GESTRA NRS 1-51, Installation Instructions Manual

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7
Functional Safety acc. to IEC 61508
Safety characteristics of the subsystem NRG 1...-51 / NRS 1-51
The level switch NRS 1-51 is certified acc. to IEC 61508 only if used in combination with level electrodes
NRG 1...-51.
The equipment combination NRG 1...-51 / NRS 1-51 corresponds to a type B subsystem with Safety
Integrity Level (SIL) 3. Type B means that the behaviour under fault conditions of the used components
cannot be completely determined. The functional safety of the equipment combination refers to the
detection and evaluation of the water level and, as a consequence, the contact position of the output
relays.
The design of the equipment combination NRG 1...-51 / NRS 1-51 corresponds to the architecture
1oo2. This architecture consists of two channels that detect and diagnose faults in each other. If a fault
is detected, the equipment combination NRG 1...-51 / NRS 1-51 will go to the safe state, which means
that the contacts of both output relays will open the control circuit.
Safety characteristics
SIL Architecture Lifetime
(a)
Proof Test
Interval (a)
General 3 1oo2 20 20
SFF PFD
av
PFH
av
λ
DU
Level switch NRS 1-51 alone 98.54 % 1.18 x 10 -4 3.73 x 10 -8 7.33 x 10 -8 /h
Level switch NRS 1-51 in conjunction with one
level electrode NRG 1...-51
98.17 % 1.69 x 10
-4
4.54 x 10
-8
9.33 x 10
-8
/h
Fig. 1
Terms and abbreviations
Terms
Abbreviations
Description
Safety Integrity Level
SIL
Classification of the Safety Integrity Level acc. to IEC 61508
Lifetime (a) Functional safety: Lifetime in years
Safe Failure Fraction
SFF
Percentage of failures without the potential to put the safety-related system into
a dangerous state
Probability Failure per Demand
(Low Demand)
PFD
av
Average probability of failure on demand for low demand mode (once a year)
Probability Failure per Hour
PFH
av
Probability of failure per hour
λ
DU
Failure rate for all dangerous undetected failures (per hour) of a channel of a
subsystem
Fig. 2