21
Detecting Gas Saving Lives
English
8. Functional Safety Manual
8.1 Introduction
The following sections provide detail on the certification of
IRmax
in accordance with the
IEC 61508 and EN 50402 Functional Safety standards. Information is given on the features
considered in the safety case, maintenance requirements and data to enable
IRmax
to be
integrated into Safety Instrumented System (SIS).
8.2 IRmax and IRmax Modbus Safety Function
To measure the concentration of flammable gas and indicate the measurement by means
of a 4-20mA output.
Failures in respect of the safety function will be detected by the hardware and associated
firmware. They will be revealed as an output signal of less than 3.6mA or more than 21mA.
8.3 Functional Safety Data
Parameter name
Symbol
Equation / source
IRmax
Proof Test Interval
T1
As defined by Crowcon.
8,760 hours
(annual)
Mean Time To Repair
MTTR
As defined by Crowcon.
8 hours
Type A/B
Type A
As defined by Crowcon.
Type B
Total failures:
λ
From FMEDA
1.04E-05
Safe diagnosed failures:
λ
SD
From FMEDA
1.95E-08
Safe undiagnosed failures:
λ
SU
From FMEDA
8.59E-08
Dangerous diagnosed failures:
λ
DD
From FMEDA
9.74E-06
Dangerous undiagnosed failures:
λ
DU
From FMEDA
5.06E-07
Safe no-effect failures:
λ
NE
From FMEDA
3.21E-09
Diagnostic coverage:
DC
λ
DD
/ (
λ
DU
+
λ
DD
)
95.06%
Safe Failure Fraction:
SFF
(
λ
SD
+
λ
SU
+
λ
DD
) /
λ
95.11%
Channel equivalent down time
t
CE
(λ
DU
/
λ
D
)(T/2 + MTTR) +
(
λ
DD
/
λ
D
) MTTR
2.24E+02
PFDAVG (using 61508-6
equation)
PFD
AVG
(λ
DU
+
λ
DD
) t
CE
2.30E-03
PFDAVG (using simplified
equation)
PFD
AVG
λ
DU
(T / 2+MTTR) + (
λ
DD
MTTR) 2.30E-03
PFDAVG (using IEC 61508-6
equation)
PFD
AVG
1-
ε
-(
λ
dd+
λ
du) tce
2.30E-03
SIL capability (Low demand
mode)
SIL2
SIL capability (High demand
mode)
SIL2
For low demand applications
, in respect of random hardware failures and safe failure
fraction, all variants of the system are suitable for use in
SIL 2
applications.