Component designs to satisfy functional safety standards
and controls to ensure intrinsically safe operation. Complementing all designs for electrical safety are surge-protective components to prevent voltage spikes from damaging electrical and electronic automation components involved in mains and drive power and/ or feedback and control-signal distribution. Built-in mechanical safety with brakes Brakes that qualify as safety brakes are also called failsafe brakes. These default to a stopped state (typically to lock or hold a motion axis) even if electrical or fluid power fails or is removed. All rely on spring-loaded or other mechanical action for this failsafe operation.
electrical or fluid power — or slow or lock a still-powered machine into a safe condition.
Another option for safety that qualifies as failsafe is the integration of dedicated safety controllers.
are noncontact RFID and magnetic safety switches that monitor the position (open or closed) of work-zone doors and disallow operator access during hazardous processes. Built-in safety with electrical breakers and isolators Safety components triggered by machine status also include those to ensure electrical safety. Circuit breakers (much like fuses) protect against the detrimental and dangerous effects of overload currents on mains, power branch, and signal circuits. Some installations include isolators for galvanic separation between field devices
Built-in lockouts with latches and switches
Switches and interlocks are essential elements on the outer perimeters of machine work cells. Safety limit switches have contacts that serve to automatically verify machine element positions or motions. In contrast, safety switches with higher functions — those called interlock safety switches — use tongue or hinge interlock mechanisms as tamper- resistant machine guards having positively driven (double-verifying NO and NC) switching contacts. Trapped-key interlock switches with mechanical keys and locks keep doors into machine workspaces closed until access is safe. Increasingly common though
Relays for hardwired safety One option for failsafe control is safety relay modules. These employ electronics with short- circuit and overvoltage protection as well as complementary relays. Hardwired electromechanical relays have been used for decades; they simply wire into automated
Case in point: Spring-set friction brakes that are pneumatically released often serve as failsafe brakes in servomotor-driven automation applications. All must carry a rating that certifies compliance with ISO 13849-1 — typically from the international product-testing organization Intertek Group. Thanks to their mechanical locking, these consume no electrical power while holding … which provides maximum reliability for safety- grade performance and avoids overheating associated with other electrically based modes of stopping. Life is rated in millions of cycles before common cause (predictable) failure to some percent of all components in the series. Where IIoT functionality is useful, failsafe brakes can also
include onboard diagnostics and sensor feedback to track operational status. Brakes having the highest functional safety ratings
incorporate multiple springs that mechanically lock machine axes via friction surfaces that interact with stationary elements inside the brake housing. Safety standards also require inclusion of sensors to confirm brake status.
controls and (in conjunction with emergency stop or light
curtains) electrically disconnect machine subsections as needed. Drawbacks include the need for extensive wiring onsite and a lack of reconfigurability. More advanced safety relays sport I/O and a modular design to facilitate flexible integration with sensors, machine controls, and automation networks. Safety controllers for programmable safety Another option for safety that qualifies as failsafe is the integration of dedicated safety controllers. Such controllers are more suitable than relays for complex automation systems because they can serve larger I/O arrays as well as PLC functions. The one caveat is that these standalone safety controllers necessitate additional
Safety relays and other safety controls
Supporting the functions of safety switches, sensors, and guards are safety relays and other controls. All share a common ability to (when needed) take the machine to a safe state through the removal of
Figure 4: Simple equipment needing just a handful of safety I/O can economically employ electromechanical safety relays such as this one. (Image source: Omron Automation)
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