Well-tried Safety Principles: what are they?

Previously on this safety blog I have addressed the topic of Basic Safety Principles, so here I will explore the requirements for well-tried safety principles.

Well-tried safety principles are requirements for safety systems designed to achieve Category 1 to 4. Well-tried safety principles set requirements for the design of the safety system and the behaviour / design of the components used.

Are you familiar with well-tried safety principles? They will probably help explain certain design features of safety components.

Let's have a look at some common Well-tried safety principles:

The circuit above shows a common safety interlock system, with some of the well-tried safety principles highlighted.
  • Positive mode actuation (direct opening action): this is a requirement for electromechanical safety devices. It requires the connection between the actuator and contacts to be mechanically rigid e.g. not relying on springs, gravity, etc.
    For NC contacts look for this symbolon your electromechanical devices such as E-Stops, Tongue Interlocks, Limit Switches, etc.
  • Positive guided auxiliaries (positive mechanically linked contacts): any contacts used for monitoring, such as contactor auxiliaries, should be mechanically linked. This ensures that the auxiliary is a true representation of the contactor state.

    Look for symbols such as:
     mechanically linked, or
    mirrored contacts on the auxiliary of the contactors.

  • Over-dimensioning: all components used in safety systems should be over-dimensioned to increase their reliability. This means that all mechanical aspects of the safety systems should have an appropriate safety factor. As for electrical components, such as contactors, they should be over-dimensioned by a factor of 2 for current, switching frequency and expected life of the product.
  • Separate safety function: keeping the safety functionality separate from the standard functionality will reduce the possibility of standard modifications contaminating validated safety systems. This can be achieved by having dedicated safety controllers carrying out the safety functions. Safety PLCs can also achieve this by separating the safety programs and standard programs in the controller and programming software.

Published: 19 February 2014