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\end{table}
\subsection{Discussion of the Proposed Primary Failure Modes}
{\em
'No-beam':} 'No-beam'} events are defined to start when the beam current drops below a given limit.
For several facilities this is currently not the case.
Table~\ref{tab:pf-limits}
shows, shows that ALBA, BESSY II and LNLS-UVX do consider a closure of
the photon shutters by an interlock equivalent to a loss of the electron beam.
Other facilities, like the SLS do not have these kind of interlocks.
We propose that these events
should can be distinguished: a 'no-beam' event would be only
defined by
the loss of beam
current; current and
an additional a 'photon-shutter-closed' event would be
recorded
only if the photon shutters are forced close while there is not a 'no-beam' event.
A similar
problem situation applies to the end of a 'no-beam' event.
A 'downtime' often only ends after the photon shutter or insertion device control is
given back to the users, which is not compatible with the 'no-beam' rule.
Again this can be solved by having additional event types for 'photon-shutter-closed' or
'insertion-devices-blocked-open' while there is not a 'no-beam' event.
This solution requires then to record
in addition if the event was preceded by another failure:
if the photon-shutters
get an interlock, are interlocked, the beam is then dumped and afterwards it takes five
minutes until the insertion-devices are unblocked, then it should be visible from the failure
data that this
were was not three
but rather only one individual interruption in user operation.
We've The authors decided to allow arbitrary limits for 'no-beam' events,
but in reality this does not make a large difference:
situations are rare
at the evaluated facilities where the current drops from
100$\,$mA below 50$\,$mA the nominal beam current to less than 50\% but not to zero.
{\em
'Low-beam-current':} 'Low-beam-current'} events can vary significantly with the definition of the $I_{\mbox{tol}}$ limit.
At SPring-8 already a beam decay of about 0.1\% causes a 'low-beam-current' event, while at BESSY II up
to 9\% additional beam decay is still tolerated as insignificant for the users.
The comparison of the number and duration of 'low-beam-current' events for BESSY II and
SPring-8 won't be useful therefore.
At least Nevertheless comparing the tolerated beam decay is still
useful;
and for useful.
For those facilities where the limits are similar, the failure rates will allow a
meaningful comparison of the reliability of the
injector system. injection process.
