A quick guide on Casting speeds and protection....
Casting Speed Caps without considering protection:
- Magery / Myst / Necromancy = Max Faster Casting Cap of 2
 - Ninja / Bush / Chiv / Spellweaving = Max Faster Casting Cap of 4  
 
What protection DOES...  "It drops the Faster Casting 
CAP by 2.
What this means...  effectively... even if you had 4/6 equipped on a Mage the max Faster casting Magery allows is 2.  Protection would then lower that 2FC cap to ZERO.. aka 0/6 Casting.   An interesting case with this is that the casting cap for chivalry is 4/6.   This means that if your using chivalry by itself (no magery over 70) and you then cast protection the CAP of 4/6 chiv casting becomes 2/6.   Which means you don't need to equip 4/6 casting if your using protection / chiv.   You'd only need to have 2/6 casting and have uninterruptible 2/6 chiv casting.
There are things that impose Faster Casting speed penalties though which you can get around by "OVER capping your FC"   
-  SC -1 Weapons
 - Stone Form -2FC I think
 
I'm sure there's a few other's which I'm missing atm.