Machinery safety experts Euchner and guarding manufacturer Troax give some practical advice about how to recognise and deal with some of the most common ‘trip-ups’ in safety engineering seen throughout many factories in the UK.
Let’s face it; we have all seen failures in well-communicated safety procedures lead to an accident or injury, anything from a slight nip on a powered roller conveyor to the devastating Buncefield refinery explosion which injured hundreds of people and led to a multi-million-pound legal claim against the plant operator and engineering contractors concerned.
Experience may not always be enough to be assured your current safety engineering practices are dependably safe – aspects such as poor maintenance, manipulation and intervention by other less-qualified people could all lead to the loss of any given safety function. Although misuse may be hard to determine at the point of design, it still needs to be considered.
We all know production facilities must be safe in accordance with applicable laws, but do we ever consider the effect management style and company culture can have in a workplace, shaping employees’ attitudes and behaviour towards safety? Piece-work is less common in UK factories today, but with cost and productivity pressures affecting every organisation, many employees feel pressurised into finding those ‘shortcuts’ that could save a second or two – such as overriding a door interlock to allow the machine to be loaded more easily, with little or no consideration of the consequences of an accident.
Modern safety devices such as light curtains and scanners can help reduce the need for physical guarding in some applications, but these also need careful consideration – they do not prevent accidentally ejected parts or materials, including coolant, and they do require periodic inspection and testing to ensure they are working correctly; further guidance can be found in the free-to-download HSE Guidebook HSG180 where six or 12-month testing schedules are recommended.
Good engineering principles should always be the starting point of any design or modification. However, many engineers do not know where to find information on what is a good principle or not. The best place to start is with the many standards, both national and international, that detail the approach an engineer would take. An additional benefit of following the standards is the presumption of conformity to the relevant legislation through the use of designated or harmonised standards. Examples include the use of non contacts to ensure broke wires on emergency stop circuits don’t lead to the loss of function.
With the plethora of different manufacturers or types of components, where does an engineer start when building or modifying a machine? Indeed, the wrong specification or selection of equipment is one of the biggest causes of failure and unreliability in the factory. The consequences resulting from the incorrect selection of a safety component could result in significant injury or death, so it is very important to get this aspect right.
Safety interlock switches are designed and installed as a measure to prevent unsafe machine operation or startup, for example, when an operator requires access to clean an area. Not all interlocks are the same, e.g. the relevant standards wouldn’t suggest the use of a washing machine door switch on a power press access panel for a number of reasons.
When door locking is required, there are generally three methods utilised; energise to lock (open circuit principle), energise to unlock (closed circuit principle), bi-stable operation (energise to lock and unlock)
Often the wrong type of locking is used in machinery with residual risk, i.e. ramp-down times. For this type of application, it would normally require an energise to unlock solution meaning if power is lost or the isolator is switched off, the doors will remain locked. This will prevent access to the hazard. This requirement is well documented within EN14119-2015, although this is often overlooked.
A further consideration where door locking is used is the protection of others; could someone be locked inside a manufacturing cell if the power goes off and the door closes? In this case, devices with a clear escape release should be adopted, such as the CTA range from Euchner.
The selection of the guard interlocks needs to be carefully considered, characteristics including operating environment, water ingress, door holding force, retaining force, size, material, as well as the electrical characteristics including code or non-coded, solid state, electrotechnical and RFID. These can all make a difference to the way the device will perform and the ease with which it can be overridden or manipulated. Euchner pioneered the use of RFID technology in safety devices to help prevent manipulation.
It is a common error, seen only too frequently, whereby the safety switch is used as the end stop for a hinge or sliding door. This does not cause a problem for devices that have been designed with this in mind, e.g. the Euchner MGB2 Gatebox with steel construction. But many standalone switches should have an additional end stop or use proprietary bolts for guards such as the Euchner Aluminium bolt for their CTP range to prevent excessive wear to the safety components and act as a mechanical stop. All safety components should include their operating parameters within their manual, and this needs to be considered by the designer. For example, the holding force of an Euchner TP device is approx. 1000nM compared to the similarly sized Euchner STA with a holding force of over 8000nM.
Prevention of manipulation
The machinery directive has considered the prevention of manipulation at the design level with the operating principle considered in BS EN ISO 14119:2013, which provides specifications and detail on how to select the correct level of coding through the actuator arrangement. This helps the machine designer take away frustrations before the desire to manipulate the safe system occurs. For example, Type 2 switches must be designed in a way so that tampering using simple tools to manipulate the safety switch is not possible. In safety engineering terms, manipulation refers to the intentional defeating of a safeguard or related component by:
- Removing or unscrewing components or the actuator
- Using a second actuator or an incorrectly mounted actuator
- Bridging contacts
Therefore, actuators should be positively mounted using rivets or positively fixed by welding. An alternative to this is safety screws that can only be tightened/undone with the aid of a special tool or that can no longer be undone again after tightening. Other safeguard measures should also be considered to reduce the desire to manipulate, including strategies such as hold to run controls or reducing the speed of the machine whilst access to the dangerous zone is required.
Another big area of concern is with the physical guarding itself, which is often seen as an afterthought, and outsourced to a local fabricator to produce, who may not consider the relevant standards when designing and manufacturing the guarding. There are many characteristics of machinery guarding that are commonly unknown to the general fabricator, and these can be found in the relevant standards such as BS EN 14120:2015 or BS EN 13857:2019. Did you know, for example, that the horizontal sections of mesh should always be fitted on the inside of a guard to prevent them from being used as a foot grip to climb over? The regulations in the standard BS EN 13857:2019 specifies that the gap below perimeter guarding should be no greater than 180mm. In many cases, this gap is too big, leaving potential access for human intervention. If you can roll a football through, then it’s too big.
Light curtains and other ESP devices are a common sight in our modern factories, and very special consideration is required regarding their type and installation. Aspects such as the number of beams, the beam spacing, distance from the hazard, reaction time of the machine and muting options all need careful consideration as light curtains are often seen far too close to the dangerous parts of the machinery or to the wrong type to suit the hazard.
Adequate workplace training
Finally, anyone working with safety systems from design concept through to maintaining the safety-related aspects must be competent to do so, but what makes someone competent? Experience; formal/informal training; and knowledge.
Engineers and designers also need to keep abreast of new technology and changes in legislation; a recent example is the transition to UKCA requirements due to the BREXIT agreement. Every employer has a legal duty to undertake a suitable and sufficient risk assessment, and for those organisations employing five or more people, this needs to be well-documented. It may be beneficial to include people from different departments to undertake the assessments and include the operators and maintenance teams as well; this will ensure any future safety concepts will be adopted by all.
Training courses from Euchner can not only help with increasing knowledge and understanding but also enable you to prove competency if you were ever required to do so. PUWER assessments can be carried out internally with competent people, or if resources are tight, then Euchner has a team of consultants who can undertake the work on your behalf and support you through the process.