Safety components are a special category of electrical switchgear that are required to comply with certain regulations regarding machine safety. Because of this requirement, these devices are designed and built to higher standards than ordinary switchgear to ensure that they operate reliably as and when required. The most common component is the safety switching device, which is usually a relay designed for a large number of operations with a very low failure probability and for use on emergency stop circuits. Many machines are fitted with safety cables that are designed to operate a safety cable control switch when activated. These switches may be fitted with latching mechanisms and indicator lights that identify which switch was operated, a particularly useful feature when a number of switches are installed on machinery such as long conveyor lines. Machine guards that prevent access to equipment and into dangerous areas on automated and robotic machinery are usually fitted with safety position switches that immediately stop the plant if access doors or removable safety guards are opened. Some door switches are used to lock doors closed when equipment is in operation, while others simply trip when operated. Manufacturers of safety components include Siemens, Omron, Honeywell and Schleicher