ESD Inquiry

I have several questions about the 'safe way' to open up and fiddle
around with a computer.
My computer user's manual lists these steps:
1. Turn computer off.
2. Wear grounding wrist strap and attach to metal chassis.
3. Touch bare metal surface on back of computer.
4. Unplug the computer's power cord.

I understand that any static charge on me would be pulled to ground in
step 2 but do I then just touch the bare metal momentarily or do I
have to hold on to it throughout (step 3), and if I do would that not
defeat the purpose of step 2?

Also when I unplug the computer's power cord, if say I am shifting
over a floor or whatever, and build up some static charge, it will
discharge into the metal chassis, but then what? The metal chassis
will then be charged, what "bank" will hold the chrage before it is
pulled to ground when I plug the cord back in? And what happens if I
do not plug it back in for some time after taking off the wrist strap?
Will the chassis stay charged, or bleed off how?

Also I heard that I can leave the power cord plugged in but turn off
the machine using a switch in the back, isolating any circuits within,
some PSU do not have this switch though. Anyway please answer the long
questions above.

tia

JJD wrote:

> I have several questions about the 'safe way' to open up and fiddle
> around with a computer.
> My computer user's manual lists these steps:
> 1. Turn computer off.
> 2. Wear grounding wrist strap and attach to metal chassis.
> 3. Touch bare metal surface on back of computer.
> 4. Unplug the computer's power cord.
>
> I understand that any static charge on me would be pulled to ground in
> step 2 but do I then just touch the bare metal momentarily or do I
> have to hold on to it throughout (step 3), and if I do would that not
> defeat the purpose of step 2?

Step 3 should not be neccessary in addition to step 2, but it can't
hurt. A momentary touch is all that's required.

> Also when I unplug the computer's power cord, if say I am shifting
> over a floor or whatever, and build up some static charge, it will
> discharge into the metal chassis, but then what? The metal chassis
> will then be charged, what "bank" will hold the chrage before it is
> pulled to ground when I plug the cord back in? And what happens if I
> do not plug it back in for some time after taking off the wrist strap?
> Will the chassis stay charged, or bleed off how?

Exactly, it's a potential problem - so I disagree with step 4.

> Also I heard that I can leave the power cord plugged in but turn off
> the machine using a switch in the back, isolating any circuits within,
> some PSU do not have this switch though. Anyway please answer the long
> questions above.

An ATX PSU sends standby power to the motherboard when it's switched off
at the front panel but still connected via the power cord (assuming the
back panel switch, if present, is on). You should not plug or unplug
components in the computer while standby power is applied as this can
cause damage.

My recommendation in place of step 4 is:

- If PSU has a back panel switch, turn it off and leave power cord
connected.
- If PSU does not have a back panel switch, replace power cord with one
that has had the active and neutral pins cut off, leaving just the
ground pin. An alternative is to attach your wrist strap to a ground
other than the computer chassis, disconnect the power cord, and repeat
step 3 every few minutes while working.

Using this method means standby power is off but chassis remains grounded.

Sunny