Design of a safety control system to improve the verification step in machinery lockout procedures: A case study

In Canada and other industrialized countries, workers who are required to carry out tasks in the danger zone of a machine when performing repairs, maintenance or unjamming activities must follow lockout procedures, unless safe alternative methods exist and can be applied. The verification step in lockout procedures is often neglected by workers for various reasons. This paper presents the design process for a new, reliable safety control system for the verification step. The design will allow automatic verification of zero electrical energy in the machine. Consequently, little effort will be required by workers to carry out this important step in the lockout procedure. Moreover, energy-saving systems create additional difficulties for lockout. This case study involves a paperboard extruder used to apply plastic on paperboard. It uses a DC bus that links the speed drives together as an energy-saving measure. This paper details for the first time the design and validation steps specified in ISO 13849-1 and 2 when using a programmable safety control system to improve the lockout verification step. The approach adopted here can be used for other machines and will help improve worker safety during interventions by ensuring correct application of the lockout procedure. The system described here was designed prior to publication of the 2015 version of ISO 13849-1. Accordingly, it was designed in compliance with the 2006 version of this standard, which directly preceded the 2015 version.