Safety Distance Calculations

 

HAZARDS MUST COME TO A SAFE STATE PRIOR TO an operator reaching the hazard. For safety distance calculations, ISO 13855 defines the distance as follows:

S = K x T + C

Where:

S is the minimum safe distance from the danger zone to the closest detection point.

K is a speed constant. The actual value of this constant is dependent on movements of the operator (i.e. hand speeds, walking speeds, and stride lengths). This parameter is based on research data which states that it is reasonable to assume a 1600mm/sec (63 in./s) operator hand speed while the body is stationary. This figure is not necessarily applicable to all applications, the specific requirements must be analysed on an individual basis. As a gerneal guide, approach speed will vary from 1600 mm/s (63 in./s) to 2500 mm/sec (100 in./s). This speed constant must be defined by an application specific risk assessment.

T is the overall stopping time of the system.  This is stated in seconds and should include all time from when the stop signal is initiated, until the hazard stops moving or ceases to be a hazard. 'T' should therefore include the response time of the interlocking/detective device and its interface, the response time of the safety rated control system and the time taken fo rthe machinery to shut down. The worst case scenario should be allowed for in all cases.

C represents the depth penetration factor. Depthe penetration is the maximum distance that can be travelled towards the hazard before detection by the safeguarding device. It will therefore vary depending on the device type and application. The relevant standard should be checked to determine the correct penetration factor.

For an approach to a light curtain or area scanner, with an object sensitivity less than 40mm (1.57 in.), the ISO and EN standards use:

C = 8 x (Object Sensitivity - 14mm), but not less than 0