$(E-Mail Removed)>, (E-Mail Removed) says...
>> 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..?

>
>Any electrician will tell you not to rely on the mains earth as being a
>true earth and totally safe. A wiring fault in a building could result
>in the earth voltage being jacked up well above zero.

Not a problem in this case. A suitable earth for anti static can be
100MOhm and work fine. In fact most professional anti static eaths
will incorporate such a resistance so they earth static charges but do
not provide a high current earth if you touch a dangerous high voltage
source. Likewise it does not matter if the earth is above true earth.
You are trying to minimise voltage differentials. If the case is at 30
volts then everything should be at 30 volts.

On all pcs I have seen unless you are working on the power supply (a
sealed system) there is no way of contacting dangerous voltages. My
advise is to leave it plugged in but switched off to get the earth.
Touching the case when it is not earthed is no protection from static
charges.
....


Bruce http://www.baggins.co.nz
http://physio.otago.ac.nz
 
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Mark Robinson
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      01-03-2006
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 don't see any advantage in grounding the machine to mains earth while
building a system, in fact this will improve the chances of damage from static
electricity as the path to ground for the static will be improved.

In the absence of a grounding strap keeping skin touching a metallic part of
the case is better than nothing but the closer you get to an antistatic
workstation the fewer random deaths you will suffer.

The idea is to keep you, the machine and all the components you are putting
into it close to the same voltage.

If you earth yourself to the case of the machine with a grounding strap, you
still need to consider how you handle components you are installing into the
machine. Touching something that's not earthed when you are doesn't protect
that thing from static electricity that it may be carrying and which may
discharge through you to earth. That's why commercial antistatic workstations
include a mat for the work surface.
 
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Rob J
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      01-03-2006
In article <(E-Mail Removed)>,
(E-Mail Removed) says...
> On Tue, 3 Jan 2006 21:22:19 +1300, Rob J <(E-Mail Removed)> wrote:


> >Any electrician will tell you not to rely on the mains earth as being a
> >true earth and totally safe. A wiring fault in a building could result
> >in the earth voltage being jacked up well above zero.

> Not a problem in this case. A suitable earth for anti static can be
> 100MOhm and work fine. In fact most professional anti static eaths
> will incorporate such a resistance so they earth static charges but do
> not provide a high current earth if you touch a dangerous high voltage
> source. Likewise it does not matter if the earth is above true earth.
> You are trying to minimise voltage differentials. If the case is at 30
> volts then everything should be at 30 volts.


If you touch this and something that is at true earth you get zapped.

> On all pcs I have seen unless you are working on the power supply (a
> sealed system) there is no way of contacting dangerous voltages.


Only true of ATX and later designs.

Earlier PCs often had a front panel 230V power switch with mains wiring
between it and the power supply.

 
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