EN954-1: It's time to let go
January 1st 2011 The death throes of the EN954-1 safety standard have been
long drawn out and it has unexpectedly rallied on more
than one occasion.Now,however, the date of its final
demise has been fixed and system integrators and machine
builders across Europe must start to deal with this change
without delay, says Paul Laidler of Laidler Associates
It has been accepted for many years that
EN 954-1, 'Safety of Machinery, Safety
related parts of control systems', which
was introduced in 1992, needed to be
replaced. It has many shortcomings, just one
example of which is its failure to deal with
the programmable safety systems that are
now becoming so popular. Accordingly, a
new standard, EN ISO 13849-1 was
produced and was published in October
2006. It was planned that it would replace
EN 954-1 in December 2009.
In the event, the new
standard was itself revised –
the current version is EN ISO
13849-1:2008 – and, in
response to objections from
machine builders and other
interested parties, it was
agreed that, although the
new standard had been
published, EN 954-1 could
continue to be used until
the end of 2011.
So what's new?
In a sense not a lot, except
that the end of 2011 isn't very
far away, and the changes introduced by the
new standard are numerous. Even the
Machinery Directive Working Group, which is
not a body noted for exaggeration, has said that
the transition 'represents a drastic evolution
in the safety philosophy for control systems.'
In other words, those machine builders who
have been burying their heads in the sand
hoping that EN 954-1 would live forever need
to start taking action now or, in just a few
months, they will have a serious problem.
Why the fuss?
The truth is that EN ISO 13849-1 adopts a
totally different approach from EN 954-1.
This means that achieving compliance with
the new standard isn't merely a matter of
tweaking the existing documentation;
instead it means starting over from scratch,
and it's by no means a simple task.
The approach to safety used by EN ISO
13849-1 is based on probabilities. Gone are
the familiar safety categories of EN 954-1,
replaced by Performance Levels (PLs). These
PLs relate directly to the probability of a
system failing to danger. To achieve PLa, for
example, the average probability of a failure
to danger per hour must be in the range
>10-5 to <10-4, while for PLe it must be in
the range >10-8 to < 10-7.
This is all well and good, but how are
those probabilities determined? The most
usual answer is that they are calculated on
the basis of MTTF (mean time to failure)
data for the components used in the safety
system. Until recently, this was a large and
sometimes insurmountable problem, as the
necessary data was often not available from
the component suppliers. This issue has now
largely been addressed, and all major
suppliers are now offering libraries of data
specifically for use with the new standard.
Even when the relevant data is available,
however, it would be misleading to pretend
that carrying out the calculations required by
EN ISO 13849-1 is a straightforward task. To
make things a little easier, several organisations
have produced software packages that guide
users through the process. Some of the
packages are produced by commercial
organisations and understandably they may
have a commercial or proprietary bias.
For those who may wish to avoid such
bias, a useful package has been produced by
a non-commercial body – SISTEMA from
IFA, the institute for research and testing that
is associated with the German insurance
industry. The package, which describes all
aspects of the analysis procedure contained
in the standard for determining the
probability of failure of control systems, can
be downloaded free of charge from the IFA
English website:
www.dguv.de/ifa/en/pra/en13849/index.jsp.
In spite of the availability of software
support, ensuring compliance with EN ISO
13849-1 is still going to be a task which few
system integrators or machine builders will
want to undertake for themselves, or indeed
have the resources available to do so. This
may change as the standard beds in and the
concepts it embodies become more familiar.
At the moment, however, a good case can
be made for calling on the help of consultants,
such as Laidler Associates, who have made a
detailed study of the new standard and who
can provide dependable advice and
assistance.While it may undoubtedly appear
that money can be saved by going it alone,
it's worth remembering that, as always when
it comes to matters involving standards,
getting it wrong can prove very expensive!
The transition from the familiarity of EN
954-1 to the relatively uncharted territories
of ISO EN 13849-1 is not something anyone
is anticipating with relish, but it's happening
for good reasons and it's not going to go
away. The message has to be to invest now in
making the appropriate preparations; a little
money spent on consultancy may well save a
lot of time, trouble and expenditure later. More articles from Laidler Associates: | 
