I don't see that anywhere in Total Warfare.
This is all I can find:
Arm Blown Off (Arm)
This critical hit occurs when the player rolls a 12 on the
Determining Critical Hits Table, if the location hit is an arm, and
is automatic (the player rolling the critical hits cannot choose
to roll the three critical hits instead). The hit blows the arm off,
and the weapons and equipment mounted in that arm are no
longer available to the ’Mech. The arm may be picked up and
used as a club per the rules for Club Attacks, p. 145.
Even if I am missing something there are mechs that can't sustain 20+ damage and lose their arm in such a fashion.
So feel free to provide a citation.
If you cannot I am at a point of expecting an apology.
You look at the wrong section and expecting an apology is somewhat offensive, since I haven't insulted you, just pointed out where you misunderstood something.
The rule for a PSR because of 20+ Damage is quite simple: You took 20+ damage, you make a PSR. (TW page 60) Nowhere is it stated, that that damage must come directly from weapon attacks or other kind of attacks. No damage and an ammo explosion because of heat that causes 20+ damage (assuming you survived)? PSR (except if you have CASE II, because that reduces the damage).
An arm that is taken of through a crit roll requires you to mark of enough bubbles that you have reached 20+ in a single phase? PSR.
If you doubt me, ask the question in the rules questions forums, you will get the same answer.
