Engineering Articles

This article presents ten things that machine specifiers and designers should know about machine guarding standards.

Ten things to know about machine guarding standards Guarding needs to be well designed if a machine is to operate safely and efficiently.

Furthermore, guards should not be expensively over-engineered, nor should they detract from the machine’s aesthetics.

In all cases, however, they should be designed in accordance with the applicable national and international standards.

These ten tips about machine guarding standards will also be useful for production managers, health and safety managers, health and safety representatives and others with an interest in the subject.

1 - Compliance is not mandatory.

This first point may come as a surprise, but there is no legal requirement (at least in the UK and Europe) to comply with machine guarding standards.

However, compliance with the relevant standards, especially those harmonised to the Machinery Directive, is generally the shortest route to demonstrating that the Essential Health and Safety Requirements have been met, so compliance is strongly recommended.

2 - PD5304, an informative ex-standard.

PD5304:2005: “Guidance on safe use of machinery”, is available from BSI and has the status of a Published Document.

Although not a standard, it is essentially the same as BS5304:1988, the old British Standard for machinery safety, and contains a wealth of useful guidance and practical examples of guard design.

Many of today’s machine safety standards incorporate principles contained in PD5304, but lack the illustrative examples found in PD5304.

3 - EN953, the main standard.

EN953 (and the British Standard equivalent BS EN953:1998): “Safety of machinery - Guards - General requirements for the design and construction of fixed and movable guards”, covers all types of machinery, from simple drive couplings to very complex installations involving robots, conveyors and processing machinery.

The standard lists those aspects of machinery, people and the design and construction of guards that need to be considered.

However, using EN953 does not mean that other standards can be ignored; for example, see point 4 below.

4 - Protecting upper limbs.

EN294 (and the British Standard equivalent BS EN294:1992): “Safety of machinery - Safety distances to prevent danger zones being reached by the upper limbs”, contains tables and data to enable guards to be designed with an acceptable combination of height, horizontal distance from the hazard, and aperture sise (for guards with mesh infill, or other openings in guards).

Unfortunately the standard is not as user-friendly as might be hoped, and the results can be ambiguous.

Procter Machine Guarding has therefore developed a spreadsheet-based safety distance calculator with simple menu selections that simplify the task and avoid ambiguities.

The safety distance calculator is available free of charge on request or can be downloaded from the company’s website.

5 - Protecting lower limbs.

EN811 (and the British Standard equivalent BS EN811:1997): “Safety of machinery - Safety distances to prevent danger zones being reached by the lower limbs”, is similar in many respects to EN294 (see point 4 above).

However, in reality it is seldom used, as guards usually extend to floor level or to a level below which access by the lower limbs is not possible.

6 - An amendment to EN1088.

In 2007 an amendment was published for EN 1088 (and the British Standard equivalent BS EN1088:1995): “Safety of machinery - Interlocking devices associated with guards, principles for design and selection”.

The aim of the amendment is to promote better safety system design so as to reduce the reliance on the correct functioning of safety interlocking devices.

There is also a recommendation to use coded interlocks, rather than simple devices that could be overridden by, for example, a maintenance worker with a “spare” actuator, or guard designs that shield the interlocks when the guards are opened, thereby preventing a second actuator from being inserted in the interlock.

7 - New risk assessment standard.

For over a decade machine builders have used EN1050 (and the British Standard equivalent BS EN1050:1997): “Safety of machinery - Principles for risk assessment”, as the starting point for designing machine guards.

It is also likely that many are still using this standard, even though it was withdrawn and superseded by EN ISO14121-1 (and BS EN ISO14121-1:2007) in 2007.

Although the new standard: “Safety of machinery - Risk assessment - Principles”, is broadly similar to EN1050, it contains some notable differences.

Procter Machine Guarding has therefore updated its risk assessment calculator, which is based on an Excel spreadsheet, to take account of the changes.

The risk assessment calculator is available free of charge on request or can be downloaded from the company’s website.

8 - Machine-specific standards.

So far we have discussed general machine safety standards, but it should be remembered that there are dozens of standards, known as Type C standards, relating to specific classes of machinery.

For example, BS5667-18:1979 is “Specification for continuous mechanical handling equipment - Safety requirements, conveyors and elevators with chain elements - Examples for guarding of nip points”.

Designers should check whether any Type C standards are available for the machine they are designing, as these standards can be extremely useful and avoid the need to work “from first principles”.

9 - Other official guidance.

Standards are very helpful when designing machine guarding, but there are also other sources of official guidance.

For example, here in the UK the HSE (Health and Safety Executive) has published a document known as HSG43: “Industrial robot safety”, which contains detailed information about how to safeguard robots.

Although this publication has not been updated for several years, meaning that some of the standards references are now out of date, it is still widely considered to be “essential reading” when safeguarding robots.

10 - A free guide to machine guarding standards.

Points 1-9 above are based on material contained in “On Your Guard - A designer’s guide to machinery guarding standards”.

This is recommended reading for anyone involved in specifying or designing machine guards, as well as production managers, health and safety managers, health and safety representatives and others that require an understanding of this important subject.

“On Your Guard” is available free of charge on request from Procter Machine Guarding or can be downloaded from the company’s website.

Syscom UK decided to use the Vital Solution from Jokab Safety as it met the Category 4 requirements and provided the ability to connect all the devices to one safety monitoring unit.

In 2003 GPS acquired two manufacturing cells that needed to be refurbished and brought up to the current safety requirements.

Syscom UK was brought in to evaluate the machine guarding and where it was needed, to increase the safety integrity.

When designing and building machinery, one has to consider the risk factor when operators are working with the machine.

This is initially started by creating a risk assessment.

The risk assessment allows the manufacturer of the machine to consider all the possible risks and dangerous areas in the machine.

Once these have been identified, the machine builder then can develop the safety system to meet the requirements put down in the standards.

There are four categories which are used to identify the reliability of the safety system.

These are called simply 1, 2, 3 and 4, depending of the type of injury, exposure to the hazard and the possibility of avoiding the hazard.

One can use these parameters to work out which category level the machine needs to comply to.

Category 4 is seen to be at the highest level one can adapt to for machine safety.

Recently there have been changes to the standards the way components and systems are created.

The new SIL levels, means that GPS has to consider the level of reliability, predictability and traceability of all components used within the safety system which is related to the failure time consequence.

At GPS UK the cells had a variety of possible dangers from Category 2 to 4.

It was decided that the manufacturing cells needed to be brought up to Category 4, SIL3 as it was simpler to keep the safety levels the same to prevent any unnecessary complex interlocking within the safety system.

The cells consisted of a gantry robot and an injection moulding machine together with other automated processes.

The focus was to prevent operators accessing the manufacturing cell during production and to allow a full integration of all the safety devices from different areas of the cell.

Syscom UK decided to use the Vital Solution from Jokab Safety as it met the Category 4 requirements and provided the ability to connect all the devices to one safety monitoring unit.

The safety system consisted of emergency stop buttons and Jokab’s JSNY9 door interlock switches.

These devices have Tina 3A interfaces, which monitor the two normally closed contacts and change the signal to the Vital unit if the safety devices are operated.

A Spot 10 safety light beam was also used in the safety circuit to monitor the gantry robot position.

The low-voltage DC LED version has a choice of nine user-selectable modes giving continuous, flashing, rotating, double strike and alternate side flash modes.

E2S has developed the Spectra beacon family to extend its industrial range of audible and visual signalling devices.

Available in three sizes, six lens colours and a choice of static, flashing filament, Xenon strobe, rotating or LED versions, Spectra offers a solution for all situations including safety warning, status indication and fire alarm.

The LED option gives longer operational life and lower life costs than a traditional filament lamp, which is particularly important for continuous use or where access or maintenance is difficult.

The low-voltage DC LED version has a choice of nine user-selectable modes giving continuous, flashing, rotating, double-strike and alternate side flash modes.

Up to three stages of alarm can be selected through the appropriate wiring connections.

The B300 has a dual-frequency option where a flash rate of 1 or 2Hz can be selected remotely through the wiring configuration to provide two stages of warning.

The Xenon strobe B300 and B400 models are optionally available in synchronised versions, so that all devices on the same power supply circuit flash in unison, a particular benefit in fire alarm applications.

Primarily intended for surface mounting, pole and wall mounting options are available to make installation neat and easy.

All units have a choice of cable entries for ease of wiring.

Manufactured from UV stabilised polycarbonate, Spectra beacons will not fade when exposed to direct sunlight and are protected to IP65, making them suitable for outdoor applications.

SensaGuard’s large sensing range and tolerance to misalignment combine to form a cost-effective solution that covers a wide range of industrial safety applications.

Guardmaster’s new SensaGuard range of noncontact interlock switches are now available from stock at Halcyon Drives.

When it comes to machine safety, protection of personnel and equipment is the main concern, but it must be balanced with flexibility and productivity.

Halcyon Drives reckons that safety system designers can optimise all three factors with the new Allen-Bradley SensaGuard family of noncontact switches.

Featuring the latest generation of RFID technology for coding and inductive technology for sensing, SensaGuard’s large sensing range and tolerance to misalignment combine to form a cost-effective solution that is ideally suited for a wide range of industrial safety applications.

The SensaGuard product line is a Category 4/SIL 3 rated switch to EN954-1, with TUV functional safety approved to IEC61508.

The switches can be connect to a standard safety relay, such as the MSR126, MSR127, MSR200/300 Family, SmartGuard and safety I/O blocks.

Multiple actuator sizes are available for large sensing distances, and all units come with IP69K environmental rating together with short-circuit and overvoltage protection.

A LED is located on the switch for door status and troubleshooting

In the event that a nut should lose clamping force and start to rotate, the mechanism triggers pins which engage with slots in the stud to prevent further rotation.

VisiLok is a dual-purpose road safety device which arrests both the movement of wheel nuts on heavy goods and commercial vehicles and flags the attention of the operator.

It virtually eliminates the possibility of wheel fixing defects and detachment which has been a cause for concern across the industry for many years.

The device can be both included in the initial customer order specification or applied as a retrofit.

The design concept starts from the principle that it is difficult, if not impossible, to eliminate loss of clamping force and the slackening of wheel nuts on commercial vehicles.

In the event that a nut should lose clamping force and start to rotate, the mechanism triggers pins which engage with slots in the stud to prevent further rotation, while at the same time raising warning flags above the surface of the nut.

Functioning as a normal nut and stud in all other respects, VisiLok is the culmination of four years of research and development, stringent laboratory testing and operating and durability trials.

Managing Director, Barry McGowan said: “Each year the UK alone has over 11,000 wheel fixing defects and around seven fatalities which can be attributed directly to accidents caused by wheel fixing defects and losses”.

“We were also aware of a number of devices on the market ranging from plastic pointers which indicate if a nut on a wheel has moved to double locks which prevent the nut from loosening”.

“However, none of these provided a true solution to the problem in our opinion”.

“Subsequently we set out to develop a comprehensive and safety-critical device which would not only help to prevent wheels from coming off, but could also help to save lives”.

“With the imminent introduction of the Corporate Manslaughter and Corporate Homicide Act in April, this aspect will become even more critical”.

VisiLok teamed up with automotive engineering consultancy, Lotus Engineering and cold-forming precision components manufacturer, Floform.

Ron Aspinall, VisiLok Operations Director said: “After going through a number of development stages which included proving the basic integrity of the design concept and function, we needed to ensure that the VisiLok nut and stud was robust in all operating conditions”.

“By working with Lotus Engineering and Floform we could demonstrate, amongst others, that the device was fully robust and would work in all tested conditions; we could select the component material combinations carefully to prevent corrosion from jamming the mechanism while keeping the component count to a minimum and ultimately produce a cost-effective, universally applicable solution which would put an end to the wheel loss problem once and for all”.

Using an existing VisiLok design, Lotus modified the locking device to enable manufacturing feasibility.

Lotus also carried out rigorous testing procedures to validate the performance of the road safety device under the most arduous operating conditions.

Philip Lawrence, Floform Managing Director said: “The product solution had to be applicable anywhere that a fastening is subject to rapid temperature change or vibration, provide an indication of the relaxation problem and arrest the movement as soon as it starts to occur”.

The process for fitting the VisiLok nut on the stud is the same as for a standard nut.

The nut will accept an appropriately sized six-point standard socket.

When applying the socket to the nut, the flags are depressed and the pins in the nut are raised clear of the stud.

The nut will then rotate freely so that it can be treated as a standard nut.

Using the existing fuel cap securing system the MPD has no moving parts and fits simply and quickly into the fuel filler neck, blocking any attempt to insert the thinner unleaded fuel nozzle.

DDN has released a product that prevents the accidental misfueling of a diesel vehicle with petrol.

There are 150,000 recorded incidences of misfueling every year in the UK.

The misfueling of a diesel vehicle with petrol creates a number of serious issues, including loss of time, the cost of the repair and environmental issues.

The Misfuel Prevention Device (MPD) was developed by two DDN engineers.

The MPD is currently undergoing Thatcham accreditation.

It is currently being used by the UK emergency services, local authorities and many fleet operators.

Using the existing fuel cap securing system the MPD has no moving parts and fits simply and quickly into the fuel filler neck, preventing the insertion of the thinner unleaded fuel nozzle.

Engineering firm Martin Brunner, working alongside Helog Heliswiss, has developed an application for monitoring loads transported by helicopters using the Pluto Safety PLC from Jakob Safety.

A Pluto controller installed in a helicopter monitors the lock of a load hook, while using resistors to check for broken cables, or that the hook has not completely closed within five seconds.

Should either situation arise, a danger signal will sound in the cockpit area.

Thanks to the referrals from some business partners, Martin Brunner became aware of the three-position enabling devices JSHD2C and JSHD4 manufactured by Jokab Safety.

The company also uses these enabling devices on applications for loads located under different types of helicopters, especially in the Puma helicopters from the Swiss Federal Border Guard using double load-hooks 3000.

Pluto is an All-Masters safety PLC concept, which simplifies the design of safety systems and reaches high safety levels including SIL3 according to IEC-62061 and PL-e according to EN-ISO-13849.

All Pluto PLCs are master devices and can see inputs, outputs and the local/global memories of the other Plutos, which can be linked with each other via the safety CANbus.

The safety system is programmed with the Pluto Manager, a programming tool based on Windows with TUV-approved safety function blocks.

Brunner has tested the complete solution in co-operation with the Swiss Federal Agency for Civil Aviation and had it certified as a complete system.

A helicopter from Helog is reported to be using the system with success.

Further projects are in the planning stage.

Jokab Safety is using the Vital coded pulse system, which can operate all components from all manufacturers in the safety-related market.

This system allows up to 30 field-safety devices to be connected in series and connected to a single control unit without compromising the safety level.

Doing this with conventional systems would only allow the user to meet category 3 / Sil level 2.

Jokab Safety’s method meets category 4 / Sil level 3.

The advantage of this is that the engineer does not have to change his field devices should they already be installed or part of a specification from the client.

The major benefits include reduced cost and eliminating the possibility of having to locate a single channel fault on the safety system.

The Safety Series range of products from Master Lock is suitable for industrial safety lockout applications.

The Safety Series helps industries maintain safe working practices during maintenance activities and complies with EU Health and Safety legislation including PUWER 1998 and EAWR 1989.

The Safety Series is a range of high-integrity safety locking-off products which, when used in conjunction with safety padlocks, create a robust lockout solution.

An essential part of all maintenance activity is the need to ensure that equipment cannot re-start or re-energise unexpectedly.

Regulation 19 (3) of the Provision and Use of Work Equipment Regulations (1998) places responsibility on employers to ensure that reconnection of any energy source does not expose any person using the equipment to any risk to his or her health or safety.

The Safety Series is suitable for all lockout applications, including electrical isolation and process valves.

Included in the Safety Series are a range of valve lockouts, electrical switch locks and key-management systems.

Euchner has released the TP and STP safety switches, which ensure machine compliance with the latest European Machinery Directive regarding ‘risk of being trapped in a machine (code 1.5.14)’.

The new directive (2006/42/EC) will become effective on 29 December 2009 and states: ‘Machinery must be designed, constructed or fitted with a means of preventing an exposed person from being enclosed within it or, if that is impossible, with a means of summoning help’.

The standard TP and STP safety switches ensure guard devices cannot be opened with machines in operation and remain closed until all potentially hazardous moving parts have stopped.

These switches are optionally available with a manual emergency release mechanism located at the rear of the switch housing.

To prevent the people being trapped in an individual machine or machining-cell enclosure, however, these switches can be adapted to incorporate an integrated bistable solenoid, operated from a separate supply voltage, which provides actuation and condition-monitoring of the guard locking.

The TP-Bi and STP-Bi types are versions of the Euchner safety switch range that incorporate the bistable solenoid feature.

The STA and twin-headed switches are also capable of the same adaptation, without alteration to their original housing dimensions.

Other provisions must be made, however, where switches are fitted in inaccessible positions, or where guard devices could trap personnel outside the reaching distance of appropriate safety devices.

In these circumstances, switches can be provided that incorporate a flexible Bowden cable for escape or emergency release of the guard locking mechanism.

The cable, combining inner wire and sleeve, can be supplied in lengths up to five metres and can be routed to cover individual installation requirements.

The cable is supplied with a convenient ring-pull, but this can be replaced with a more substantial pull handle if the cable needs to be routed through a machine bulkhead, rigid partition or guarding.

A pre-assembled Bowden cable emergency release mechanism incorporating replacement switch-cover is also available as a retro-fit item for Euchner TP and STP switches where they have already been built into existing safety circuitry.

This will ensure that even currently operating machine installations can be upgraded to comply with the latest machinery directive requirements.