Successive machinery safety directives and associated standards in the EU have led to one of the most comprehensive safety regimes in the world. It has also addressed the complexities of applying the highest possible standards across national borders.
However, there is still no room for complacency where moving machinery and its interaction with the people who operate it are concerned.
New safety positioning requirements
The recent introduction of BS EN ISO 13855:2010 ‘Safety of machinery – positioning of safeguards with respect to the approach speeds of parts of the human body’, has highlighted how machinery operators are putting their safety at risk, without knowing it.
We have carried out thousands of safety inspections across Europe, which illustrate how some protective measures are deployed without reference to the standard Incorrect positioning of the safety device is one of the most common reasons for compliance failure. Yet the provisions and implementation are simple and compliance can easily be achieved.
Reaching for safety
The aspect commonly overlooked with protective measures, such as light curtains or two hand control buttons, is the distance between the hazard and the preventive protection. This is designated as the minimum safe distance to ensure any sensing devices can detect an approach and bring machinery to a stop before injury occurs.
A common assumption is that if the machinery stops ‘in the blink of an eye’, typically 250 to 350 milliseconds, all is well. For a light curtain to stop a machine in that time would require a safety distance of 500mm to 650mm. Fortunately, the majority of hazards stop much faster than this allowing light curtains to be positioned much closer to the hazard. Getting the timing wrong could have serious implications to operator safety.
Distance calculations
A good example is a typical hand fed machine press. In the event that work is misplaced, a common operator response is to try and grab the item during the down stroke of the press. The desire to save on wastage, and reduce penalties on piece rate wages, could be dangerous.
The operator’s hand speed could easily reach two metres a second, if the protective light curtain is positioned too close to the press a hand could reach the hazardous zone before the press can react to the curtain’s stop command, the consequences could be devastating.
Calculating the correct safety distance to prevent such mishaps is important.
See Fig. 1; a typical set up with a vertical light curtain, where the safety distance is shown as S
For a vertical light curtain, the safety distance (Safety reach through or Srt) is calculated from the Speed of approach (K) multiplied by total response time of the machine (T) plus a constant (C) dependent on the specification of the light curtain, Srt = (K*T) + C.
With a full body access detection multiple beam light guard (typically three or four beams), the equation becomes, Srt = (K * T) + 850mm.
When using multiple beams, the minimum safety distance should be greater than 850mm to accommodate the arm length when reaching through the light beam grid.
The standard also describes the calculations to be performed when correctly positioning horizontal light curtains, safety mats and two-hand controls. The orientation and mounting height of the devices are important parameters in the calculation.
Eliminate hazardous reaching over
The completely new element of the 2010 standards tackles the problem of operatives reaching over light curtains. The height of the hazard from ground level, when reaching from a standing position, is a major deciding factor in the height of the top beam of the light curtain.
Positioning preventive devices
BS EN ISO13855 also applies to other preventative measures such as pressure sensitive mats, two-hand control systems and interlocking guards without guard locking.
In simple terms, the standard is intended to ensure accessible hazards with mechanical movement are protected by an electro sensitive protective device (ESPE). This device can detect an approach to the hazard and initiate a stop command which brings the mechanical motion to a safe condition before the operative approach can reach the hazard.
See Fig. 2; the reach over safety distance calculation is new to the standard
The reach over safety distance (Sro) is calculated by adding a specially measured constant (Cro) into the equation, Sro = (K*T) + Cro.
Cro is derived from the table below:
Fig. 3; Cro[l1] table
Essentially, as a ready guide, areas of the table marked ‘0’ show where the hazard cannot be accessed by overreaching. This means a hazard height of 800mm, with the upper edge of the detection zone at 1600mm, like the light beam, cannot be accessed by overreaching.
If the standard Srt safety distance is also calculated for the set up, the resultant safety distance adopted should be whichever is greater; Srt or Sro.
Achieving compliance with confidence
The new standard, BS EN ISO 13855:2010, is an excellent step forward, and fills an important gap in the previous standard. However, the precision required for complex safe distance calculations and the need for accuracy in stop time can be critical to achieving safety compliance.
Manufacturer derived safety distance calculation tools and stop time measurement services can remove the uncertainty from the required measurements, and establish the exact distances needed for compliance. By taking advantage of these, the engineer can gain peace of mind and reduce the risk of accident.