Keep Your Distance With Safety Light Curtains

By Seb Strutt, Safety Specialist, SICK (UK)

To ensure any machinery or automated process equipment is safe for operators, a health and safety engineer must analyse and identify the key movements which present a ‘clear and present’ hazard.

Such dangers are present in processes like metal and wood machining, pressing, robotics, plastics moulding, packaging, printing, textiles and general fabrication.  Safety light curtains, pressure sensitive mats, two-hand control systems and guards without guard interlocking must be carefully analysed for effectiveness.

Successive machinery safety legislation and associated technical standards in the EU have created one of the most comprehensive safety regimes in the world. The legislation needs to be applied rigorously.


Speed of human movement

To guard against unrecognised risk to the safety of machinery operators, the 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’ highlights the hidden dangers.

Incorrect positioning of the safety device is one of the most common reasons for compliance failure.   A machine designer must also ensure any protective measure can react quickly enough to forestall human injury.

But there is no need to panic. The simplicity of the provisions and their implementation means compliance can easily be achieved.  To begin with, however, it is essential that accurate measurement by a competent person is made of the distance from the hazard point to the safety device and its orientation relative to the hazard. 

It is critical for safety compliance for the safety engineer to achieve precision in the complex distance calculations and accuracy in estimating the safe stop time of the hazardous movement.


Calculating distance

For example, in a typical hand-fed machine press, if a work piece 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.

With the operator’s hand speed easily reaching two metres a second, a protective light curtain positioned too close to the press means a hand could reach the hazardous zone before the press can react to the curtain’s stop command; the consequences could be devastating. 

It is therefore critical to calculate the correct safety distance to prevent injury.

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.

For a full body access detection multiple beam light guard (typically three or four beams):

  • the equation becomes Srt = (K * T) + 850mm.

With 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.


Hazardous reaching over

The problem of operatives reaching over light curtains was the completely new element of the 2010 standards. 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.

The reach over safety distance calculation is new to the standard


Reach over safety distance:

  • the 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:


Cro 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 light curtain detection zone at 1600mm, cannot be accessed by overreaching. The standard tackles in more detail under reach and indirect reach into hazardous areas.

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. The results can be attached to the machine under scrutiny, as an immediate guide to its safe operation and safety status.