Functional Safety Manual; Introduction; Irmax And Irmax Modbus Safety Function; Functional Safety Data - Crowcon IRmax Instructions D'installation, D'utilisation Et De Maintenance

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
Proof Test Interval
Mean Time To Repair
Type A/B
Total failures:
Safe diagnosed failures:
Safe undiagnosed failures:
Dangerous diagnosed failures:
Dangerous undiagnosed failures: λ
Safe no-effect failures:
Diagnostic coverage:
Safe Failure Fraction:
Channel equivalent down time
PFDAVG (using 61508-6
equation)
PFDAVG (using simplified
equation)
PFDAVG (using IEC 61508-6
equation)
SIL capability (Low demand
mode)
SIL capability (High demand
mode)
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.
Detecting Gas Saving Lives

8. Functional Safety Manual

Symbol Equation / source
T1 As defined by Crowcon.
MTTR As defined by Crowcon.
Type A As defined by Crowcon.
λ
From FMEDA
λ
From FMEDA
SD
λ
From FMEDA
SU
λ
From FMEDA
DD
From FMEDA
DU
λ
From FMEDA
NE
λ
DC
DD
+ λ
SFF (λ
SD
(λ / λ
t
DU
/ λ
(λ
CE
DD
(λ + λ
PFD
AVG DU
λ
PFD (T / 2+MTTR) + (λ
AVG DU
PFD 1-ε -(λdd+λdu) tce
AVG
21
+ λ
/ (λ )
DU DD
+ λ ) / λ
SU DD
)(T/2 + MTTR) +
D
) MTTR
D
) t
DD CE
MTTR) 2.30E-03
DD
IRmax
8,760 hours
(annual)
8 hours
Type B
1.04E-05
1.95E-08
8.59E-08
9.74E-06
5.06E-07
3.21E-09
95.06%
95.11%
2.24E+02
2.30E-03
2.30E-03
SIL2
SIL2