Safety of Machinery and Control Systems - Directive for

Safety of Machinery and Control Systems Directive for Surge Protection of Control Systems
EN/ISO 13849-1
Harmonized in the Directive on Machinery since May 2007
EN ISO 13849-1 follows EN 954-1 as main standard for the design of safety related control systems in
the field of “Machinery Safety”.
The standard, ratified in November 2006, has been harmonised in the Directive on Machinery since May
2007. The previous standard EN 954-1 is thus formally withdrawn, however, it may still be used until the
end of December 2011, although there is a controversial discussion among experts whether EN 954-1, in
spite of its harmonization, still is the state of the art.
In addition to the generally approved statements of the previous standard, especially the considerations
of probabilities and the possible consequences are new. Additional requirements for programmable
electronic systems have been defined and a so-called Performance Level (PL) has been introduced.
Basically the hazard situation affects the achievable PL (see graphic). The intensity of each hazard situation is scaled into five steps ranging from “a” to “e”. At PL “a” the control function is just a little contribution to reduce the risk, whereas it is high at PL “e”. The higher the risk, the higher the requirements for
the control system, the more im-portant are the safety related measures to avoid failures.
In order to determine the achieved Performance Level the safety function is to be structured in three
parts, namely the sensor, the logic and the actuator. Similar to the Safety Integrity Level (SIL) of IEC EN
61508 calculations to determine the failure probability are made in order to provide further measures
for the safety of each subsystem, if necessary. In this context the systems or sub-systems often are also
designed redundantly.
Even if the fail safety of each sub-system has already been optimised, or sub-systems are used redundantly, simultaneously arising faults may have serious consequences in different parts.
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One single cause, such as a lightning strike, often can have a lot of concurrent effects. In order to prevent
such causes and their effects, the Common-Cause-Failure-Management (CCF) is described in EN 13849-13.1.6. It refers to systems designed with at least two or more channels.
For avoiding that, due to a common cause, different parts of the system fail at the same time, the CCF
approach offers a simple method.
List of measures with points system (maximum: 100 points)
- Separation of signal paths
15 pts
- Diversity 20 pts
- Protection against surges, overpressure, etc.
15 pts
- Approved components 5 pts
- FMEA 5 pts
- Competence/training of the developers 5 pts
- EMC or filtering of the pressure medium and
Protection against pollution 25 pts
- Temperature, humidity, shock, vibration etc.
10 pts
There is a point rating of measurements which contribute to the general safety of the channels. Out of 100
possible points at least 65 are to be achieved.
Strikingly, already 15 points are granted for surge protection, which is almost one quar-ter of the required
65 points.
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Surge protection
for increasing the safety of machinery
EN ISO 13849 describes the “Safety of Machinery”
Part 1 - Safety-related parts of control systems
For risk minimisation a necessary performance level PL is specified for the system.
EN ISO 13849-1
Table 3
Performance Level
(PL)
Probability of a dangerous failure per hour
10-4
10-5
a
10-6
b
c
Protection of
low risks
10-7
d
10-8
e
Protection of
high risks
Systematic evaluations of the achieved PL. One of the resulting measures is among others
the so-called Common-Cause-Failure-Management (CCF).
Purpose for 2-channel systems is to minimise the probability that just one single cause may turn both
channels at the same time into a safety critical state, e.g. a lightning strike!
Surge protection
for increasing the safety of machinery
EN 13849-1-3.1.6: Common-Cause-Failure-Management
An evaluation method of the measures taken against CC Failures is the scoring according to
Table F1
CC Failures of different parts due to common causes
List of measures with points system (maximum: 100 points)
- Separation of the signal paths
- Diversity
- Protection against surges, overpressure, etc.
- Approved components
- FMEA
- Competence/training of the developers
- EMC or filtering of the pressure medium
and protection against pollution
- Temperature, humidity, shock, Vibration etc.
Target: at least 65 points
15 pts
20 pts
15 pts
5 pts
5 pts
5 pts
25 pts
10 pts
There is a points rating of the
performed measures.
Out of a possiblity of 100 points
the aim is to achieve at least 65!