Power Loss Design Coordination

The power-loss response must be coordinated across the complete installation design so the mechanical design, electrical design, controls hardware, and PLC program all support the same intended behavior.

The completed installation must then be observed and verified to confirm that its actual power-loss response matches the intended design.

The intended power-loss response must be defined to protect people, prevent equipment damage, and avoid unacceptable installation or process conditions.

Every actuator and other machine element capable of producing motion or applying force must be evaluated individually. Its de-energized behavior, stored-energy response, and dependence on electrical, pneumatic, hydraulic, or other services must be understood so the intended power-loss response can be established for that equipment.

Much of a machine’s physical response to electrical power loss has little or nothing to do with the PLC program. Mechanical design, electrical design, actuator selection, drive configuration, stored energy, and the distribution of control and field power determine how the machine physically responds when electrical power is lost.

The intended response does not necessarily require every machine element to stop or remain in place when electrical power is lost. Some components may need to move to a defined position, retain a load, continue operating briefly, or otherwise assume a deliberately selected condition. Those behaviors must be established as part of the coordinated design rather than left to the incidental de-energized behavior of the selected hardware.

For example, a transfer table or other machine element may be raised by a pneumatic or hydraulic cylinder during normal operation. If loss of electrical power changes the valve state or removes the means holding the load in position, the raised equipment may fall and create a crushing hazard. The power-loss design must prevent that unintended movement.

Consider an inclined conveyor carrying product. The PLC Pro™ may recognize that removing motor torque during power loss could allow the loaded conveyor to roll backward, moving product into other equipment or creating a hazard to personnel. That observation may lead to a mechanical backstop, holding brake, different drive configuration, or other design change that prevents uncontrolled movement when electrical power is lost.

Power loss may also leave a process in an unacceptable condition even when no immediate personnel hazard or equipment damage results. For example, a machining center may lose power while a cutting tool is engaged with the workpiece. Simply allowing the machine to stop in that condition may ruin the workpiece, so the power-loss design may provide enough available energy to retract the cutting head or otherwise leave the process in an acceptable condition.

The PLC Pro should be included when the power-loss response is defined. Because he or she understands how the installation operates, how actions are sequenced, and what loads, materials, or products may be present when electrical power is lost, the PLC Pro may identify hardware interactions and power-loss risks that are not apparent from the mechanical or electrical design viewpoints alone.

This participation includes reviewing actuator defaults, drive and brake behavior, stored energy, utility dependencies, and the expected physical state of the installation after electrical power is lost. The objective is to ensure that the selected hardware produces the intended response and that the controls design does not rely on behavior the hardware cannot provide.

Much of the machine response to electrical power loss may be determined outside the ordinary PLC program. Safety-related control systems may remove drive enable, de-energize contactors, remove power from selected outputs, or otherwise place equipment into its defined safety response. The PLC Pro must understand these actions and coordinate the ordinary control program with the response already provided by the safety-related control system.

Power distribution must also be considered as part of the coordinated design. The design determines which portions of the installation lose power together, which may remain powered, and whether the PLC can continue executing during a particular power-loss condition. Those decisions establish what physical and programmatic responses are available to the completed installation.

A product roll-down table with pneumatic pin-stop cylinders provides a practical example. The PLC Pro, understanding how product is staged and released during operation, may recognize that spring-return cylinders could retract the pin stops when electrical power is lost and allow product to roll unexpectedly. That observation may lead to detented valves, mechanically retained stops, or another hardware change so the table assumes the intended condition during power loss.

Consider a pneumatic gripper carrying a suspended part. The PLC Pro may recognize that electrical power could be lost while the part is being transferred, causing the gripper to release and drop the load. That observation may lead to a mechanically retained gripper, load-holding valve, stored-air arrangement, or other hardware change that prevents the suspended part from being released during power loss.

Programming and commissioning often continue after the installation has been mechanically and electrically completed. Observing the completed installation during electrical power loss may reveal interactions or movements that were not apparent during the design process.

Because the PLC Pro may be the last person in a position to observe how the completed installation actually responds to electrical power loss, she or he has a responsibility to identify any power-loss behavior that creates a safety risk or other unacceptable installation or process condition.

Whenever the PLC Pro identifies a safety risk or other unacceptable installation or process condition, he or she must raise the concern with the mechanical designer, controls designer, or both for resolution before the installation is placed into service.

A vertical servo axis provides a practical example. During commissioning, the PLC Pro may observe that the axis sags or drops when electrical power is removed because drive torque disappears before the holding brake is fully applied, putting the operator at risk. The cause may involve brake timing, brake-control wiring, the selected brake, or the completed mechanical transmission. Regardless of the cause, that power-loss behavior is unacceptable and must be corrected before the installation is placed into service.

• Coordinate the intended power-loss response across the complete installation design.
• Evaluate every actuator and other machine element capable of producing motion or applying force for its power-loss behavior.
• Whenever practical, include the PLC Pro when the power-loss response is defined.
• Do not rely on the PLC program to compensate for unacceptable power-loss behavior in the hardware design.
• Observe and verify the completed installation’s actual response to electrical power loss.
• Identify and report any unacceptable installation or process condition, and ensure it is resolved before the installation is placed into service.