How to Select Aspects of Machine Guarding – Strength of the Guarding System


To guard or not to guard? Or be more precise to use physical guarding or presence sensing safety devices? The answer is probably both, but it raises the question about the design, selection and implementation of physical guarding and what the designer of a machine safeguarding system needs to consider.

At face value physical guarding seems the easy option, it can help reduce the manufacturing footprint, protect the machine operator from any hazards, and prevent egress of material from the safeguarded space. It is, however, not as simple, as just buying some fencing and erecting it around the machine.

ISO 14120 (2015) Safety of machinery — Guards — General requirements for the design and construction of fixed and movable guards is the standard to turn to and it specifies the general requirements for the design, construction, and selection of guards provided to protect persons from mechanical hazards. As with the whole safeguarding process the starting point should, however, be ISO 12100 (2010) General principles for design -- Risk assessment and risk reduction. This standard guides the reader through the risk assessment process and the selection of safeguarding methods.

If the risk assessment leads the designer to the selection of guards then ISO12100 breaks them into the flowing categories, fixed, movable, adjustable, interlocking, interlocking with guard locking and interlocking with start function. These are all defined in ISO12100 and their application detailed in ISO14120.

In general, if you are asking “What are interlocked guards?” they are those that:

- Ensures a machine cannot operate until a guard is closed.

- Provides a stop command if the guard is opened while machine is operating

- Allows hazardous functions to operate when the guard is closed.

The stop command being driven by the interlocked guard makes them a popular vehicle for lock out/tagout (LOTO) as padlocks can mechanically block the guard being closed and the machinery starting. 

On often overlooked area of guarding selection is impact testing. Guarding should both ensure an operator cannot enter the safeguarding space from the outside until it is safe to do so and protect operators stood outside the guard from projectiles and emissions from within.

For peace of mind aside from general wear and tear guarding should be able to resist impacts from equipment, such as forklifts moving about the facility, without compromising on the machine’s safety.

The new Annexes (B and C) of ISO14120, providing methodology for both projectile and pendulum tests provide much needed guidance for manufacturers and users alike. Whilst ISO14120 specifically excludes interlocking devices from its scope it is difficult to imagine fitting such devices that cannot withstand the same impact testing. An interlock device as part of the interlocked guard should not be the weak link when it comes to impact.

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