Emergency stop switches are devices that users manipulate to initiate the complete shutdown of a machine, system, or process. Unlike regular stop switches, emergency stop switches are not solely dependent on springs. Instead, they generate electrical signals that latch the actuator. Several types of emergency stop switches are available. Pushbuttons are relatively large devices that close a switch when pressed. Trip wires or rope pulls can be actuated at any point along a line that continues to operate even if the cable breaks or goes slack. Combinations of pushbutton and trip wire switches are also available. Typically, these devices incorporate a trip wire for actuation along any point of the cable and a pushbutton for emergency stopping at the end of the installation. By contrast, emergency stop switches that use a telescopic antenna are actuated by the movement of the antenna in any direction.
Emergency stop switches vary in terms of minimum operating or trip force, the minimum force required to initiate switching. They also vary in terms of contacts. Normally open contacts consist of a relay contact pair that is open when the coil is not energized. Conversely, normally closed contacts consist of a relay contact pair that is closed when the coil is not energized. Changeover contacts are single-pole, double-throw (SPDT) devices that are used to turn on equipment in either of two positions. For example, SPDT switches can be used to turn on a machine in one position and an audible alarm in another. Positive break contacts keep the contact points open in the activated position even in the event of a mechanical failure such as a broken spring or weld.
There are several important specifications for emergency stop switches. Electrical switches carry a maximum current rating, a maximum AC voltage rating, and a maximum DC voltage rating. Pneumatic switches vary in terms of pressure range. For both types of switches, there are four basic types of reset mechanisms. Automatic switches do not require reset action and should never be used to restart a machine. Pull-to-reset and twist-to-reset switches require users to pull or twist a button or cable in order to reset the switch and resume operation. With key switches, users must insert and turn a key in order to reset the device. Special pushbutton reset switches are also available.
Emergency stop switches provide many features. Some devices are labeled to indicate their purpose. Others provide illumination for visibility in low light situations. Visual indicators such as light emitting diodes (LEDs) are commonly available. Hermetically sealed devices provide a vacuum or leak-tight seal such as a brazed glass-to-metal seal. Lockable and tamper-resistant devices can be used only by authorized personnel. Explosion-proof emergency stop switches provide a rugged housing that can withstand sparking and flames.