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.

Is TÜV Rheinland FS Engineer certification relevant?

In recent years there has been a large increase in certified safety engineers, but are these certifications necessary or even relevant?

The international safety standards such as IEC 61508 require people involved with safety systems to have the appropriate competence. Part of being competent is determining that the person has the required understanding of standards, theory and technology however the standard doesn't specify a particular qualification.

This is where certification courses can come into play. There are multiple certification courses available however the TÜV Rheinland FS Engineer course has become the most widespread in Australia and New Zealand.  The certification is globally accepted with over 6100 certified TÜV Rheinland FS Engineers around the world, around six times more than any other certification program.

The TÜV Rheinland FS Engineer course is available for engineers (or equivalent qualification) with at least three years of functional safety experience. There are five streams of the certification:

1. Safety Instrumented Systems
2. HW/SW Design acc. to IEC 61508
3. Functional Safety of Machinery
4. Automotive – Systems Design acc. to ISO 26262 and IEC 61508
5. Process Hazard and Risk Analysis (Starting in May 2013)

As functional safety becomes more prevalent in both Process and Machine Safety these certifications will be an essential starting point for ensuring personnel are competent.

 

Editor’s note: NHP has three TUV certified staff employed, including Craig who is the author of this blog. 

What are Basic Safety Principles for Machine Safety?

Basic Safety Principles are fundamental requirements for all safety systems. From Category B to 4, Basic Safety Principles are the first step in building reliable systems. Do you pay attention to Basic Safety Principles when designing your safety systems?

Let's have a look at some common Basic Safety Principles in the below diagram of a circuit. Here we can see protection of control circuit, de-energisation principle, protection against unexpected start-up, transient suppression and sequential switching.

As you can see, the circuit above shows a common safety interlock system with some of the Basic Safety Principles highlighted. These have been explained below:

• De-energisation principle – This principle dictates that the safe state should always be initiated by the contact opening, voltage going low, output opening, pressure lowering, etc. This principle ensures that a loss of energy will default the system to a safe state when possible.
• Protection of the control circuits – The control circuits should have all relevant protection to ensure that any supply faults can’t cause the system to fail in a dangerous state.
• Transient Suppression – Transient suppression should be used in parallel with all loads. This will reduce the chance of transient voltages affecting the safety system.
• Sequential Switching – Timing the outputs so that one switching device always operates without current will reduce the chance of common mode failure.
• Protection against unexpected start-up – The system should be designed to avoid unexpected start-ups.

The above are some examples of Basic Safety Principles that are relevant for electrical systems. The full list can be found in Table D1 of AS 4024.1502-2006. Basic Safety Principles can also be found for mechanical, hydraulic and pneumatic systems in the Appendix sections of this standard as well.