ANSI B11.0 and ISO 13849 define the control systems to include, the mechanical, electromechanical, electronic, hydraulic, and pneumatic components.  Most safety considerations regarding machinery are linked to the movements of equipment and production materials. Think about it like this; how many safety concerns would you have about a piece of equipment that had no moving parts? You would have very few concerns except for non-contact hazards such as radiation. However, it is the inherent characteristic of machinery that machine parts move and their movements are a source of a great portion of the dangers associated with machinery. Control of these potentially dangerous movements is ultimately bestowed upon the control valves for pneumatic and hydraulic systems, and control relays for electrical devices. Because this final control lies with the valves, many safety standards exist that concern valves and valves will continue to be at the forefront of safety considerations.