As noted - 4mA is a good 'zero' that is also a valid signal. I use
4-20mA loop devices and zero current implies (quite reasonably) that the sender is either broken or not connected.
Shorting the leads causes no damage whatever - the current will still be whatever it was, regardless of where the short occurs, provided it is beyond the sender, of course. For an industrial situation, this is perfect.
Another good reason for using this vs. a higher current (besides the fact that this is easily sufficient) is it gives greater range for a given output driver. I have used (and seen used) 4-20mA loops for over
1km, where the compliance requirements can exceed 100V, but as this is simply I*R(loop), a higher current would have a higher compliance (Vout range) for a given loop.
I will note that many pieces of industrial equipment can actually be switched between 4-20mA and 0-20mA, which can actually cause a great deal of grief for remote monitoring if one is not aware of it, and the switch is on the wrong position.
Cheers
PeteS