Working safely with machines

Terry Hayward of Schmersal UK discusses the options available to specifiers of safety equipment to comply with the standards of risk assessment and machine safety required under EN60204.

The area around manufacturing robots, automated handling machines and machine tools will always be hazardous and a potential threat to operator safety.

To reduce the risk of accidents, it is essential that plant engineers analyse carefully the safety implications and specify the most appropriate protection measures.

Traditionally, a mechanical fence with switches fitted to opening doors and gates is the first solution to be considered.

For these applications Schmersal has recently introduced a new range of safety interlock switches that are activated by an individually coded, geometrically unique, actuating key with 15,000 different combinations.

This ensures that engineers are safe in the knowledge that the switch can never be bypassed with a key from another machine.

Conventional safety interlock switches are only available with a standard key (actuator), designed to prevent the guard switch being bypassed using screwdrivers, paper clips and other objects.

However, these switches can be bypassed using a similar standard key obtained from another machine or from the stores department.

This practice is particularly common in plants where a large number of machines are fitted with the same type of switch.

The switches are designed for use as safety interlocks on applications such as protective gratings, hinged or swivelling covers, access panels and other types of moveable guards.

An IP67 rating makes them ideal for hostile environments.

All versions are of two-piece design, consisting of a switch mechanism and an individually coded actuator key that is hard-mounted directly to the moveable guard.

To allow flexibility when installing the switches, up to four key entry locations and a high-speed key withdrawal facility are available.

All switches meet stringent international safety standards including BG, UL and CSA.

However, for many applications the traditional guard fence may be more expensive, take up more space and reduce operational flexibility.

Access to the zones in modern processes or machines cannot be categorised as simply "allowed" or "disallowed".

To accommodate variables, such as the stage the machine or process has reached in its operating cycle, there are other, often more appropriate, safety solutions that should be considered.

The following notes explain the functionality and features of the latest tactile and electrosensitive devices and should help plant engineers evaluate their performance against traditional mechanical components.

The safety mat is a tactile device that offers a simple yet flexible solution to machine safety.

Their robust design combines reliability and long mechanical life with a nonslip and chemical-resistant surface.

Safety mats are designed to provide operator protection around moving machinery and are sensitive enough to detect a single person and robust enough to carry the weight of a forklift truck, and either will be detected immediately.

Tactile safety mats consist of two separated electrodes held apart with insulating strips.

Electrode strips cause an electrical short circuit when the pressure sensitive mat is activated and a safety-monitoring module evaluates the signal and switches off the machine movement.

Safety mats comply with safety requirements up to Category 3 of EN954-1.

The main advantage of most optical devices is that they can be programmed to allow access to machinery at specific times during the process and in specific areas.

This important facility allows designers to tailor a safety system to particular hazards, whilst allowing the machine or process to operate at its maximum capacity.

When compared with machines fitted with standard mechanical guarding, the increase in efficiency can greatly improve the profitability of the process.

Optoelectric safety devices fall into three main categories - laser scanners, light barriers and light curtains.

Similar to a radar system, the laser scanner is a flexible and powerful device that is able to continuously scan a 180-degree detection zone up to a maximum radius of 60m using a beam that is absolutely safe and harmless to the operator.

Safety zones and warning zones can be user-defined.

When a safety zone is entered, the scanner detects the person or object and machine motion is immediately stopped.

When a warning zone is entered, the scanner issues a warning signal to prevent inadvertent machine stops.

The laser scanner can also be used to detect objects or as the sensor for an automatic transport system.

Light barriers are designed for use as entry controls or movement detectors.

They are ideal for guarding: powered windows, doors and gates; danger zones in conveyor systems; printing, packaging and paper-processing machinery; storage equipment and machinery; elevator platforms; and palletising, stacking and production machinery.

Any interruption of the safety light barrier produces a signal, which stops dangerous machine operations.

Light barriers can be used as perimeter guards or as direct guards for smaller danger zones.

Their small size means they can be installed almost anywhere without interfering with machine operations.

They comply with safety requirements up to Category 4 of EN954-1.

Using a master/slave configuration, a series of cascading safety light curtains can be created to protect operators of equipment such as power presses, industrial robots, welding machines and many more.

Under normal circumstances, when the light beam of the curtain is interrupted a signal is triggered to switch off the machine.

However, where the process requires it, curtains can be programmed to allow the light beam to be interrupted without generating a stop signal.

Light curtains comply with safety requirements up to Category 4 of EN954-1.

Schmersal specialises in man-machine safety systems and offer further information on a range of approved safety products that will enable process engineers to make an informed decision on individual installations or a comprehensive machine safety strategy.

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