1          Additional aspects regarding the selection of risk reduction measures for laser devices

1.1      Introduction

Reducing the risk of laser radiation emission at the machine design stage involves mainly the use of appropriate engineering measures to restrict human access to the beam or attenuate the beam.

PN-EN 12100 describes the general principles of risk reduction, which takes place in three steps:

Step 1. Inherently safe solutions

Step 2. Use of engineering control measures and/or complementary protective measures.

Step 3. Information on the use

1.2      Step 1. Inherently safe solutions

Step 1 is the only one where the risk can be eliminated, which prevents the use of additional protective measures such as engineering control measures or complementary protective measures. Inherently safe design solutions eliminate or mitigate the risk to a low level due to the selection of the design features of the machine itself.

In the case of laser radiation, which is a physical factor, this relates primarily to reducing radiation emission by affecting the characteristics of its source. This is achieved by the use of laser sources:

–          with radiation power reduced to the lowest level sufficient for the proper operation of the machine (this requires the knowledge of emission values necessary for a manufacturing process);

–          designed in such a way that the beam is focused on its target;

–          placed at a long distance from the operator or remotely controlled by being placed in such a way that there is no operator exposure while operating the machine.

It is also important to restrict exposure to hazards by equipment reliability, which applies in particular to safety guards, which should be reliable, because their damage exposes people to laser radiation risks.

1.3      Step 2. Use of engineering control measures and/or complementary protective measures

If an inherently safe solution fails to eliminate or insufficiently mitigates a risk, protection of people should be provided by the use of guards and protective devices. It may also be necessary to use supplementary protective measures, such as emergency stop equipment.

Engineering control measures limiting the emission of laser radiation include the following:

–          the use of filtering and absorption;

–          the use of attenuating screens or guards.

The primary engineering control measure for protection against laser radiation are laser guards, which protect the zone of machine operation for laser processing. The principles for design and selection of such guards are described in the standard PN-EN 60825-4. This standard applies to all guard components including screens and viewing windows, panels, laser curtains and walls.

The guards of the control panel position and operator intervention areas while operating the machine should also be taken into account.

Another important protective measure are screens for laser workstations, the requirements for which are described in PN-EN 12254.

Guards and screens may have two functions:

–          preventing access;

–          reducing the emission of human accessible radiation.

A laser guard should not increase any associated hazard on or beyond its rear surface (high temperature, release of toxic materials, fire, explosion, etc.) when exposed to laser radiation up to the foreseeable exposure limit (FEL). The foreseeable exposure limit (FEL) at a particular location where a laser guard is to be sited is the maximum exposure estimated by the manufacturer of the laser processing machine, assessed under normal and reasonably foreseeable fault conditions. The protective exposure limit (PEL) indicates the capability of a laser guard to protect against incident laser radiation.

Passive and active laser guards are distinguished:

–          a passive laser guard relies for its operation on its physical properties only (e.g. a metal panel relying on thermal conduction or a transparent sheet, opaque at the certain wavelength and power density/radiation energy);

–          active, when a laser guard is part of a safety-related control system; the control system generates an active guard interlocking signal in response to the effect of laser radiation on the front surface of the laser guard (e.g. a guard with embedded thermal sensors, which detect overheating).

–          Selection of laser guards consists in:

–          determining the preferred laser guards position and the estimation of the FEL for this position;

–          if necessary, minimising the FEL under fault conditions.

Fig. 1. Illustration of guards around a laser processing machine [PN-EN 60825-4]

The FEL is estimated based on a measurement or calculations. The full FEL specification comprises the following:

–          the maximum irradiance at the front surface of the laser guard (determined assuming specular or diffuse reflections, in accordance with PN-EN 60825-4, Appendix B);

–          any upper limit to the area of exposure of the front surface at this level of irradiance;

–          the temporal characteristics of the exposure (laser operation mode and parameters);

–          the full duration of exposure;

–          the wavelength of the radiation;

–          the angle of incidence and the polarisation of the radiation;

–          any minimum dimensions to the irradiated area;

–          the active guard protection time.

a)

b)c)

Fig. 2. Examples of a foreseeable guard fault condition [PN-EN 60825-4]

It is recommended that FELs be estimated for the worst reasonably foreseeable combination(s) of available laser parameters, workpiece materials, geometry and processes likely to be encountered during normal operation. Examples of foreseeable guard fault conditions due to software failure (Fig. 4.2a), inadequately clamped workpiece (Fig. 4.2b) or workpiece missing (Fig. 4.2c) are presented in Fig. 2.

Below there are presented examples of risk reduction through the use of appropriate laser guards.

1.3.1    Examples

1.3.1.1    Example 1

Laser processing machine mounted over a conveyor belt. High laser radiation emission risk was found in the area of personnel presence. During production, controlled access is possible for authorised personnel, but exceptionally this area can be entered by an unauthorised and untrained person.

Risk reducing measure – the use of the laser guards, which should be positioned so as to cover the area of insertion and removal and enable continuous feeding of components to the process zone.

Solution:

Introduce a sliding interlocked laser guard, which opens to allow insertion of a component and closes before the laser processing starts.

1.3.1.2    Example 2

Table for cutting flat surfaces using a laser.

a)   High laser radiation emission risk was found in the area of personnel presence. During production access cannot be controlled so as to allow only authorised personnel.

Risk reducing measure – the use of a laser guard.

Solution:

Ensure that outer area is fully guarded to protect the operator by means of a sliding guard with an opening interlock in order to allow movement of a component and closing before the laser processing starts.

b)   Moderate laser radiation emission risk was found in the area of personnel presence. During production access cannot be controlled so as to allow only authorised personnel.

Risk reducing measure – the use of a laser guard.

Solution:

–          Provide a free-standing guard to protect the operator.

–          Provide personal protective equipment required for all persons within the accessible zone.

1.4      Step 3. Information on the use

Developing information on the use is an integral part of the machine design [PN-EN 12100] and applies to both the use of appropriate warning signs or signals on the machine or warning devices, and providing their description in the machine instruction manual. The scope of information and method of presentation as well as its placement on a machine incorporating a laser device is presented in PN-EN 60825-1. This information should also be contained in the instruction manual. The manufacturer is responsible for marking the laser class and place all the necessary information and warning labelling.

The accompanying documentation should include information on the parameters of the emitted laser radiation.

The user should be provided with information on how to use the machine as intended, having regard to all types of work. The user should be made aware of the residual risk and warned against it. The information should indicate whether the use of personal protective equipment against laser radiation is necessary (e.g. goggles) and whether the user may have to use additional protective equipment.

Documentation concerning risk reduction, in accordance with PN-EN 12100, should include:

–          risk reduction targets, to be achieved with the use of protective measures;

–          a list of protective measures implemented to eliminate identified hazards or to reduce risks;

–          residual risk due to laser radiation emission;

–          risk assessment result.