Ashcroft A Series - N4 Pressure Switch - Manual (Page 5)

A-S ERIES S WITCH S AFETY M ANUAL
I&M009-10210
REV A - 4/19/2013
3.2 Environmental limits
The designer of a SIF must check that the product is rated for use within the
expected environmental limits. Refer to the temperature limits labeled on the
product, or the A-Series pressure switch datasheet available at www.Ashcroft.com.
3.3 Application limits
The materials of construction of an A-Series switch are specified in the Ashcroft A-
Series pressure switch datasheet. It is especially important that the designer check
for material compatibility considering on-site chemical contaminants and air
supply conditions. If the A-Series pressure switch is used outside of the application
limits or with incompatible materials, the reliability data provided becomes invalid.
3.4 Design Verification
A detailed Failure Mode, Effects, and Diagnostics Analysis (FMEDA) report is
available from Ashcroft Inc. This report details all failure rates and failure modes
as well as the expected lifetime.
The achieved Safety Integrity Level (SIL) of an entire Safety Instrumented Function
(SIF) design must be verified by the designer via a calculation of PFDavg
considering architecture, proof test interval, proof test effectiveness, any automatic
diagnostics, average repair time and the specific failure rates of all products
included in the SIF. Each subsystem must be checked to assure compliance with
minimum hardware fault tolerance (HFT) requirements. The exida exSILentia® tool
is recommended for this purpose as it contains accurate models for the A-Series
pressure switch and its failure rates.
When using an A-Series pressure switch in a redundant configuration, a common
cause factor of at least 5% should be included in safety integrity calculations.
The failure rate data listed the FMEDA report is only valid for the useful life time of
an A-Series pressure switch. The failure rates will increase sometime after this
time period. Reliability calculations based on the data listed in the FMEDA report
for mission times beyond the lifetime may yield results that are too optimistic, i.e.
the calculated Safety Integrity Level will not be achieved.
3.5 SIL Capability
3.5.1 Systematic Integrity