SGII Question Armour and Cover

Hi,

Probably a question that has come again and again, but it worried me
that I 
coudln't decide what the answer was.

When a unit is in cover it recieves an armour bonus, 1 shift for soft, 2
for 
hard cover. Is this an open shift? The book does not (IIRC) say so, but
if 
it isn't, then does this mean that Heavy power (for example) are 
invulnerable from ranged fire while in soft cover?

TIA

Richard