MRO Magazine

Reducing machinery hazards

Today, new machines must be designed with safety in mind and existing machines assessed/evaluated for compliance to new and applicable safety standards....

December 1, 2004 | By Simon Fridlyand, P.Eng

Today, new machines must be designed with safety in mind and existing machines assessed/evaluated for compliance to new and applicable safety standards.

Canadian machine, press and robot standards talk about the reliability of safety circuits. Safety components must be reliable, and their degree of reliability is established based on hazard analysis.

The concept of hazard analysis is the measurement of the possibility and severity of injury that may be caused by a machine or work cell. In other words, the hazard analysis measures how safe it is for a worker to work on a machine.

An injury caused by a hazard in a machine action, such as motion, can be rated to a level of severity. The level of severity the hazard can cause could be rated as minor or major, and reversible or irreversible.


The probability of a worker being injured by the hazard is determined by many factors, such as the characteristics of the machine (speed and mass, for example), the task that needs to be done, how frequently this task must be performed, etc.

Based on the hazard analysis, standards prescribe the degrees of redundancies that are required to be built into the safety of the machine. Minimum performance standards are also set out for safety circuit integrity, which encompasses the electrical, electronic, hydraulic, pneumatic and mechanical aspects of the system. There are four levels of circuit safety reliability applied according to the hazard analysis.

Because of the need for safety integrity, safety circuitry typically has been hard-wired with redundancy and monitoring. The use of traditional PLCs (programmable logic controllers) for this purpose was not considered an option, since they could fail at any time. The signal going through the PLC, for example, could be lost, repeated, appear in the wrong order, or even be corrupted.

However, Safety PLCs are available on the market today. A Safety PLC contains at least two microprocessors and has diversity built into its design — different microprocessors, different operating software, and different codes for each microprocessor. In order to be called a Safety PLC, the device has to be certified by a third-party nationally recognized testing laboratory.

Safety PLCs offer the highest I/O (input/output) count of all — up to several hundred digital I/O points if necessary. They are also the only solution if analogue I/O is required as part of the system.

Safety PLCs are a cost-effective solution for systems requiring large numbers of I/O points. As one might expect, Safety PLCs offer the highest degree of communication functionality, producing diagnostic status, controller status and communication status information.

If changes to program logic were anticipated due to alterations in the process or expansion of the system, the programming capability of a Safety PLC would lead to optimal cost-effectiveness and flexibility.

The following are common benefits of Safety PLCs:

* Easier wiring

* Faster machine startup

* Elimination of hardwire logic design

* Smaller enclosures

* Fewer components to fail

* Better diagnostics

* Easier troubleshooting

* Less machine downtime

* Decreased maintenance effort.

A Safety PLC is an excellent tool to use for new or retrofitted machinery as far as safety is concerned.

Simon Fridlyand, P.Eng., is president of S.A.F.E. Engineering, a Toronto-based company specializing in industrial health and safety issues and compliance. He can be reached at 416-447-9757 or For more information, visit


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