Lockout Tagout – Sequence of Deactivation and De-Energization of Electrical Circuits

Today, most of the processes and much of the operational control occurs under the guidance of a microprocessor and often a graphical user interface that is accessible to the operator.

The normal configuration in systems controlled by microprocessors is that the control circuit passes from the PLC (programmable logic controller) through the control switch to the contactor for the component. The operator can perform the primary deactivation through the computer interface. Moving the control switch to the OFF position isolates the contactor from the PLC. This is a hardware -based isolation.

Confounding this expectation is the situation where there is a second connection directly from the PLC to the contactor. In these cases, the PLC can maintain the contactor in an energized condition, contrary to the operational expectation that deactivation and de-energization of the device occurs through the control switch.

In circumstances where the circuit between the PLC and the contactor remains live following isolation through the disconnect, signals from the PLC are evident. One means to remove this influence is to remove the fuse in the circuit board through which the signal passes. Since some of these boards can process four circuits, this decision requires thorough analysis and deliberation of consequences of leaving the fuse in place and allowing the control circuit to remain active. The situation emphasizes the diligence required in creating procedures for disconnection and the need for verification.

Information provided by the graphical user interface is not always reliable or true. Deficiencies in control systems can occur in a number of ways. (check our blog: failure modes in electric control circuits)

Although, software control plays an obvious role in deactivation of equipment involved in processes, but Safety Demands Certainty. Therefore, operators must go beyond software deactivation as the sole means of preparing for work on equipment, machines, and system where contact with hazardous energy can occur. The operator must be aware of the four levels of control that now exist in computer-based control systems as compared to the two or three levels that existed in switch-based control systems operating through hard-wired relays. The operator should consider shutting down these systems in a sequence that involves all possible levels of control.

The situation posed by computer-based control highlights issues expressed in the requirements for isolation and lockout in 29 CFR 1910.147 and in subsequent interpretive documents (OSHA 1989, OSHA 2004, OSHA 2008). This regulatory standard contains hardware basis for electrical isolation, that requires severance of energy source in the circuit. There is no allowance for circuit-powered devices to participate in the isolation and Lockout.