Feature Machine building, frameworks & safety EN ISO 14119 explained When published, EN ISO 14119 will not only
provide designers with more comprehensive and up to date guidance on interlocking of guards, but it will clear up some of the confusion surrounding the application of EN ISO 13849-1 to safety functions which involve interlocked guards. David Collier, business development manager at Pilz, and chairperson of the Machinery Safety Alliance, explains
T
he final draft of what is to become EN ISO 14119 is being voted upon. Once harmonised to the Machinery Directive, it will replace the older EN 1088 for interlocking devices and their application, providing greater clarity on the use of state-of-the-art technologies and practices which have evolved since EN 1088 was originally published. But what kinds of topics will be covered within the new standards?
One example is the phenomenon of ‘fault masking’ (unintended resetting) when connecting multiple volt-free contacts in series into a safety monitoring device. In simple terms, if we have more than one frequently opened guard – once per hour – the level of Diagnostic Coverage falls to zero, which in EN ISO 13849-1 results in a max PLc. The same applies if there is one frequently opened guard connected in series with more than four infrequently opened guards. The new requirements will force us to consider how to handle guard monitoring, and to look at technologies which aren’t volt-free contact based (e.g. have built in diagnostics such as RFID types), which have network diagnostics, or which employ distributed/decentralised I/O.
Interlocking devices
The standard will also address the topic of defeating of interlocked guards, and offer guidance on the various types of interlocking device now available: • Type 1 uncoded mechanical switches (e.g. Limit switches) • Type 2 coded mechanical switches (e.g. Tongue actuated interlock switches)
•Type 3 uncoded non-contact switches (e.g. Inductive or capacitive proximity switches/sensors used to sense safe position)
• Type 4 coded non-contact switches (e.g. Magnetically actuated or RFID actuated switches).
Guard locking Guard locking types have also been expanded from power to lock or power to
release, to include bistable locks where power can be applied to lock and release a solenoid guard switch. Furthermore, it considers the circumstances under which the use of electromagnetic locks are allowed for machine safety – for example, taking into account the distance to the hazard, the stopping time in the event of power loss, monitoring the holding force and providing clear indication when forced entry has been attempted.
Guard locking switches not only prevent entry by maintaining the locking of the guard, but also prevent the interlocked actuators (typically contactors) from being energised until the guard is closed to prevent unexpected startup. The former safety function will almost always have a lower PL than that of the latter, although it is application dependent and should be assessed.
For example, even if the machine is running and the guard is open, a person outside the machine might be able to see the hazard and therefore avoid it (hence max PL d for locking). But, if they are inside the machine for maintenance, they won’t be able to avoid the hazard in the event of unexpected startup (hence a likely PL e for the detection that the guard is open). This won’t be the case, however, where hazards are not apparent to the person standing outside the open guard – such as where parts might be ejected or invisible phenomenae are present, ie. ionising radiation. In this case the locking function will be just as critical as the prevention of unexpected startup.
Fault exclusions
The use of fault exclusions has long been covered in EN 62061 (max SIL 2), TR ISO 23849 (PLd) and now also in EN ISO 13849-2 – Annex D.8 a single mechanical point of failure (the tongue or cam) can not be fault excluded for PLe. This limitation to PLd for fault exclusions now appears in ISO 14119. To achieve PLe, using at least two devices is mandatory. As a result we are seeing more non-contact devices being used for PLe as they have no single mechanical point of failure. Interestingly, the locking function, although dependent upon a single mechanical channel (the tongue), is allowed to perform up to PLe with the proviso that it is defined as locking up to a maximum stated extraction force. The manufacturer must demonstrate this through repeatable tests at 130% of the specified hold force, e.g. for 2500N, test at 3250N.
Additional topics covered
Additional topics include emergencies where persons are trapped behind the closed guard. Here, the means of emergency release from the outside and emergency escape from the inside on guard locking switches will also be standardised.
Furthermore, for where some interlocked guards aren’t opened often, 14119 specifies for PLe a monthly test and for PLd a 12 monthly test. This is important, even in dual channel systems, because faults can only be revealed by placing a demand on the guard. Guidance will also be given on the levels of holding force, in Newtons, required in typical applications. Levels of coding to prevent defeat are defined: high/unique 1000 variants covering RFID systems, medium 10 to 1000 variants covering trapped key systems, and less than 10 variants covering magnetic types and reteachable RFID types.
Pilz
www.pilz.co.uk
Machinery Safety Alliance
http://msawww.com
12
Enter 205 Enter 206
OCTOBER 2013 Design Solutions
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60