ISO/DTR 24119:
Il nuovo Standard per il calcolo del PL di serie di dispositivi di interblocco

This is currently under committee review, with an expectation that it will be available quite soon as ISO/TR 24119. 

This TR has not been published but it will deal with.

The Technical Report illustrates and explains principles of fault masking in applications where multiple interlocking devices with potential free contacts are connected in series to one logic unit which does the diagnostics. 

It provides also a guide how to estimate the probability of fault masking and the maximum DC for the involved position switches.

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 maximum PLc. 

It remains to be seen exactly what ISO/PDTR 24119 has to say about the maximum PL achievable when several infrequently operated guards are connected, but it is very likely to be PLd where careful analysis is possible (for example, the number of guards, the type of switches, type of wiring, distance between guards, and accessibility of guards), otherwise it is more likely to be PLc. 

It is definitely not possible to achieve PL e with more than one guard switch connected in series, at least not when using volt-free based interlock switch technology.

ISO/TR 24119
Safety of machinery - Evaluation of fault masking serial connection of guard interlocking devices with potential free contacts.

Fonte: Pilz


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http://www.iso.org/iso/catalogue_detail.htm?csnumber=63160

http://www.machinebuilding.net/ta/t0402.htm