Or like I suggested above, a frangible shackle padlock..
These are the padlocks used on standpipes, etc where the lock is designed to prevent easy tampering, but the shackles are designed to break easily. (ie: you hit it with the butt of an axe, hammer, valve wrench, etc.).
They can be identified by the groove around the shackle where the weak point is. Some are available where the weak point in hidden inside the lock body.
http://www.masterlock.com/cgi-bin/p...&sub_cat_id=D500BRK&template=app
