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.
There are three basic interlock mechanisms for mechanical safety interlock switches. Keyed (tongue-style, guard) products are mechanical interlocks in which the switching element and actuator do not form a design unit, but are combined or separated during actuation. These mechanical safety interlock switches often use a solenoid that applies a controlled voltage. Hinged interlocks are mounted over the pin of a hinged guard that opens with the positive mode of operation. Hinged interlocks with a pre-bored actuator shaft allow the existing hinge pin to be used as the mounting point. For certain types of guards, solid actuator shafts can also be used as hinge pins. Manually-operated bolt interlocks are retracted by hand and provide a time delay for machine run-down. The bolt locks the machine guard in place and operates the contacts. The first few turns of the bolt opens the contacts; however, the bolt is not fully-retracted until the knob is turned repeatedly.
Mechanical safety interlock switches vary in terms of electrical specifications and special features. Electrical specifications include maximum current, maximum AC voltage, and maximum DC voltage. Features include emergency overrides and visual indicators. Devices with integral control units can be used to monitor multiple switches. Products with manual time delays continue to protect equipment even after machine controls are turned off. Some mechanical safety interlock switches are hermetically-sealed or explosion-proof. Others are tamper-resistant or include a rotating head for multiple entry points.
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