Electromechanical guard locking devices in practice Proven Systems Proven Safe

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1 PRESS INFORMATION Electromechanical guard locking devices in practice Proven Systems Proven Safe Electromechanical guard locking devices are proven safety components to safeguard hazardous locations on a machine. As these safety components prevent access to the hazard, and processing in the machine is also protected against interruption, traditional guard locking is still a popular and frequently used safety precaution for machines and installations. The new EN ISO14119 standard therefore distinguishes between two different types of guard locking, namely guard locking for process protection and guard locking for personnel protection. The first type of guard locking merely serves to prevent interruptions in the working process. No demands are made on the safety technology with regard to the guard locking function in this case. However, at least all requirements for an interlocking device must be met. The machine therefore does not have to stop safely when the guard locking device is opened, but instead, only when the safety door is opened. Guard locking devices for personnel protection prevent people from entering a machine as long as the hazard has not been eliminated. The machine must have already stopped safely when the guard locking device is unlocked. High demands Standards place various demands on these safety components. The first demand, naturally, is the reliable monitoring of the guard locking device's position. Another demand concerns the guard locking principle. The standard makes a distinction between four different Page 1 of 7

2 principles. Two of these principles are suitable for guard locking devices for personnel protection. The first of these two options is the principle of "actuated by spring force, unlocked with energy on." A mechanical spring moves the locking device to locked position, and the guard locking is released by applying voltage to a solenoid. The door remains reliably locked upon complete shutdown of the machine, if the safety guard was closed. The machine cannot be entered in this case. In case it is necessary to open the machine in this situation, e.g. to clean it, the EN ISO14119 standard calls for the installation of additional unlocking options or the use of a different principle. Bistable principle A further possible principle for guard locking with personnel protection function is called "unlocked with energy on, locked with energy on." With this type of guard locking device, the locking device remains in the position it was in when the power supply was switched off and does not change status. So-called bistable versions of such guard locking devices, e.g. the STP-BI model, have been available from Euchner for years. In practice, the machine is frequently also stopped when the guard locking device is opened, even if process protection is adequate. However, the demand for one of the above mentioned principles does not have to be met in this case. The two additional principles are "actuated with energy on, released by spring force" and "actuated with energy on, released with energy off." With both of these principles, it is possible to open the door and thereby perform cleaning and servicing work easily when the machine is switched off, or in case of a power failure. Different Performance Levels A safety door can be secured in any desired or required Performance Level (PL) in accordance with EN ISO , using an electromechanical guard locking device for personnel protection. It is therefore not always necessary to use a modern guard locking device with integrated electronics. All that is necessary is the correct circuit technology and the correct selection of safety components. Three practical examples demonstrate how this can be implemented. Only one guard locking device, whose two contacts are connected to a safety evaluation system, is used in many applications for Category 3, as shown in Figure 2. This circuit clearly meets the main requirement of Category 3 for a redundant design, because the electrical system is designed with two channels throughout. The diagnostic function also seems to be adequate at first glance. What about the mechanical aspects? What if, for example, the actuator on the guard locking device breaks? Then this circuit would fail on occurrence of the first fault, namely failure of the mechanical equipment. In turn, this means that not all demands from Category 3 can be met after all. The EN ISO standard not only describes the electrical system of a safety circuit; the mechanical, hydraulic Page 2 of 7

3 and pneumatic aspects are also listed. Nevertheless, the example represents a correct application of a guard locking device and can also satisfy Category 3. The key to this is a socalled fault exclusion, i.e. the finding that a certain fault cannot occur. These exclusions are permissible in EN ISO Part 2 of the standard includes long lists concerning the various concepts. In addition to the possible faults, it can also be determined whether the associated fault exclusion is permissible. In order to execute a fault exclusion for failure of the mechanical equipment, it is necessary to rule out the occurrence of this fault. For example, the mechanical equipment can be protected in such a way that no forces able to cause a failure can act on the guard locking device. If this can be ensured, a fault exclusion is easily justified and permissible. List of fault exclusions A good guide on the topic of fault exclusion has long since been available from Euchner in the form of the "Proven Systems Proven Safe" brochure (Figure 3). This overview lists all faults that can occur on an interlocking device or a guard locking device. Furthermore, the brochure contains information on provisions in the standards to justify a possible fault exclusion. The EN ISO standard does not permit fault exclusion for failure of the mechanical equipment in PLe for interlocking devices designed with electromechanical switches. This standard does not address guard locking devices. This has now also been regulated in the new EN ISO14119 standard. In case of guard locking devices, a fault exclusion for the locking mechanism of a guard locking device is possible in PLe, depending on the risk assessment. However, this is possible only for the locking mechanism, i.e. the bolt that absorbs the actual mechanical force. This does not apply to the interlocking function included in every guard locking device. The above circuit, which includes a fault exclusion, therefore cannot achieve PLe. Instead, it can achieve only PLd. Page 3 of 7

4 Category 3 without fault exclusion If such a fault exclusion cannot or must not be made, an additional sensor must be used in the second channel. It this sensor's task to issue at least one more switching signal to switch the machine off in the event of a mechanical defect in the guard locking device. In case of guard locking devices, the question arises as to what kind of sensor must be used for the second channel. Ultimately, this question can be answered only by a risk analysis. However, one-time opening of a safety door in combination with run-on of a machine is acceptable in many cases. In other words, if the guard locking device no longer functions due to a mechanical defect, at least a signal to shut the machine down must be issued when the safety door is opened. This function can be realized with a simple second sensor, e.g. a position switch from Euchner, which does not include a guard locking function or safety function. An example of this is shown in Figure 4. The additional sensor P1 is connected in series with one of the two contacts that monitors the position of the guard locking device. As a result, only one of the channels switches off if the guard locking device is defective a fault is detected by safety evaluation unit K1, and the machine no longer starts up after the safety door is opened. Information on calculating the PL: In this example, the block diagram used to determine the failure probability is identical to the block diagram with fault exclusion in Figure 2. The additional sensor is not part of the safety function and therefore does not affect the failure probability. The sensor is only necessary for achieving the diagnostic coverage. Guard locking in Category 4 The drawback of the above circuit is that failure of sensor P1 cannot be detected. The circuit therefore achieves only Category 3 and not Category 4. If failure of P1 has to be detected as well, the circuit is much more complex. The circuit diagram for this is shown in Figure 5. A second safety evaluation unit and another contact in the guard locking device will now be needed to detect failure of P1 as well. Safety evaluation unit K1 performs this function in this example. The evaluation unit K2 monitors the guard locking device position using two Page 4 of 7

5 channels. This design is very complicated. The same result can be achieved much more easily using a safe control system, because both the position switch and the guard locking device can be connected directly to the control system. Guard locking for personnel protection The standards EN ISO14119 and EN ISO were already mentioned in the examples above. EN ISO13855 is another standard that refers to interlocking devices and therefore also to guard locking devices. EN ISO14119 references it if a decision has to be reached as to whether a guard locking device is required for personnel protection, or whether an interlocking device is sufficient. The selection is made according to a simple flowchart (Figure 6). Monitoring the locked position As already mentioned, the position of the locking device must be monitored for a guard locking device for personnel protection. This ultimately means that the position of the locking mechanism is monitored. As has also been mentioned, every guard locking device also includes an interlocking function. The basis for this is that two completely different safety functions are realized in the same component. One of them is obvious: the machine switches off when the guard locking device is opened. Another function associated with the interlocking function is the prevention of an unanticipated machine start-up. As long as the safety door is open, the machine must be incapable of starting up. So-called protection against unintentional closing is integrated in most, but not all, guard locking devices for this purpose. This mechanical function ensures that the guard locking device can lock only if the safety door is actually closed. This ensures that the door is closed and locked even if the Page 5 of 7

6 guard locking device was activated at the wrong time. Conversely, this function also ensures that the interlocking function is active whenever the guard locking device is active. For this reason, it is sufficient if the contacts for monitoring the locked position are integrated into the safety circuit. The locking position is thereby automatically monitored as well. Only a guard locking device featuring this function is suitable as an interlocking system without extra effort being necessary. Electromechanical guard locking devices are still up-to-date products, offering many advantages. The one-time effort required to integrate and assess these safety components properly is surely higher than that required for the electronic products. But the reliability of the mechanical equipment and the effectiveness of these products speak for themselves. Figure 1: Overview of the EUCHNER range of electromechanical safety switches with guard locking Figure 2: Guard locking in Category 3 with fault exclusion Figure 3: "Proven Systems Proven Safe" brochure Figure 4: Guard locking in Category 3 with additional sensor Figure 5: Guard locking in Category 4 with second safety evaluation unit Figure 6: Guard locking intended for personnel protection EUCHNER More than safety. [Characters with blanks: 11,425] Page 6 of 7

7 A brief profile: EUCHNER GmbH + Co. KG EUCHNER GmbH + Co. KG in Leinfelden is an international family business and employs more than 600 staff worldwide. Fifteen sales subsidiaries, of which nine are in Europe, four in Asia and two in North and South America, cover the world along with 24 authorized sales offices. The Swabian company is managed by Mr. Stefan Euchner. Switchgear has been developed at EUCHNER for more than 60 years. These devices are used primarily in the field of mechanical engineering. The company has a leading position in safety products. EUCHNER safety switches monitor the position of safety guards on machines and installations electro-mechanically and electronically with a high degree of reliability. Further information on the company is available on the Internet at EUCHNER GmbH + Co. KG Kohlhammerstrasse Leinfelden-Echterdingen Germany Tel Fax info@euchner.de Press contact: Dipl.-Betriebsw. (BA) Frank Kretzschmar Group leader Marketing Tel Fax press@euchner.de Page 7 of 7