Safeguarding Against Machine Hazards

Safety light screens offer numerous features and should be selected according to application requirements. For example, light screens vary greatly in detection capability (also known as resolution or minimum object sensitivity), which determines the ability of an optical system to reliably detect an object. Safety light screens are grouped based on this sensing potential. High-resolution light screens can detect a finger or hand, while medium-resolution light screens can detect an arm or ankle, and low-resolution light screens can be used to detect a torso.

When selecting a safety light screen, also consider the size and mounting options; the cascading capabilities to safeguard multiple areas or sides with a single system; the sensing range; reduced resolution and fixed blanking to ignore objects such as tooling; selectable automatic or manual reset; and the capability for external device monitoring.

Two-Hand Controls

Another common safeguarding device is a two-hand control, comprised of two actuating buttons (or hand controls) and specific logic used to start and control a machine cycle when an individual concurrently actuates both buttons within 0.5 seconds. In doing so, two-hand controls provide a safety function for the machine operator by physically occupying both of the operator's hands and keeping them out of the hazardous area when the machine is activated. Since two-hand controls only protect the individual who is operating the buttons, additional two-hand control devices or other safeguarding is required for other individuals.

A two-hand control safeguarding system is comprised of two basic elements. The hand controls typically are mechanical palm buttons, touch-activated photoelectric devices or capacitive touch switches. Ergonomically designed touch buttons requiring little or no physical pressure to operate are gaining popularity to eliminate hand, wrist and arm stresses associated with old-style mechanical pushbuttons.

The control logic should be provided by a safety module, safety controller or safety PLC. Many accidents have been reported that resulted from a standard PLC monitoring the palm buttons.

Emergency Stops

It should be noted that an emergency stop device is not considered a safeguarding device, as described above but fulfills a safety function as a safety device under the heading of complementary protective equipment. This is an important distinction because an emergency stop device, such as a button or rope/cable pull, should not be used in place or as an alternative for proper safeguarding of a hazard. As described by ANSI B11.19, “a safeguarding device detects or prevents inadvertent access to a hazard, typically without overt action by the individual or others. Since an individual must manually actuate an emergency stop device to issue the stop command, usually in reaction to an event or hazardous situation, it neither detects nor prevents exposure to a hazard.”

Among the requirements, an emergency stop device must have a red-colored actuator and, when possible, a yellow background, such as in the case of a palm button. The device also must have a positive-opening design (similar to safety interlocking switches). The emergency stop function must be initiated by single human action, override all other functions and operations and remove power to the machine actuators as quickly as possible without creating additional hazards. Resetting the emergency stop device (or function) must not restart or otherwise initiate a hazardous motion or situation.