The guarding of machines, including laser processing machines, should comply with a number of EU Directives before the machine can be put into service within the EU or countries within the European Free Trade Area (EFTA). The specialist standard for laser guards is EN60825-4 which is the European adoption of IEC 60825-4. However, this standard only deals with the laser radiation hazard. Depending on the machine, there are likely to be other standards that the guards have have to satisfy to provide the protection needed from other machine hazards. This paper looks at the standards and European Directives that are relevant to guarding of industrial laser processing machines. The requirements are viewed from a European perspective; however the international standard references are given so that these requirements can be easily applied internationally.

Standard EN60825-4 (IEC60825-4)
At low levels of radiation or radiant exposure, the selection of material for giving protection against exposure to excessive laser radiation is determined primarily by a need to provide a high enough optical attenuation. At higher levels, the guard may be damaged by the exposure and thus attention is necessary to ensure that the guard material will tolerate the foreseeable exposure without loss of performance. Standard EN 60825-4 (IEC 60825- 4) deals only with protection against laser radiation.

The standard requires that the guard must satisfy those relevant parts of standard ISO 12100-2 with respect to general equirements.
It can be applied for all laser protective housing applications but specifically relates only to requirements for laser guards that enclose the process zone of a laser processing machine. The main requirement is that when the front surface of the guard (the one facing the process zone) is subjected to exposure of laser radiation (exposure level and duration) at the foreseeable exposure limit (FEL), it prevents laser radiation accessible at its rear surface from exceeding the Class 1 AEL at any time over the period of the maintenance inspection interval. For automatic machines this period should be 8 hours i.e. a working shift. Of course, the foreseeable exposure needs to encompass both normal and foreseeable fault conditions. When using a proprietary or pre-designed guard, the performance can be assessed knowing the guard's Protective Exposure Limit (PEL) as this limit indicates the guard's maximum protective characteristics.

The standard gives performance requirements for the design of guards, the use of proprietary guards, viewing windows and active guarding systems. Testing protocols are given together with a wide range of guidance.

The Machinery and Low Voltage Directives
All machinery put into service within the European Union is required to conform to the Machinery Directive. This Directive issued by the European Council of Ministers as Directive 98/37/EC and its amendments, specifies the Essential Health and Safety Requirements (ESHR's) for all machinery. Lasers that are independent of a machine will almost certainly be required to conform to the Low Voltage Directive (73/23/EC and amendments). This Directive is also incorporated into National legislation and specifies the health and safety requirements for equipment where the predominant hazard is electrical rather than mechanical. This Directive demands that all hazards are considered.

Standard EN ISO12100 (ISO 12100) Series
The primary purpose of this standard is to provide designers with guidance to enable them to produce machines that are safe for their intended use. EN ISO 12100 is a type-A standard giving basic concepts, principles for design and general aspects that can be applied to all machines. Part 1 of this standard defines the terms used. These include the hazards generated by radiation, including laser radiation, whose effects can be acute (e.g. burns) or long-term.

It is assumed that a hazard will sooner or later lead to harm unless protective measures are taken. The standard recommends that protective measures are included at the design stage and should take into account the experience of the user. Thus the designer should take the following actions in order indicated:
o Specify the limits and intended use of the machinery;
o Identify the hazards;
o Estimate the risk for each hazard;
o Evaluate the risk and apply risk reductions if required;
o Eliminate the hazard or reduce the risk by using protective measures.

The objective is to apply the greatest risk reduction taking into account:
o The safety of the machine during ALL phases of its lifecycle;
o The ability of the machine to perform its function;
o The usability of the machine;
o The manufacturing and operational costs.

To meet some of these requirements the protective measures taken may include guards. The primary hazard associated with laser processing machines is likely to be the laser radiation hazard, but equal attention must clearly be given to any other hazards that may be present.

Part 2 of the standard EN ISO 12100 gives general guidance on the selection of the appropriate guard. The choice of a safeguard for a particular machine is made on the basis of the risk assessment for that machine. In selecting an appropriate safeguard, it should be remembered that a fixed guard is simple and should be used where access of an operator to the danger zone is not required during nor mal operation. Where there is a need for frequent access, the fixed guard is not a practicable solution and an alternative protective measure must be adopted such as a movable interlocking guard. Often a combination of safeguards is required. The standard gives excellent advice covering a wide number of situations.

With the introduction of the Noise Directive, the design of the machine, the guards in particular, now needs to consider noise reduction or containment within the guard specification. (Note: requirements for noise limitation will be included in the next amendment to ISO 11553-1, the standard for laser processing machines, which will probably be published in 2006.)

It should be noted that EN ISO 12100 series of standards has replaced the EN 292 series, which until they were withdrawn in December 2003, were the main supporting standards for the Machinery Directive.

Risk Assessment
Human exposure to a typical laser beam used for laser materials processing can produce a moderate to severe injury, depending on laser wavelength, tissue exposed and the response of the victim. The probability of such an exposure occurring becomes the key
variable element in assessing the risk of injury.

The reduction of risk to tolerable levels is an iterative process. There is no standard approach to procedure and documentation for this process. Nevertheless, the steps involved are universal and are described in EN 1050 (ISO 14121).

General considerations
A risk assessment should be performed to identify hazardous situations and to assess the FEL on vulnerable areas of laser guards. This assessment should take into account a number of factors, including the following.
o Features of the laser process zone
Relevant features include the laser power and wavelength, the focal length of optics in relation to separation from the guard, the degrees of freedom of the beam delivery (e.g. number of axes of movement).
o Process
The nature of the process, such as cutting, drilling, welding, marking is important with regard to beam movement, workpiece reflectivity, quantity of fume generated. The machine may be dedicated or offer several processes.
o Process control
The detection of a fault condition and the means and effectiveness of automatic process control intervention is a major consideration in determining the time for which a laser guard may be exposed under fault conditions. Otherwise, the duration of worst reasonably foreseeable exposure of the guard may be determined by the process cycle time or the inspection period (e.g. per item or per time period/ number of items).
o Manual operations
Operator intervention considerations include the need and provision for manual control, the means and effectiveness of process observation (including the location of viewing windows or cameras) and the accessibility and effectiveness of intervention in the event of a fault condition.
o Robot operations
Considerations include the full range of robot movements, impact protection for the robot head and general protection of service lines and the beam delivery to the robot, and the means of limiting robot head movement and direction (e.g. software limits, hardware limits and physical limits), in particular the closest approach of the exposed laser beam to laser guards.
o Workpiece
Workpiece considerations include the geometry, composition and surface finish of the workpiece, and how it can affect the direction and strength of reflections during laser processing.
o Clamping and fixturing
Relevant features include the holding and positioning of the work piece and the related issues of reflections from surfaces and collisions of the focusing head.
o Loading and unloading
Considerations include the method by which the workpiece is introduced and removed, in particular whether it is manual or automatic, single piece or continuous; and the method (e.g. sliding, rolling or lifting door) and control of access to the process zone.
o Beam delivery
Beam delivery considerations include the optical method (mirror or fibre) and means of inspection, positioning and movement of optical components. Considerations include the structural integrity of the mounting of beam path components, the means of maintaining the condition of optical components (e.g. clean dry gas purge plus cooling supply), means of maintenance of beam alignment, provision of on-line errant and non-errant beam monitoring, and means of construction of the beam delivery enclosure. Particular attention should be given to the use of novel (unproven) design of laser beam delivery, the exposure of the beam delivery structure to external mechanical forces (e.g. vibration) which may give rise to optical misalignment. Attention should also be given to tampering with optics or anomalous performance of lasers, especially in regard to beam pointing, and situations where the laser power is so high that the performance of beam delivery optics is uncertain.
o Location of workers
Considerations of worker position include the setting of a defined work area, in particular the minimum distance of permitted approach to the machine. Included in this consideration are overhead locations (e.g. crane operators, office workers on elevated walkways), steps and ladders in the vicinity.
o Maintenance provision
This consideration includes the means and control of access to maintenance positions (e.g. removable panels, key control) and the provision of interlock overrides and emergency stops.
o Guarding properties
The assessment of FEL under normal conditions and reasonably foreseeable fault conditions should be made for each element of guarding, including fixed and moveable walls and windows.
o Guarding environment
Considerations include environmental factors that may influence the effectiveness of the guarding, including access for fork lift trucks and other moving objects that could cause significant mechanical damage; and dusty environments that could adversely affect the performance of optics and/or the protective properties of the guard.

ALARP
This method is to reduce risks to “As Low As Reasonably Practicable” (ALARP) by means of a structured approach to design and implementation. The main tool is the implementation of good practice, which in the present context means satisfying standards for controlling risk.

Written good practice may take many forms; its scope and detail will reflect the nature of the hazards and risks, the complexity of the activity or process and the nature of the relevant legal requirements. Examples of recognised sources include guidance produced by government departments, standards produced by Standards-making organisations (e.g. BS, CEN, CENELEC, ISO, IEC) and guidance agreed by industrial/occupational sector bodies (e.g. trade federations, professional institutions, sports governing bodies).
The table below shows how ALARP could be applied.

Project stage	Elements in demonstration that risks are as low as is reasonably practicable
Choosing between options or concepts	Risk assessment and management according to good design principles. Demonstration that duty-holder's design safety principles meet legal requirements Demonstration that chosen option is the lowest risk or justification if not Comparison of option with best practice, and confirmation that residual risks are no greater than the best of existing installations for comparable functions. Risk considered over life of facility and all affected groups considered Societal concerns met, if required to consider.
Detailed design	Risk assessment and management according to good design principles Risk considered over life of facility and all affected groups considered Use of appropriate standards, codes, good practice etc. and any deviations justified Identification of practicable risk reduction measures and their implementation unless demonstrated not reasonably practicable.

Risk assessment as suggested in Standard EN 954-1 (ISO 13849-1)
Safety related parts of control systems - Part 1 General design principles. The requirements for those parts of a machinery control system assigned to provide safety functions are defined in this standard and are described in detail below. They can consist of hardware and/or software and include the control of machine performance in relation to the state of safeguards.

The performance of a safety-related part of a control system with respect to the occurrence of faults is allocated in the standard EN954-1 into five categories (B, 1, 2, 3, 4) which can be used as reference points. These categories give some indication of the fault tolerance of the safety-related controls.

A risk assessment process is described in Annex B of the standard. This risk assessment leads to the determination of a safety category. These Categories are used as the basis for the design of the safety-related controls and define their resistance to faults.

Following the risk assessment, the controls need to be designed taking into account all intended use and foreseeable misuse, when faults occur and when foreseeable human mistakes are made during the intended use of the machine as a whole. The objective here is to ensure that the safety-related parts of the control system produce outputs which achieve the risk reductions desired. This objective is not always attainable and thus other safety measures such as guards may be necessary.

The greater the risk reduction is dependant on the safety-related parts of the control system, the higher must be the ability of those parts to resist faults. The higher the reliability to faults, the lower the probability that the safety related parts will fail to carry out the required safety functions. Note, however, that reliability and safety are not the same and safety must have the highest priority regardless of the reliability achieved especially when the consequences of failure are serious, possibly irreversible.

The following parameters need to be considered
o The reliability to perform the safety function;
o The structure of the control system;
o The quality of safety-related documentation;
o The completeness of the specification;
o The design, manufacture and maintenance;
o The quality and accuracy of software;
o The extent of functional testing;
o The operating characteristic of the machine.

Like other types of design the process is as follows:
o Hazard analysis and risk assessment;
o Decide the measures for risk reduction by control means;
o Specify the safety requirements for the safety-related parts of the control system and determine how they will be realized;
o Complete the design and verify the design at each stage to ensure that the requirements for fulfiled;
o Validate the overall system to ensure the safety functions are achieved and redesign as necessary. It is also important to validate the system when incorporated in the complete machine.

It is noted that at present, consensus opinion suggests that the use of a single channel of programmable electronic equipment may not be adequate for safety-related applications due to the difficulty of ensuring correct operation in situations when a significant hazard can occur due to the mal-operation of the control system.

Standard EN953 (ISO 14120)
The general principles of design and construction of guards is given in a standard EN953 (ISO 14120).

Inadequate consideration of foreseeable aspects of the machine environment and its operation through the foreseeable life of the machine can lead to unsafe or inoperable machinery. This in turn can lead to persons being able to defeat the guards provided, thus exposing them to greater risk. To minimise access to danger zones where practicable, guards should therefore be designed to enable routine adjustments, lubrication and maintenance to be carried out without opening or removing the guards. Access may be required for a number of reasons including:
o Loading and unloading;
o Tool changing and setting;
o Measurement, gauging and sampling;
o Process observation;
o Maintenance and repair;
o Removal of waste material;
o Obstruction removal;
o Cleaning and hygiene.

In addition to the containment requirements of guards, it is important to consider the ergonomic aspects. Size and weight must be suitable to allow ease of handling. Guards that cannot be readily moved by hand may well be bypassed. Guards that require regular and frequent opening especially need to be designed to reduce the stress and the physical effort of the operator as this improves the performance and reliability of the operation reducing the probability of errors at all stages of machine use.

In general, materials selected should be suitable for the construction envisaged during the foreseeable life of the guard. Of particular importance are:
o Impact resistance - guards should be capable of withstanding reasonably foreseeable impacts from parts of the machinery, work piece, broken tools etc., as well as impact by the operator.
o Rigidity - Support posts, guard frames and infill materials should be selected to provide a rigid stable structure and to resist deformation.
o Security of fixing - All guards should be secured by fixing points of adequate strength, spacing and number to remain secure during their foreseeable use.
o Reliability of moving parts - Moving parts, e.g. hinges, slides, catches etc. should be selected to ensure reliable operation given their foreseeable usage and working environment.

The following aspects should be considered with regard to the construction of guards:
o Removal of all sharp edges;
o Ensuring joints are of sufficient strength;
o Making demountable parts of guards only removable with the aid of a tool;
o Providing a positive location for removable guards and preferably making the guard unable to remain in place without their fixings;
o Positively determining the closed position of movable guards i.e. held in position by gravity, a spring, catch, locking device etc.;
o Limiting the size of self-closing guards so that it should not be possible to lock these guards in the open position;
o Restricting the opening of adjustable guards to the minimum amount needed to fulfil its function and adjustment should demand the use of a tool;
o Positive action for the operation of movable guards, which should preferably be permanently fixed to the machine;

Control guards i.e. those that cause the machine to start when closed, should only be used if:
o There is no possibility of the operator being in the danger zone;
o The environment allows the operator a global view of the machine/process;
o Opening the control guard or an interlocking guard is the only way to enter the danger zone;
o The interlocking device associated with the control guard is of the highest possible reliability (as its failure can lead to unintended or unexpected start-up);
o Where starting the machine with the control guard is limited to one of the possible control modes of the machine by MODE selection.

Guards should be selected from the following order of priority:
o Local guards enclosing individual danger zones if the number of danger zones to protect is low. This can provide an acceptable residual risk and permits access to non-hazardous machine parts for maintenance, setting etc.;
o Aguard enclosing all the danger zones if the number or size of the danger zones is high. In this case, setting and maintenance points should, as far as possible, be located outside the guarded area;
o Partial distance guard, if the use of an enclosing guard is impracticable and the number or size of the danger zone is high;
o Fully enclosing distance guard, if the use of an enclosing guard is impracticable and the number or size of the danger zone is high.

With laser processing machines the use of distance guards is rarely possible. However, as lasers become incorporated in more complex machines, the overall machine may be protected using this method of guarding. Generally a combination of guarding types will form the optimum solution.

Frequency of access is an important factor. Where access is not required during use, fixed guards should be used on account of their simplicity and reliability. Where access is required during use but limited to setting, process correction or maintenance, movable guards are the answer if frequency of access is high. Fixed guards should only be used if the foreseeable frequency of access is very low, and even then only if replacement is easy with removal and replacement controlled under a safe system of work. Where access is required during the working cycle, a movable guard may be used with an interlock or with an interlock and a locking device on the guard. Under certain conditions a control guard may be acceptable. Clearly, a detailed risk assessment with a documented justification of the solution should be carefully considered during the design's review.

Standard EN1088 (ISO 14119)
The primary purpose of this standard is to give guidance to designers on how to design or select interlocking devices associated with guards in order to meet the requirements of the Machinery Directive.

Interlocking principles are described to allow an understanding of control interlocking and power interlocking. Several examples with descriptions in text and with diagrams are given to explain various typical forms of interlocking devices. These forms of interlocking include those with and without guard locking.

Interlocking techniques involve a broad spectrum of technological aspects. The main forms of interlocking devices that are given include:

Interlocking devices with mechanically actuated detectors such as:
o Cam-operated detectors;
o Tongue-operated detectors;

Interlocking devices with non-mechanically actuated detectors such as:
o Magnetically actuated switches;
o Electronic proximity switches;

Systems incorporating keys including:
o Captive-key systems;
o Trapped-key systems;
o Plug and socket systems;
o Mechanical interlocking between guard and movable parts.

Regardless of the nature of the energy source for the control system (electrical, pneumatic etc.), there are a number of essentials that should be incorporated into the design of the interlock device, as detailed below.

When a single detector is used to generate a stop command, it needs to be activated in a positive mode. This means that the detector is caused to move directly or by rigid elements so that operation of the detector is inevitable. Non-positive mode actuation is only allowed in conjunction with a detector having positive mode actuation, notably to avoid common cause failures. The design of the actuator should be a simple as possible, since this may reduce the probability of failure.

Position detectors require to be arranged to that they are protected against a change in their position either intentionally or unintentionally. Position detectors should not be used as mechanical stops and should be located so that damage from foreseeable external causes is avoided.

Interlocking devices should always be designed so that they cannot be defeated in a simple manner. Design examples include the use of interlocking devices or systems that are coded, or the use of physical shielding to prevent access to the interlocking device.
Proximity switches and magnetic switches, which rely solely on the presence or absence of detectable materials or of a magnet for their actuation, can be easily defeated. Therefore the method of mounting is vital to minimise defeat. Where there is the risk of a substitute actuator being used to defeat the system a suitable obstruction should be incorporated into the arrangement.

The use of plug and socket interlocking devices requires very careful design to minimize the potential for defeat. In the UK, this type of interlock is particularly disliked. If plug and socket interlock devices are to be used, the design should prevent access when the guard is open. Often multi-pin connectors are preferred, making it more difficult to restore the continuity circuit.

The informative Annexes in this standard give examples of a variety of different types of interlocking device and how these devices can be successfully incorporated into equipment. Advantages and disadvantages are highlighted for each example. This aids design decisions and indicates the potential complexity of the controls that will be necessary to make a specific type of interlock system meet the overall reliability and security requirements of a particular application.

Selection of Materials for Guards
In selecting the material to be used for the construction of a guard, consideration should be given to the following:
o Its ability to withstand the forces of any foreseeable mechanical hazard associated with the laser processing machine. The guard will fulfil a combination of functions such as the prevention of access and containment of hazards. These hazards include laser radiation, ejected particles, dust, fumes, noise etc. and one or more of these considerations may govern the selection of guard materials.
o Its weight and size in relation to the need to remove and replace it for routine maintenance.
o Its compatibility with the material being processed. This is particularly important in the food processing or pharmaceutical industry where the guard material should not be a source of contamination.
o Its ability to maintain its physical and mechanical properties after coming into contact with potential contaminants generated or used during processing operations or cleaning or sterilising substances used during maintenance.

Solid sheet metal
Metal has the advantage of strength and rigidity and in solid sheet form is particularly suitable for guarding where adjustments are rarely needed and there is no advantage in being able to see the working operation within the process zone. However, care should be taken to ensure that, where necessary, sufficient ventilation is provided in the guard to prevent overheating within the process zone and the guard does not create a resonance.

Data has been established as part of a research project to determine the typical burn-through times for a variety of metals using both typical CO2 and Nd:YAG industrial lasers. This data, samples of which are provided in figures 1 and 2, is being incorporated into an additional informative Annex to the standard IEC 60825-4, publication of which is expected during 2005. Although this data is only typical and should therefore not be relied upon, it can be helpful to determining practical passive guarding solutions.

Glass
Ordinary glass is unsuitable for guard manufacture due to its brittleness but where a laser process is required to be observed and the material is likely to be exposed to high temperatures or abrasive action, a safety glass, which provides adequate protection from laser radiation (by internal absorption of the laser radiation within the material or suitable reflective optical coatings on the surface of the guard material) may be suitable. IEC 60825-4 gives methods for determining the suitability of such materials.

Plastics
Transparent plastic sheet materials may be used in laser guarding as an alternative to opaque materials especially where observation is required during the processing operation. Plastic materials available for guarding purposes include polycarbonate and specially dye-impregnated acrylic sheet. It is essential that these materials are selected with the appropriate optical protective properties for the wavelength and power of the laser source fitted to the laser processing machine.

The mechanical properties of many plastics are adversely affected by contaminants, by incorrect cold working and by continuous exposure to high temperatures or UV radiation. Continuous exposure to high temperature (polycarbonate: 135degC, acrylic sheet: 900C) will cause softening and consequently lowering of both impact strength and optical properties.

As these materials are generally used as absorbent filters, any removal of the surface material will reduce the optical protective properties of the material at laser wavelengths and the provision of additional sacrificial mechanical protective layers should be
considered.

Most plastics have an ability to hold an electrostatic charge. This can create a risk of electrostatic ignition of flammable materials
and can also attract dust. This characteristic can be mitigated by the use of an anti-static preparation.

Other materials
Concrete block work may be an effective material for some guard construction and is frequently used for large CO2 laser processing machine enclosures. Details of the protective performance together with other building materials are shown the Annex of standard IEC 60825-4.

 

Standard EN12626 (ISO 11553-1)
This European standard gives the complete requirements for laser processing machines. This standard is mandated under the Machinery Directive and thus should be the primary document applicable to laser processing machines when they are to be put into service in Europe. The European and the international ISO standard are essentially identical.

The standard describes the hazards generated by laser processing machines and specifies the safety requirements relating to radiation hazards and hazards generated by materials and substances. Particular reference is made to the ISO 12100 series of standards and thus the information contained in this paper above is very relevant. The standard requires manufacturers to consider a wide range of hazards and lists them in a general form to act as an aide mémoire. The list is augmented in the informative Annex A to give guidance regarding the hazards derived from materials and substances particularly relevant to laser processing. The standard follows the principles of risk assessment to formulate the required design features and gives guidance for the protective measures that should be put into place during the various stages of the machine life-cycle.

In early 2005 the standard EN12626 was withdrawn and replaced with EN ISO 11553-1, which is now identical with the ISO standard for laser processing machines. Whereas the earlier standard was only applicable to machines that conform to the Class 1 conditions specified in standard EN60825-1, the revision allows the standard to be applied to all type of laser processing machines including those that do not conform to Class 1 conditions. (See AILU magazine Issue 37 Dec 2004)

A further revision is under development to include requirements for the limitation of noise as required by the European Noise at
Work Directive.

Conclusion
The guarding of laser installations and particularly industrial laser processing machines needs to consider much more than just the protective requirements for laser radiation. It is very rare for the laser radiation hazard to be the only hazard that the equipment designer is required to take into account.

There are a number of relevant and extremely useful standards that can be used to aid the design of guarding. In the early 1990s, when the Machinery Directive was first introduced into the UK, there was considerable reticence among machine designers to review their designs in the light of the, then, “new” requirements. However it soon became apparent that the requirements were extremely sensible. Following a period of re-education, designers acknowledged the advantages, and machine design in general has significantly improved. The improvements have not been limited to just the safety aspects although this has perhaps been the main driver.