How strong must guard fencing be?

Many mechanical engineers appear to think that safety standards are defining a particular impact and ejection resistance for guard fencing. But that is not the case.  

ISO 14120 in section 5.4.2 says that “guards shall resist static and dynamic forces (pressure, impacts) according to the risk assessment.” Thus, the machine designer needs to specify the requirements as part of the risk assessment.  

How can machine designer specify the requirements as part of the risk assessment?

The designer must find out which hazards can come up from parts ejected or from uncontrolled movement of the machinery itself or any objects handled by it (inside fencing). Risks from movement of machinery, objects, and even people outside the safeguarded area must also be considered (outside fencing). Once the hazards have been found, one can compute the impact energy the guard will have to withstand (weight of object multiplied with the square of its velocity, divided by two). 

ISO 14120 does not specify a minimum value for impact from inside the safeguarded space. Also, the “projectile test” described in Annex B is not suitable for guard fencing. Annex C describes pendulum testing with a “hard body” simulating impact of a machine part from inside the safeguarded area and with a “soft body” simulating a human body from the outside. (All tests specified are optional since the annexes are “informative” only). 

A minimum value has been specified for impact testing with a soft body from outside the safeguarded area. The soft body shall have a mass of 90 kg and hit the guard at a speed of 1.6 m/s (5.76 km/h), which corresponds to swift walking speed. This corresponds to 115.2 Joules. This is a relatively low value. Standard guard fencing will typically resist impact of hard bodies between 1200 and 2500 Joules depending on the make and version.  

This is sufficient for most applications. However, where impact of very large objects or machinery parts, or impact at very high velocity is expected, stronger guard fencing may be required. It is the responsibility of the machinery designer to specify stability values. 

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