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Appendix: Safety of machinery
The PL must therefore always be calculated for an entire safety function from the individual compon-
ents or components.
Instructions for simplified determination of the PL for a safety function comprising several components
can be found in the standard è EN ISO 13849-1 (Chapter 6.3, Appendix H and Appendix I).
It should be noted that in the case of a series connection of safety components the component with
the lowest PL in the safety function determines the PL of the safety function. Thus, a safety function
composed of 3 components with category 4 PL e, category 3 PL d and the third component with cat-
egory 2 PL c specifies PL c for the entire safety function. It is also important that an error results in the
loss of safety although category 4 PL e components are integrated in the safety function, since one of
the components used only conforms with category 2.
If several PLs linked, the total PL can be reduced èEN 13849-1 (chapter 6.3).
Evidence that an error does not result in loss of safety can be established with the aid of a FMEA (Fail-
ure Mode and Effects Analysis), in which all possible assumed errors are tested theoretically or prac-
tically and it is demonstrated that the requirements of the category is enough.
14.7
Use of handheld terminals in special operating modes
In the case of manual control of machines in special operating modes, where safety depends on the
prompt response of the operating personnel, it is essential that the operating area can be viewed by
the operator. The handheld terminal offers the advantage of being very close to the operating area. At
the same time, mobility increases the risk of misuse because the handheld terminal can be used to
start machine movements consciously or unconsciously even at more remote locations where the
operating area can no longer be perceived. By selecting the appropriate cable length for the handheld
terminal, the machine operator must therefore find the right compromise between the necessary flex-
ibility and a reasonable working range limitation. The working range of wireless handheld terminals
cannot be restricted with the cable; therefore, additional technical solutions are required for this type
of handheld terminal.
If the machine or system is operated with the handheld terminal, it must be ensured that it can only be
operated from the handheld terminal and not from any other point in the system. The danger zone
may only be entered by the person operating the handheld terminal. If more than one person is work-
ing in the danger zone at the same time, each person present needs a 3-position enabling switch. The
movement may only be enabled if all 3-position enabling switches are actuated.
14.8
Notes on the switch for stopping in an emergency (emergency stop func-
tion)
Theoretically, a perfectly designed machine should not require an emergency stop function, since the
machinery directive requires that a machine is safe in all phases of its life and operating modes.
However, it is known from practice that unforeseeable situations occur despite all precautionary
measures. To enable the machine to be stopped quickly in these cases or to eliminate the danger, an
emergency stop device is provided on most machines.
As can be seen from the following illustration, a machine is constantly in a dangerous state from the
point of view of the emergency stop as long as it is not actuated.
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Festo — CDSA-D3-RV — 2020-08
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