I've noticed that some abilities don't seem to share cooldowns with each other. The various grenade abilities, for instance, and the Vanguard's Nova biotic. The Sentinel's Tech Armor also doesn't cause a cooldown to turn on (though it does to detonate).
Which abilities are off the shared cooldown? What about ammo powers?
Best Answer
There's technically three types of powers in Mass Effect: Active Powers, Grenades and Passive Abilities.
Active powers a single Global Cooldown. A skill either activates the Global Cooldown or it doesn't. Skills that don't activate a cooldown are only limited by how quickly you can perform the action (the time it takes to switch ammo/throw a grenade).
Most powers listed as a base power here qualify as Active. These are your standard attacks and always cause a cooldown.
Passive powers are always active and don't have cooldowns, simple enough. Ammo powers are technically passive, and you can always switch ammo regardless of cooldown. Tech Armor is basically a passive power unless you detonate it.
Grenades don't use a cooldown either, and if something seems like an "active" power and it doesn't activate the cooldown, that's because it's technically a grenade. Proximity mines, Nova and Sentry Turrets are all technically grenades.
Nova has a "cooldown" that's actually based on your Biotic Barrier (the purple "shield" Vanguards have). You can't use Nova if your Biotic Barrier is empty, but it otherwise ignores cooldowns like a grenade.