User-Friendly Guards

It's important to recognize that safeguards – even interlocked guards – are always willingly accepted and are not manipulated when they do not obstruct but actually support or even simplify the workflow. Faults in the safety concept which force operators to manipulate safeguards are genuine design faults, for which the machine manufacturer is liable in some circumstances. Safety-related solutions with an acceptable residual risk must be put in place, not just for fault-free normal operation, but also for setup, testing, fault removal and troubleshooting.

Simply to make manipulation attempts more difficult on a technical level, as laid out in the supplement to EN 1088 for example, only appears to solve the problem. For if there is enough pressure, a “solution” will be found. It's more important to eliminate the reason for manipulation. What's needed is not excessive functionality (even in terms of safety technology), but user friendliness. If there's any doubt as to whether the safety concept is adequate, it's recommend that you seek expert advice from the relevant employer's liability insurance association or from the safety component manufacturer.

Guards use physical barriers to stop people and hazardous situations coinciding in time and space. Their essential design requirements are stated in EN 953 and EN 1088. Safety-related and ergonomic aspects must be taken into account alongside questions regarding the choice of materials and consideration of mechanical aspects such as stability. These factors are decisive, not just in terms of the quality of the guard function but also in determining whether the safeguards, designed and constructed at considerable expense, will be used
willingly by employees or be defeated and even manipulated.

Experience shows that despite all the protestations, almost every safeguard has to be removed or opened at some point over the course of time. When safeguards are opened, it's fundamentally important that hazards are avoided where possible and that employees are protected from danger. The reason for opening, the frequency of opening and the actual risk involved in carrying out activities behind open safeguards (see the following illustrations) will determine the procedures used to attach and monitor safeguards.

Where safeguards are opened as a condition of operation or more frequently (for example: at least once per shift), this must be possible without using tools. Where there are hazardous situations, use of an interlock or guard locking device must be guaranteed. Further protective measures must be adjusted to suit the resulting risk and the drive/technological conditions, to ensure that the activities
which need to be carried out while the safeguards are open can be performed at an acceptable level of risk. This procedure conforms to the EC Machinery Directive. It allows work to be carried out while the safeguards are open as a special operating mode and gives this practice a legal basis.

Just some final words in conclusion for all designers: Designing interlocks so that absolutely no movement of the machine or subsections is possible once the safeguard has been opened actually encourages the type of conduct which is contrary to safety and, ultimately, leads to accidents. Nevertheless it is the causes you have to combat, not the people. If a machine does not operate as intended, users will feel they have no choice but to intervene. In all probability, the machine will “reciprocate” some time with an accident. Which is not actually what is was designed to do!

How can a machine control system designer consider that "reasonably foreseeable human error during operation does not lead to hazardous situations"?

The obligation to consider 'reasonably foreseeable misuse' related not only to control systems but is reflected in the general principles of the "Essential Health and Safety Requirements". The term 'reasonably foreseeable misuse' means the use of machinery in a way not intended in the instructions for use, but which may result from readily predictable human behavior.

While similar principles are enunciated in other standards including EN ISO 12100, this statement is particularly onerous. The control system designer needs to be conscious of the risk assessment and the measures for risk reduction that are dependent on the control system. The ease ofuse and ergonomics of the machine interface and its relationship to the control system are critical. Studies show that the probability of a safety system being "bypassed" is proportional to the "benefit" of bypassing the system. Therefore to avoid predictable behavior of system bypass, the design of the safety system should be considered in a manner where machine operators do not benefit or gain incentive from bypassing safety systems. Intelligent safety concepts including zoned safety systems, boundary less guarding, safe mode enabling and safe monitoring of machine parameters should be considered by the control system designer.

What are the important changes impacting machine control systems?

The obligations on machine control system designers are more explicitly and in some cases more stringently defined. Consider the following:
"Control systems must be designed and constructed in such a way as to prevent hazardous situations from arising. Above all, they must be designed and constructed in such a way that:

• they can withstand the intended operating stresses and external influences
• a fault in the hardware or software of the control system does not lead to hazardous situations
• errors in the control system logic do not lead to hazardous situations
• reasonably foreseeable human error during operation does not lead to hazardous situations."

These requirements restate with clarity that a failure of or any other cause in a machine control system should not lead to a hazardous situation. The new wording reflects the state of the art in control system technology, where there is a distinction between software and hardware.

Can I demand a copy of a risk assessment from the machine builder?

Certain social partners including European trade unions lobbied for a requirement that Risk Assessments be supplied with machinery to create more transparency and to compensate for such variations in assessment. However, it is not mandatory for machine builders to provide a risk assessment.