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Are you switched on to the new door safety standards?
Important changes to safety standards for door and gate switching have set tougher rules that all designers, builders and operators of machinery must be aware of. Martin Kidman, machinery safety specialist at Sick UK, explains.
EN ISO 14119 (Safety of Machinery – Interlocking devices associated with guards – principles for Design and Selection) replaces EN 1088 which covered the use of safety door switches used in interlocking safety systems, electro-mechanical safety switches, Type 2 Coded Cam Switches and non-contact safety switches. The old standard was purely European, whereas the new standard is published by ISO and therefore it is valid worldwide.
Underlying the new standard is the principle that electromechanical key-actuated guard switches on doors are no longer acceptable as a comprehensive solution to protect personnel and achieve the highest levels of safety with the recent changes to EN ISO 13849-2 (Validation) and the new EN ISO 14119. The main focus is to minimise any possibility of safety switches being disabled by operator manipulation and a new symbol [ ] must be applied to guard locking devices which comply.
Safety engineers were previously assured that the use of a dual channel circuit with cross circuit monitoring and a single mechanical link was acceptable for high levels of safety (Category 4, PLe etc.). However, the latest version of EN ISO 13849-2: 2012, Part 2: Validation has an additional entry in Table 8 of Annex D (Faults and fault exclusions) which states that: “For PLe, a fault exclusion for mechanical (for example the mechanical link between an actuator and a contact element) and electrical aspects is not allowed. In this case redundancy is necessary.”
The EN ISO 13849:2006 standard is used to help people comply with the Machinery Directive (98/37/EC) and it covers the safety-related parts of control systems (SRP/CS) on machines. Looking at the ability of SRP/CS to perform under foreseeable conditions, the standard assigns the system one of five ‘Performance Levels’ (PL a to e); a PL is defined in terms of probability of dangerous failure per hour. So the 2012 additional entry in Annex D means one safety switch with a single mechanical link is no longer sufficient means to achieve PLe on a door because it is normally not possible to exclude broken actuators from faults. The same applies to the lesser level PLd unless a full justification is provided in accordance with the standard.
This new standard contains the following information that should be taken into consideration, Clause 8.2 Assessment of faults. Where an interlocking system requires PLre in accordance with ISO 13849-1 or SIL3 in accordance with IEC 62061 a minimum fault tolerance of 1 is required. In order to achieve this it is normally not justifiable to exclude faults, such as broken actuators. The same requirement applies for PLr d and SIL 2 unless a full justification is provided in accordance with ISO 13849-1 or IEC 62061. This clause reinforces the change in EN ISO 13849-2:2012, i.e. for PLe applications, one mechanical link is no longer sufficient.
The EN ISO 14119 standard goes on to talk about interlocks that incorporate locking. However, to save having to use two locking devices, EN ISO 14119 states in Clause 8.5 Fault Exclusion: “There should be a proper selection of the device ensuring that the force of the guard locking device is sufficient to withstand forces on the locking bolt and that shearing forces on the locking bolt by a bouncing action of the movable guard are prevented. In this case, the use of fault exclusion for breakage of the locking bolt does not necessarily limit the PL or SIL for the guard locking function.”
In other words, fault exclusion on breaking of guard locking is permissible for PLe and SIL 3 applications by installing a single locking device in conjunction with a non-locking device (non-contact, key activated, etc).
There are two types of electromechanical key actuated switches; with or without solenoid locking. The choice of device depends on a number of factors such as the inertia of a machine and its stopping time. For each type of switch, there are measures that can be introduced in order to comply with the new requirements.
Without solenoid locking, there are two ways in which the safety of doors can be improved if locking is not required. You can either double up to achieve redundancy by using two Type 2 devices (as described in DIS/ISO 14119) or use a Type 3 or 4 device. A key actuated electromechanical switch is a Type 2 device, i.e. single channel. One solution is to opt for a device of a higher type such as a magnetic coded switch or a transponder which are both non-contact coded switches with dual channels.
With solenoid locking, if solenoid locking is required, then a non-contact device is impractical as the door needs to be kept in place. As described earlier there is no need to use two solenoid locking devices so an additional device such as a key actuated switch could be used, or a magnetic coded switch which could be more practical and cost effective. It should also be noted that if two devices are used on a door, the switches can be ‘daisy chained’ without having to worry about fault masking. The reason for this is that both devices would be connected to the same door and therefore would always be actuated at the same time.
In summary, when modifying or designing new machines that have door safety requirements with a required performance level of PLd or PLe, it vital to take account of the recent changes and the impact that the choice of switch could have on compliance. It is no longer acceptable for one mechanical link to be used for PLe applications and for PLd, a full justification for one mechanical link must be provided in accordance with EN ISO 13829-1.
For most of us, the simplest way to achieve compliance on existing applications incorporating a single channel device is to connect a non-contact safety switch in series with the electromechanical device. For new applications, choosing a Type 4 over a Type 2 device will achieve the required PL and, for applications that require solenoid locking, a second device must be used which does not have to be locking also.
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