Fire hydrant lock

A fire hydrant security arrangement includes a center post which is retained on the operating nut of the fire hydrant by a central bolt which is threaded into the operating nut. The center post has a pentagonal recess therein which receives an insert, the insert having an inner recess which corresponds in shape to the operating nut. A domed housing is slid over the operating nut and can rotate with respect thereto. A circular groove in the center post receives a projection from a security plug which has been threaded through a threaded bore in the domed housing. The security plug has a socket therein which includes a recess which is off-set slightly from the axis of the plug and is only operable by a security tool having an off-set projection that is receivable in the recess. Without possession of the security tool, it is very difficult to achieve sufficient purchase on the plug to rotate the plug and withdraw the plug sufficiently away from the circular groove in the center post to remove the domed housing. A person with the security tool can readily back the plug out of the socket so that the domed housing may be removed and the water valve of the fire hydrant opened by applying a standard fire hydrant wrench to a pentagonal surface on the center post.