Reliance Valves Potable Water Shock Arrestor is a compact mini vessel designed to be fitted close to an appliance or fixture to eliminate the noise created by water hammer. Water hammer is caused during a pressure surge within a potable water system. The hammer is caused by a sudden stop or change of water flow within the system and is extremely common in domestic plumbing. Water hammer usually happens when the pressure within the system is too high.. Our shock arrestor can be fitted at the point of use to prevent water hammer within the system.