Overcoming plant safety challenges

ACCORDING TO the Health & Safety Executive, despite robust legislation, numerous injuries are still caused by machinery across many engineering and manufacturing sectors. Safeguarding plant and machinery is essential to meet the legal obligations already in place, which includes the PUWER regulations for managing risks and avoiding injuries.

The machinery used in these environments must be durable enough to withstand the daily rigours of the tasks-in-hand and be maintained and operated in a safe manner. With machinery safety, one-size does not fit all and when compared with other industries, processing equipment has a number of additional challenges to deal with not often witnessed elsewhere:

- Nothing is Uniform

Natural products for example often cause jams and breakages, requiring frequent access to machinery and processing lines.

- Throughput is King

Incentivised operators often push the equipment to or even beyond its original design limit. Anything that slows things down is seen as a hinderance.

- When one machine stops, we all stop

Space required for stockpiling is often limited on site so halting one link in the process chain often results in a site-wide shut-down shortly afterwards.

- A long way to walk

Material handling conveyors for example often stretching hundreds of metres, short-cuts are often considered when something does go wrong.

- Stop when you can

High loads and long over-run times means emergency-stop commands are likely to occur too late to make a difference. Here, preventing access is often the more suitable solution!

- Shake & vibration

Processing lines and their associated equipment are not well-known for quiet and smooth operation.

- Not taking prisoners today

Powerful, high-torque equipment requires high performance safety-related control systems. Escaping with only ‘broken-bones’ could be seen as a good outcome following an accident in some facilities.

Current legislation identifies the need for taking a hierarchical approach towards safeguarding the dangerous elements of equipment. Due to the inherently unsafe nature of the  some processes, it is often not possible to completely design-out the hazard and simply providing a padlock or warning sign to an opening access-door would not be deemed suitable if better methods can be used. If the guard-door can be replaced with a fixed-panel then this should be used; if access is needed frequently then an interlocked door should be provided instead, e.g. where the hazard is removed upon opening the door by bringing motion to an instant stop, or delaying the unlocking of the guard-door where longer run-down times are experienced.

Too often the safety interlocks fitted to a machine end up being subject to manipulation or are disconnected completely. This frequently occurs when the interlocking device is deemed to reduce operator efficiency or when regular access to an aging machine is required for maintenance. This highlights the increased need for a considered selection of safety devices for any particular application, plus adequate maintenance procedures and safety checks to be in place. The interlock used on a food packaging line is unlikely to be suitable for safeguarding a large milling machine for example.

Ramp type safety interlock with full metal body

Keeping the plant running is on everyone’s mind. A single failure can lead to the entire operation grinding to a halt and considering safety interlocks, emergency-stop devices and rope-pull switches are crucial in allowing the plant to operate. Comprehensive diagnostics and interlock reliability also become a very important feature.
Unlike a company making widgets, a mill operator doesn’t usually have the option of switching production to a different site when things go wrong.

At the fundamental on-machine level, bright daylight-visible LEDs give clear indication of current status as well as additional diagnostics to the machine control system, all helping to increase the ‘up-time’ of the operations. For the more complex equipment, safety and diagnostic information can often be sent via proprietary or conventional fieldbus systems including Siemens PROFINET & PROFISAFE, substantially reducing the amount of wiring required and even providing information, e.g., to allow preventative maintenance programmes to advise when a guard-door may need to be serviced.

Although not always ideal, maintenance operations often require access to the dangerous parts of equipment and preventing inadvertent start-up or reducing the hazard to an acceptable level needs to be considered – the Lock-Out-Tag-Out system is well established, but the thought of the long walk back to the control panel to place the padlock may provide motivation to ignore the safety procedures in place and ‘risk-it’. Using local interlocking devices that incorporate a local padlock ‘lock-out’ facility (a number of interlocks can accommodate up to six padlocks on a single device), the likelihood of an operator side-stepping the safety procedures in place is greatly reduced.

Emergency-stop with padlock facility

The ability to ‘lock-out’ is great when the person involved recognises the danger to themselves or others, but what do you do when an operator or maintenance staff try to deliberately override an interlocking system, often to make things easier for themselves? The most common method of overriding a guarding system is using a spare actuator (key) tricking the guarding system to believe the door is closed. This is easily overcome by using RFID ‘coded’ actuators paired to the safety-switch – only one actuator can be ‘paired’ at a time and if the operator tries to use a different actuator the machine will not run.

“As the company responsible for developing the first RFID locking guard-switch, Euchner knows a thing or two about preventing defeat or manipulation of safety systems. With greater focus being placed on this subject within the relevant safety-standards, including BS EN 14119, it is vitally important all equipment manufacturers and end-users consider their existing arrangements are adequate,” says David Dearden.

Some may feel the use of electronic safety devices unsuitable for the arduous applications within a feed mill, but these devices can handle pull-out forces up to 8KN (much more than even the strongest person could manage), often use vibration tolerant encapsulated solid-state failsafe electronics and achieve very high ingress protection ratings, including IP65, IP67 & IP69. 

Lockout methods for local safeguarding

The selection of the safety interlock is only part of the story, plant and equipment used in industrial facilities is often extremely dangerous and therefore the safety-related control system used to safeguard many of the dangerous parts needs to be designed to meet a high performance level and detect faults when they occur. It is no good to only find out your guard interlock has failed when the machine doesn’t stop on opening the guard door!

This is one of the most common issues Euchner comes across when visiting sites, with machinery operators not aware of their responsibilities and the risk assessments that have been undertaken not thorough enough into the safeguarding measures being provided by a particular safety control system. “Many site-based risk assessments highlight the safeguard measures that are fitted to a machine, such as interlocks, pull-cords, etc., but has someone actually assessed the design of the electrical control system behind the devices?” suggests Dearden.

“The standards, including BS EN 13849-1, assist in the process of determining the Performance Level (PL) required for the control system protecting the hazard. And although it is sometimes seen as too complex, there is plenty of help from companies including Euchner to assist with this much-too-often missing step. The Safety Manager needs to be sure that the safety related control systems have been designed and verified irrespective of how much functional testing is taking place during the working week”.

The functional test often used at the start of a shift only shows the system is working at that single moment in time, it does not give any guarantee the system will operate next time it is called upon, which may be when someone’s life is depending on its operation!

Not all applications are the same and it is always important for the correct components to be selected for any given application. Because of the sometimes-heavy nature of products being processed, combined with a very demanding operational environment, the machinery used throughout industry creates a lot of additional challenges for both users of and original equipment designers alike.

This article has shown that by focussing on good practice with suitable equipment, it is possible to achieve increased plant efficiency at the same time as running a safer site, quashing the long-considered opinion that machinery safety reduces throughput - and not even considering the distressing consequences of not getting plant safety right in the first instance.

David Dearden is country manager, UK&I at EUCHNER

www.euchner.co.uk