Power Loss Response Method
The power-loss response must be coordinated across the machine design so mechanical, electrical, controls, and programming decisions all support the same intended behavior.
When electrical power to the PLC is restored, the control program provides the defined programmatic response as the PLC resumes execution.
When the PLC remains powered during a power-loss condition elsewhere in the installation, the control program provides the defined programmatic response to the affected condition.
Coordinate and Verify the Complete Machine Response
The intended power-loss response must be coordinated across the machine design. Mechanical, electrical, controls, and programming decisions must support the same intended behavior when electrical power is lost.
The PLC Pro contributes an understanding of how the machine operates, how its actions are sequenced, and what loads or products may be present when power is lost. This may reveal hardware interactions or unacceptable power-loss behavior that are not apparent from the mechanical or electrical design viewpoints alone.
The completed machine’s actual response must also be observed and verified. Any safety risk or other unacceptable machine or process condition must be raised with the machine designer, controls designer, or both and corrected before the machine is placed into service.
PLC Startup Response
When the PLC loses electrical power along with the rest of the machine, no programmatic response is available until power is restored and the PLC resumes execution. At startup, the control program provides the programmatic response to the power-loss condition.
The programmatic startup response establishes the proper control-program state before recovery from the power-loss condition is allowed to proceed.
Field I/O, communications, drives, and other controlled resources may not be available at the same time as the PLC. The programmatic startup response must account for their actual availability before the control program relies on their status or allows full recovery to proceed.
Partial Power Loss Response
If the PLC remains powered while electrical power is lost elsewhere in the installation, the control program can respond to the affected condition while the power-loss condition is occurring.
The control program must determine what portion of the installation is affected and respond to that condition while preserving control of equipment that remains powered and under control whenever practical.
Recovery must then establish that the affected resources are again available, determine the actual equipment state, and reconcile the control-program state with it before normal operation is allowed to resume.