Originally Posted by frieza
one other tip, make sure the motherboard is drained of charge before trying to add the card, you can do that by pressing the power button with the system unplugged (or the switch on the back of the PSU in the off position), this ensures that any residual charge is drained from the capacitors on the board.
Look at this ATX_PSU_Schematic
as a general reference (Not all designs are the same but are very similar).
Most designs for modern ATX systems will not drain the caps on board by the method you mentioned. Look at this ATX Power supply schematic, you will not drain the caps within the PSU. Switch S1 is a normally open contact (on the back of the PSU) when closed the power from the power source will be applied to the input of the PSU. Now look at the output plug pins voltage rails, that is the supply to the system. Any residual voltage after power down will decay by the PSU control circuit over time also via the system board and peripherals . The ATX voltage rails are isolated via the Power transformer. PWM IC does sense the VAC for control for a clean shutdown and control. System power on/off button is a NO(normally open) contact that controls the PSU control circuitry for on/off condition.
From experience, I have checked the Motherboard capacitors and the charge level is nil over a short time after turn off. Most of the peripheral buss are tri-state anyway. Rail capacitor(s) within the PSU do maintain a very small charge but certainly not enough to damage anything since the drain over a short period will reduce that charge by the control circuitry.
The biggest issue would be STATIC charge on the individual. One should be sure to be at the same level as the card & system, this can be done simply with a isolation static wrist strap to maintain one at the same level as the system. Usually this is nothing more than a piece of wire with a attachment strap , 10-20MΩ
ohm resistor in series with the wire to isolate the user with another piece of wire attached to the opposite end of the resistor then the end of the wire has a alligator clip attached. The wrist strap will be at the same potential when the alligator clip is connected to the chassis/ground of the system. It will then be safe to work on the system.
Older AT PSU supplies could be safely drained, new ATX PSU use a low voltage sense scheme to control power on/off via the system power button. For those that want to get into a ATX_PSU, USE caution when powered on or off when the AC source is attached.