Activity: maintenance (installing/replacing hardware parts, like motherboard, CPU, DIMMs, graphics cards) inside an ATX case. Goal: ESD protection and user safety.
Original question: Is 'grounding a desktop PC via the power cable' safe when working inside the case? Answer: in theory, maybe. In practice: NO!
Edited question: Is 'grounding a desktop PC via low impedance (less than 1Ω) path' safe when working inside the case? (Considering the PSU is completely disconnected from live and neutral.) Answer: if a dissipative antistatic mat should be grounded via 1MΩ resistor, then a good conductive surface like the unpainted inside of an ATX case should too.
New question: Is 'grounding a desktop PC via 1MΩ resistor' unsafe when working inside the case? (Considering the PSU is completely disconnected from live and neutral.) Answer: probably not as PE (Protective Earth) is only needed when an appliance is plugged into a wall outlet.
People often advice 2 things to prevent ESD damage:
- Antistatic wrist strap, connected to the case of your desktop PC.
- The PSU should be turned off, but still plugged into the wall outlet.
I do understand a low impedance path is desirable in case of a residual-current. But when a PC is powered on, you should not be working inside the case anyway.
If the power cable is disconnected, then is a low impedance path to mains earth safe when you are building a PC or doing maintenance (installing/replacing hardware parts) inside the case? Can the case become a shock hazard in a worst case scenario?
After all, you're probably going to touch the unpainted conductive inner surface of the case at least a few times, while (un)screwing things. Should there be another resistor?
Answer
NO! You really don't want to work inside the case with the power cable plugged in period. You are relying on a switch for safety, which is simply not enough isolation even if it is open.
PC should be unplugged and grounded through a standard anti-stat cable with alligator clip end.
Wrist-strap should go direct to ground, not through PC case.
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