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Machinery safety under special circumstances

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article image The integrated RFID technology of the electronic RSS 36 safety sensor provides for increased protection against tampering

The use of safety switchgear is recommended for the unique requirements set by the food industry to ensure machine safety. Selection of components and systems used for machine safety is governed by application-dependent requirements such as hygiene, sealing to prevent penetration of liquids and dust, compliance with industry-specific prescriptions, explosion protection and the suitability of the devices for particularly high or low temperatures.

Machinery safety is an extremely complex issue, and selection of the required components is defined by a number of standards based upon the Machinery Directive.

Special requirements for hygiene

The selection criteria for safety guards used on and in machines of the food industry is guided by the generally applicable machinery safety regulations, while also taking into account the requirements placed on hygiene. It must first be determined, in which of the four hygiene areas the safety switchgear to be selected will be used.

For instance, in the dry area, only protection against soiling is required. In the splash zone, there is risk of contamination or cross-contamination, considering that the staff touches both the foodstuffs and the machine. The wet area is exposed to a high risk of bacterial contamination; therefore, regular low-pressure cleaning operations with chemicals or hot cleaning activities are an integral part of the prescribed hygienic measures. The contamination risk is even higher in the aggressive area, where frequent hot-steam cleaning or high-pressure cleaning with aggressive cleaning agents takes place. This means that the switchgear used in this area should be capable of withstanding water jets with a pressure of at least 80 bar and a temperature of 80°C.

Hygienic design and efficient sealing

Conventional safety switchgear cannot be used to meet hygienic requirements. First, the switchgear must be able to resist the frequent cleaning activities, placing high requirements on the material selection and sealing. Secondly, the components must be designed in accordance with the principles of hygienic design, which means that the generation of dirt pockets must be excluded. The standard EN 1672-2 (Foodstuff machinery - General design principles - Hygienic requirements) gives appropriate indications for this requirement.

Switchgear programme for the food industry

For this reason, foodstuff machinery construction was the first industry, which used safety sensors instead of the conventional electromechanical safety switches, with the non-contact safety switchgear providing trouble-free cleaning thanks to the smooth surfaces of the sensor and actuator that can also be installed in concealed mounting, e.g. behind plastic covers.

The current innovations in this industry-specific field of machinery safety include the BNS 40S magnetic safety switches that have been developed for high-end applications such as in wet areas in the immediate vicinity of the product. Their stainless steel enclosure is resistant to corrosion and cleaning liquids while the slim design and fine-polished surface allow smooth integration in the typical surrounding construction of foodstuff machines.

As the BNS 40S is IP 69 K, it can also be considered as ‘high-pressure cleaner-resistant’. The high switching distance enables a concealed mounting behind non-ferromagnetic covers. The design process also focussed on avoidance of dirt pockets while the device’s compatibility with commonly used cleaning agents was tested in accordance with ECOLAB.

For concealed mounting: safety sensor with CSS technology

Also developed for high-end applications, the CSS 30 S safety sensor with cylindrical design uses the CSS (Coded Safety Sensor) technology developed and patented by Schmersal for the bi-directional communication between the sensor and the actuator. This operating principle features, amongst others, the advantage that up to 31 sensors can be wired in series and evaluated through a single safety-monitoring module. Even in this series-wiring format, safety circuits with Performance Level e to EN 13849-1 (SIL 3, control category 4) can be set up.

Additionally, the sensors are capable of detecting misalignments of the guard door and emitting an alarm at an early stage, avoiding undesired machine stops.

The CSS 30 S is completely encapsulated in a stainless steel enclosure and has protection class IP 69K. Capable of functioning even through stainless steel covers, the sensor is also available as a low-cost variant, the CSS 300, which features a thermoplastic enclosure.

Integrated RFID technology offers new possibilities

The RSS 36 safety sensor is also increasingly becoming popular in the foodstuff machine construction industry as well as in the packaging industry. Now also available in a version with integrated AS-i Safety interface, this safety sensor features an RFID chip in addition to the CSS technology, allowing the use of different coding variants to enhance protection against tampering.

Key features of the RSS 36 include universal mounting possibilities, high switching distance and protection class IP 69 K. Its robust design allows use as an end stop on small doors and flaps while the optionally integrated, fully thermoplastic-encapsulated magnetic latching eliminates the need for additional hygiene-compliant latching stops.

All these features make the RSS 36 very popular in many fields of application including the protection of hazardous points on foodstuff machinery.

Under extreme circumstances however, the use of a safety sensor with stainless steel enclosure is a more appropriate solution.

Solenoid interlock with innovative operating principle

Though not specifically developed for the foodstuff and packaging industries, the new AZM 300 solenoid interlock recently launched by Schmersal will open up many applications in these fields, thanks to its new and flexible interlocking system in the form of a rotary Maltese cross as well as the numerous mounting possibilities. The codification through the RFID sensor technology is another standard feature of the RSS 36, just like the adjustable latching force. Moreover, the device has been designed in accordance with the Hygienic Design principles and to protection class IP 69 K requirements, making the AZM 300 also suitable for use in hygiene-sensitive areas. 

Control Logic is a 100% Australian owned company specialising in the distribution, technical sales and support of industrial electrical automation control solutions and systems.

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