“Methodology for specification of safety requirements during designing and manufacturing of machinery equipped with laser kit.”

Type of machinery and function intended for laser beam emitted on that machinery determine type and strength of the installed laser. Time- and energy-related parameters of the laser beam are adjusted to a given technological process (welding, cutting, engraving, marking, drilling, etc.) or another application of laser on a given machine (e.g. laser pointer, laser guide, etc.).

During designing and manufacturing of machinery equipped with laser equipment the designer determines its laser safety class, and they provide relevant protective means according to this class. In the case of laser processing machines also additional laser guards are accounted for.

Designer of a given machine selects specific laser or laser kit which constitutes a component of a given machine. At this stage, provided that time- and energy-related data of a given laser are known and available (power, energy, wavelength, operating mode, etc.), one can determine its safety class, or, respectively, such a class has already been determined by the manufacturer of a given laser kit.

Characteristics and criteria for determination of laser safety class are set out in detail in the standard PN-EN 60825-1:2010 Safety of laser products -- Part 1: Equipment classification and requirements. The table below contains description of individual safety classes.

Note:

Laser processing machines (for welding, cutting, etc.) usually use Class 4 lasers (less commonly Class 3B lasers). Laser guides and laser pointers used in machinery are usually Class 2 lasers (less commonly Class 3R lasers).

Depending on laser beam enclosure, introduction of beam limiters, expanders or optical instruments through which the beam passes, laser safety class of a given machine might be different than the one of the laser integrated into this machine. Therefore, it is necessary to determine laser safety class for machinery.

Ø    Determination of laser safety class (based on the PN-EN 60825-1:2010 standard)

After designing a machine the designer shall determine laser class by means of calculations. This class is verified by measurements after manufacturing a prototype of the machine.

Classification of laser safety takes place based on accessible emission limit (AEL), i.e. the maximum level of radiation emitted from the laser allowed within a given laser class. In order to determine laser safety class, one has to specify the level of laser radiation – the so-called accessible emission (when access of people to a given piece of equipment is considered). This is compared to accessible emission limit (AEL).

Level of accessible emission shall be determined for operating, maintenance and servicing conditions for a given machine. The determined laser safety class can be different for different stages of machine operation listed above.

AEL values are referenced to laser radiation wavelength and emission duration. When determining them one needs to consider whether laser source is of local or extended type (based on the determined viewing angle of observable source, determination of which is set out in the PN-EN 60825-1:2010 standard). Appropriate emission times shall be used when determining laser safety class, namely: 0.25 s, 100 s, and 30000 s, and for pulsed lasers additionally duration of a single pulse plus time T for which groups of pulses are summed up are used (table 3 in the PN-EN 60825-1:2010 standard).

AEL values are usually expressed as radiation power [W] or energy [J], and in the case of ultraviolet radiation in the range of 180 – 302.5 nm for Classes 1, 1M and 3R, as irradiance [W/m²] or radiant exposure [J/m²]. AEL values are listed in tables 4 to 9 in the PN-EN 60825-1 standard.

Ø    Determination of other safety requirements depending on the determined laser safety class (based on the PN-EN 60825-1:2010 standard)

Protective housing is required for all classes of laser equipment as it limits access required for ensuring safe operation of the equipment.

Class 1 shall be equipped with the following:

Ø  Safety interlock in protective housing designed to prevent removal of the panel until accessible emission values are below those for Class 3R

Ø  Safeguards ensuring that scan failure does not result in exceeding classification of equipment

Ø  Class label – inscription is required

Ø  Override interlock label, if applicable

Ø  Wavelength range label for invisible radiation range

Ø  Operating instructions including user manual for safe operation of equipment

Ø  Purchasing and service booklet shall specify product class and contain safety-related information

Class 2 shall be equipped with the following:

Ø  Such visual optics so that emission from all visual systems does not exceed AEL for Class 1M

Ø  Class label – laser radiation hazard symbol and inscription

Ø  Service entry label

Class 2M shall be equipped with the following:

Ø  Operating instructions including user manual for safe operation of equipment and additional warning against viewing laser beam directly with optical instruments

Class 3R shall be equipped with the following:

Ø  Safety interlock in protective housing designed to prevent removal of the panel until accessible emission values are below those for Class 3B or 3R for particular devices

Ø  Emission warning device (visible or audible warning) if invisible radiation is emitted

Ø  Location adjustment ensuring that controls are located in such a way so that there is no danger of exposure to AEL above Classes 1 or 2 when adjustments are made

Ø  Aperture label – specified wording is required

Class 3B shall be equipped with the following:

Ø  Remote interlock switch allowing for easy addition of external interlock in the laser system

Ø  Key control ensuring that laser is inoperative when key is removed

Class 4 shall be equipped with the following:

Ø  Manual resetting in case power interruption occurs or remote interlock switch is operated

Ø  Beam attenuator which gives means to temporarily block the beam

Designing of guards for laser processing machines

One of the important technical standards when designing laser-equipped machinery is the following standard: PN-EN 60825-4:2010 Safety of laser products – Part 4. Laser guards. This part specifies the requirements for laser guards that enclose the process zone of a laser processing machine, and specifications for proprietary laser guards. These are guards protecting against harmful effects of laser radiation reflected in diffused or directional way. It contains information on how to select a laser guard and how to assess and specify protective properties of a laser guard. This standard applies to all component parts of a guard including clear (visibly transmitting) screens and viewing windows, panels, laser curtains and walls.

Moreover, the designed laser guard shall comply with requirements of the following standard: PN-EN ISO 12100: 2012 Safety of machinery. General principles for design. Risk assessment and risk reduction (p. 6.3.2) including general requirements concerning guards and also more specialist requirements, taking into account guard location and method of mounting.

When designing guards the following shall be ensured:

Ø  When the front surface of a laser guard is subjected to exposure to laser radiation at the foreseeable exposure limit, the laser guard shall prevent 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 automated laser processing machines, the minimum value of the maintenance inspection interval shall be 8 h).

Ø  A laser guard, in its intended location, shall not give rise to any associated hazard (e.g. high temperature, release of toxic materials, fire, explosion) at or beyond its rear surface when exposed to laser radiation up to the foreseeable exposure limit.

It shall be noted that some materials may lose their protective properties due to ageing, exposure to ultraviolet radiation, certain gases, temperature, etc.

Laser guards are divided into the following categories: passive laser guards (relying for their operation on their physical properties only) and active laser guards (laser guards which are part of a safety-related control system; the control system generates an active guard termination signal in response to the effect of laser radiation on the front surface of the laser guard).

Selection of laser guards consists in (PN-EN 60825-4:2010):

-        identifying the preferred position for the laser guard and estimating the foreseeable exposure limit (FEL, i.e. the maximum laser exposure on the front surface of the laser guard, within the maintenance inspection interval, assessed under normal and reasonably foreseeable fault conditions) at this position;

-        if necessary, minimizing the FEL under fault conditions by:

ü   including automatic monitoring in the machine, or

ü    ensuring that the laser guard is sufficiently far away from beam focus produced by focusing optics, or

ü   installing vulnerable parts of the laser guard away from regions that could be exposed to high irradiance, or

ü   moving the laser guard farther away from the laser process zone, or

ü   incorporating in the design of the machine beam control features to facilitate improved laser beam control.

Passive laser guard is the simplest option. If the FEL cannot be reduced to a value where common guarding materials provide adequate protection in the form of a passive laser guard, an active laser guard can always be used.

Assessment of foreseeable exposure limit (FEL) is outlined in Annex B to the PN-EN 60825-4:2010 standard.

When designing laser processing machines, one needs to take into account safety requirements set out in two parts of the PN-EN ISO 11553 standard: 2010 Safety of machinery. Laser processing machines.

Ø  Part 1: General safety requirements – concerns laser equipment manufactured exclusively and specifically for application in photolithography, stereolithography, holography, medicine, and data storage.

Ø  Part 2: Safety requirements for hand-held laser processing devices – concerns hand-held or hand-operated equipment not listed in Part 1 of the above standard.

When designing type of protection, the following need to be determined:

-        direction of propagation of a laser beam (fixed, variable) with respect to a workpiece

-        type of performed operation (cutting, welding, etc.)

-        material and shape of a workpiece

-        fixing of a workpiece

-        visibility of a processing area

EN 60825-4:2010 standard contains the following guidelines:

-        Guidelines on the arrangement and installation of laser guards – Annex E

-        Guidelines for designing and construction of laser guards – Annex F

-        Guidelines on requirements for access panels and safety interlock – Annex G

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·                     CO2-laser beam machines for welding and cutting

When designing CO2-laser beam machine for welding or cutting in two-axis configuration, the designer shall take into account requirements set out in three parts of the following standard: PN-EN ISO 15616 Acceptance tests for CO2-laser beam machines for high quality welding and cutting:

-        Part 1: General principles, acceptance conditions

-        Part 2: Measurement of static and dynamic accuracy

-        Part 3: Calibration of instruments for measurement of gas flow and pressure