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How to choose the right guard safety switch technology

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Alex Bryce, the Sales Manager at Pilz Automation Technology, discusses the various switching technologies currently available for machine guarding, and the selection criteria that designers should consider.

How to choose the right guard safety switch technologyDesigners of plant and machinery are normally responsible for implementing the safety-related requirements and for selecting the components used for safe monitoring. However, designers are spoilt for choice, which makes choosing the most appropriate safe sensor or actuator/sensor combination a difficult task.

Sensor systems operate using different principles and so it is important that designers understand fully the implications of using each type. The challenge is to try to harmonise productivity with machine safety and user-friendliness. If a safeguard is not user-friendly, there may be a temptation to manipulate it, putting the machine operator at risk. Designers need to identify potential cases in which safety devices might be manipulated and then to eliminate these at the machine development stage.

Certain considerations will help the designer to decide which components should be used for the safe monitoring of movable guards in accordance with EN 60947-5-3, Low-voltage switchgear and controlgear - Part 5-3: Control circuit devices and switching elements - Requirements for proximity devices with defined behaviour under fault conditions. Typical questions that should be asked include: Is the cover frequently opened and closed? Does the process need protecting as well as the operator? Are components exposed to high vibration or extreme temperatures? Are they used in potentially explosive areas?

Mechanical safety switches

For simple safety gate applications where there is no increased risk of manipulation, mechanical safety gate switches are widely used. These are used for simple safety gate monitoring or with a mechanical guard locking device, which uses increased extraction force on the actuator to prevent the gate or cover from being opened unintentionally. Many applications require two mechanical switches to guarantee the required level of safety, which increases engineering costs significantly.

Another consideration here is that swarf can quickly be deposited on the switch or actuator, restricting its function. Also, if safety gates drop over the course of time, any misalignment could damage a mechanical switch housing and/or actuator.

Mechanical safety bolts comprise a safety switch, handle and bolt. These provide protection where safety gates are difficult to adjust, as well as on safety gates that are frequently opened and closed. A mechanical guide on the actuator ensures that the actuator engages with the safety switch correctly when the safeguard is closed.

Mechanical hinge switches provide an elegant concept for rotating or hinged gates and flaps. These provide better protection against manipulation compared to other mechanical solutions because they are concealed within the hinge; they also save on installation space.

Non-contact safety switches

Gates, covers and flaps often need to be opened repeatedly for operational or assembly purposes, so that operators can access or reach into the protected area. Here, the risk of a gate being mechanically misaligned is high.

The disadvantage of mechanical guard locking devices is that they only have limited tolerance to misalignment. For this reason, non-contact safety switches provide an attractive alternative, particularly if there are high vibrations. These switches have a higher response tolerance, allowing greater flexibility in the way they are attached to the machine.

Rugged non-contact switches are also suitable for applications that require protection types higher than IP67. These switches are able to meet safety requirements up to PLe of EN ISO 13849-1, and SIL 3 of EN/IEC 62061, or Category 4 of EN 954-1.

When it comes to deciding on the operating principle of the sensor, important distinctions need to be made. While magnetic safety switches provide only limited protection against manipulation due to the technology used, coded safety switches achieve up to 100 per cent protection against manipulation. As a result, magnetic safety switches should be concealed or installed in such a way that they are invisible to operators.

If a safety switch cannot be concealed or if 100 per cent protection is required, RFID coded safety switches are preferred. RFID technology enables a clear assignment to be made between sensor and actuator. RFID coded safety switches operate in accordance with the 'key lock principle' whereby only one key – a unique version – fits a lock. Given that the actuator is installed using one-way screws – which can only be loosened by means of a special tool – a very high level of protection against manipulation is achieved.

It is also important to understand that magnetic safety switches have limited suitability if metal swarf is present, as the swarf can be attracted to the magnet and deposited on the switch, leading to operational problems or even failures. Although a safety-critical situation cannot arise as a result, plant availability and productivity will suffer. Here, coded switches are better, as RFID is unaffected by metal swarf.

Various coding types mean that different levels can be achieved in terms of security against manipulation. The greatest protection is provided by systems in which an actuator is uniquely assigned to a switch. Sensors with 'normal' coding are also available if the demands on manipulation protection are less stringent (one lock, many keys).

Gate closed in case of danger

Risks can remain even after a machine has been shut down, usually due to moving parts. In 'hazardous overrun' cases such as these, the use of a safe guard locking device is mandatory. This means that the gate cannot be reopened until standstill has been safely detected or a time delay has elapsed. Here, mechanical guard locking devices with solenoid interlocks are typically used. The solenoid interlock guarantees that the gate can only be opened when it is safe to do so.

Integrated systems

More can be achieved by using integrated safety gate systems; these safety units combine safe interlock, monitoring and control functions, plus additional functions such as emergency stop and escape release. All components – whether mechanical or electronic – are integrated.

Complete systems such as these help to reduce the time and costs associated with project configuration, design, documentation, purchasing and installation. Furthermore, if sensors and evaluation devices are sourced from a single supplier, this means that the interfaces between the various components are already compatible.

Pilz offers the PSENsgate safety gate system that may be used up to PLe of EN ISO 13849-1, SIL 3 of EN/IEC 62061 and Category 4 of EN 954-1, even when connected in series with other devices. Follow the link for more information about the Pilz PSENsgate safety gate system.

13 June 2011

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