Feature Machine Safety
Traditionally it was an accepted solution to use electromechanical key actuated guard switches on doors to protect the operating safety of personnel. That is until now, as SICK’s Martin Kidman explains
ue to recent changes to EN ISO 13849-2 (Validation) and the forthcoming EN ISO 14119, machine builders and end users should consider additional or alternative measures when modifying old machines or designing new ones. 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.) - see Figure 1. 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 (e.g. 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 by the industry 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. However, the 2012 additional entry in Annex D means one safety switch with a single mechanical link is no longer a sufficient means to achieve PLe on a door because it is normally not possible to exclude broken actua- tors from faults. The same applies to the lesser level PLd unless a full justi- fication is provided in accordance with the standard.
EN ISO 14119
Later this year EN ISO 14119 will super- sede EN 1088 as the main standard for safety doors (Safety of Machinery - Interlocking devices associated with guards - Principles for design and selec- tion). This new standard, according to ISO/DIS 14119 (E), will contain the fol- lowing information that should be taken into consideration - Clause 8.2 Assessment of faults, ‘Broken actuators are normally not possible to exclude from faults. For example: Where an interlocking system requires PLr = e.
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Closing the door on unsafe practice D
The same applies for required 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 incorpo- rate locking. However, to save having to use two locking devices, EN ISO 14119 states in Clause 8.5 Fault Exclusion, that ‘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 bounc- ing 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 con- junction with a non-locking device (non- contact, key activated etc). It should be noted that ISO/DIS 14119 is a working document and EN ISO 14119 has not been released yet at the time of writing.
Above: Figure 1 - a dual channel safety switch (2 N/C contacts)
Below: Table 1 - Performance Level ranges (EN ISO 13849-1:2006)
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. 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 mag- netic 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 safety device such as a key actuated switch could be used, or a magnetic coded switch which would 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’ with- out having to worry about fault mask- ing. 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.
Summary
When modifying or designing new machines that have door safety requirements with a required perfor- mance level of PLd or PLe, it is 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 appli- cations incorporating a single channel device is to connect a non-contact safety switch in series with the electro- mechanical device.
For new applications, choosing a Switch selection
There are two types of electromechani- cal key actuated switches - with or without solenoid locking. The choice of device depends on a number of fac- tors such as the inertia of a machine
Type 4 over a Type 2 device will achieve the required PL and, for appli- cations that require solenoid locking, a second device must be used which does not have to be locking also.
SICK
www.sick.co.uk T: 01727 831 121
Enter 217 JULY/AUGUST 2013 Electrical Engineering
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