Manufacturing AUTOMATION

Features Machine Safety Opinion
When can you use interlocks to protect the worker?


September 20, 2012
By Franco Tomei

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In my experience one of the questions that arises the most is “when is interlocking permissible to protect the worker?”

 Ideally, using interlocking to protect the worker would never be acceptable since, as stated in CSA Z460-05, lockout is always the preferred method of protecting a worker, as long as it is practicable. Practicable may mean, for example, that providing a guard completely over a grinder is not practicable, since no grinding would be possible.

However it is practicable to allow the wheel to be exposed sufficiently so as to permit the grinding to take place.

Another example would be that it may not be practicable economically to perform a full lockout, as would be the case on a CNC machine. In this case, requiring a worker to perform a full lockout each time a workpiece needs to be unloaded may make the operation so economically unviable that the work would be lost. In such a case, we would need to provide adequate protection to the worker so that the risk is as low as reasonably practical (ALARP). In essence, ALARP involves weighing a risk against the trouble, time and money needed to control the risk. This long-standing issue has been tackled by a technical committee that developed CSA Z460-05 (R2010) Controlling Hazardous Energy – Lockout and Other Methods.

In this standard, the distinction is made between tasks that are integral to the production process and, by implication, tasks that are not integral to the production process. In short, the standard distinguishes that lockout is not always doable in an economic sense and there is a need to use “other methods” to control the hazardous energy.

This “other method” of controlling hazardous energy is, for the purpose of this article, the use of interlocks. However, interlocks can only be used if – and only if – the task to be performed is integral to the production process.

At this juncture it is worth reviewing the regulatory requirements of section 24 and 25, which can be surmised to state that where there is an exposed moving part or nip point that endangers the safety of a worker, the worker must be prevented from gaining access to the exposed moving part and/or nip point. It is to be noted that the emphasis is on the employer to prevent access and not on the worker not to gain access. We can hope and pray all we want that nobody will access the exposed dangerous parts, but at the end of the day, the regulations only require that a person have access to be non-compliant with the regulatory requirement. That is the point of interlocking – that with the interlocking, there are no exposed moving parts, thereby removing the source of the hazard that has the potential to cause harm to the worker.

This reliability of the functioning of the interlock is not in itself absolute, as there is some risk of failure of the interlocking system. But the risk must be reduced to as low as reasonably practicable under the circumstances to protect the safety of the worker.

As referenced earlier, the interlocking should be applied for specific tasks under specific circumstances and, by implication, not all tasks under all circumstances. It is clear therefore that one must be able to assess whether or not a task can be considered as integral to the production process or is part of some other activity.

To be considered integral to the production process, the designed task will exhibit most of the following characteristics:
1. It must be of short duration.
2. It must be relatively minor in nature.
3. It must occur frequently during the shift or production day.
4. It is usually performed by operators or others functioning as operators.
5. It represents pre-determined cyclical activities.
6. It minimally interrupts the operation
of the production process.
7. It must exist even when optimum operating levels are achieved.
8. It requires task-specific personnel training.

Each of these tasks should be analyzed within the context of their application but the following analysis is useful:

1. Duration: Of course, the question then becomes how short is short? This may depend on the nature of the activity, but one must recognize that if a machine needs to be fiddled with for a disproportionate period of time, that task is not part and parcel of the production process.
2. Minor in nature: This is, once again, relative, but one could define minor as meaning that no tools, or perhaps a specific tool only (to keep parts of the body out of hazardous areas) only, are to be used.
3. Occurrence: If the task needs to be performed infrequently or sporadically, then the task is required not because of production requirements but because of defects within the machine itself. Clearly the root cause of the required task needs to be addressed and not have a worker subject him or herself to a potentially hazardous event because of the machine deficiency.
4. Operator skill level: If the task requires a person with specific skill sets not normally attributable to the operator, then the task itself is distinct from the production task. Clearly, such a task would not be integral to the production process.
5. Pre-determined cyclical activities: As an example, we can look at a spot welder, whereby the operator is required to change the welding tip every 5,000 weld cycles. Changing the tip may be considered integral to the production process.
6. Production interruption: If the task to be performed requires a lengthy amount of time, then that task cannot be said to be integral to the production process.
7. Exists all the time: It sometimes happens that an operator needs to make some adjustment on the machine and that the adjustment is the result of a defect due to a defective or worn part. These things start slowly and the operator tolerates the deficiency. Over time, it is no longer deficiency but becomes part of the “normal” operation of the machine. Clearly the task necessary to overcome this deficiency cannot be considered to be integral to the production process and the worker should not be subject to undue risk because the machine is not operating within is normal operating specifications.
8. Personnel training: As was noted earlier, these tasks are designed tasks, not tasks merely performed at the whim of the operator. The task must be designed so as to minimize worker exposure in the course of performing a specific task.

One method of conducting the assessment is to give specific quantitative (or qualitative) values to each of the characteristics. Then, you can draw conclusions to assess whether or not the task is integral to the production process. If it is, you may use an appropriate interlock. If it is not, you must redesign the task.

The preceding is all fine and well, but please remember that if an incident with consequences occurs, labour officials in your province will need to look at any violation.

If an injury has occurred, it becomes difficult to state that access to exposed moving parts or pinch points has been prevented.

Franco Tomei, B.A.Sc. P.Eng, is a professional engineer with more than 40 years of industrial experience — 12 years of that directly in the safety field. He can be reached at ftomei@yahoo.com.

This column originally appeared in the September 2012 issue of Manufacturing AUTOMATION.