Mechanical safety interlock switches couple a moveable guard door with the power source of the hazard. When the guard door is opened, the power is isolated, ensuring that the machine does not pose a hazard while an operator requires access. Mechanical safety interlock switches use several types of contacts. Positive-break contacts are normally closed (NC) electrical contacts which, upon actuation, are forced open by a non-resilient, mechanical drive mechanism. In the event of a mechanical failure such as the breakage of a spring or weld, the contact point remains in an activated position. True spring-actuated safety interlock switches are not considered to be positive-break devices. Mechanical safety interlock switches with normally open (NO) contact pairs and pairs of changeover contacts are also available. Typically, changeover contacts are used in single-pole double-throw (SPDT) devices.