There are some misunderstandings here. But first,
I am curious, how did you measure 80ms since that is
less than 1/10th of 1 second and pretty darn quick! I note 100ms (1/10th of 1 second), from the
human perspective, is "perceived as instantaneous". In other words, unless one has super powers, they typically could not "see" something that happened in less than 100, let alone 80ms - at least not with their eyeballs and "normal" human brain.
joemamo is correct and residual voltages lasting for that short period of time are not dangerous -
as long as you are talking about the PSU's "
output" voltages - which would (should!) only be a maximum of +12VDC.
However, it is important to remember that
anything that plugs into the wall can KILL! So, are PSUs dangerous? Of course! Deadly? Absolutely! It is for this reason that
only qualified electronics technicians should ever open up a PSU case. So keep your fingers out of the internals of the PSU for that 80ms and you (and your electronics) will live another day!
Now, with that out of the way,
Sorry, but that is not a rule. It is an old wives tale. A myth - at least when it comes to standard, ATX compliant PCs (laptops are a different issue).
It is, however, like most myths, based on historical truths - in this case, from
before 1995.
The background:
When PCs first came about, they were based on the old, obsolete "AT" Form Factor. In 1995, the new (and current)
ATX Form Factor standard was adopted. It supplanted and made obsolete the old, antiquated AT standard. One of the major differences between the AT and the current ATX Form Factor is AT power supplies used a "wiring harness", a bunch of power cables hardwired in and running directly from the PSU to the front panel power switch. With AT, the front panel power switch actually powered on and off the PSU. And with the AT PSUs, pressing and holding down the power button did indeed, help ensure any residual voltages in the PSU's filter caps were bled off to safe levels.
That is
NOT how it works today.
The ATX Form Factor requires ATX Form Factor PSUs to supply +5Vsb standby voltage to several points on the motherboard
at all times whenever the PSU is plugged into the wall (and, if equipped, the master power switch on back is set to on). With ATX power supplies,
there is no direct connection to the front panel power switch. That is just a "remote" switch that carries that +5Vsb voltage through it when pressed, directly from the motherboard. Pressing the front panel power button simply signals the motherboard to, in turn, signal the PSU to power up completely and boot the computer.
The front panel power switch is, for safely reasons, totally isolated from the power supply. So, pressing and holding that button does nothing, but tire your finger.
But there are still deadly voltages inside today's ATX PSUs. So, today's power supplies have "bleeder" circuits that are specifically designed to rapidly discharge those large capacitors to quickly "decay" any residual voltages to safe levels once power is removed. Again, the front panel power switch is NOT in that circuit, and has no effect on those voltages.
Having said all that - as in all safety features for all electronics, they assume everything is working properly, that there is no physical damage from accidental (or intentional!) abuse, or any manufacturing defects. So even if the PSU has been sitting on the shelf for days and days, DO NOT ASSUME IT IS SAFE. Do not open the case!
I hope that clears up some misconceptions.
