Functional Safety M-LB-(Ex-)2000-System
Planning
201
9-02
11
3.3
Safety Function and Safe State
The safety function of the surge protection barriers depends on the signal loop to which
it is attached. The interference on safety relevant signals (e. g. 4 mA to 20 mA analog signal)
that pass through the devices was evaluated.
Observe the PFH/PFD
avg
values in the functional safety manual and the specified calculation
rules. The devices fulfil the requirements for SIL 3 and can be used to pass safety relevant
signals through in applications up to SIL 3.
The surge protection barriers limit induced transients of different causes, e. g. lightning
or switching operations. This protection function itself is not the safety function of the device.
Safe State
The safe state depends on the application. There are 6 different applications:
•
Digital input (NAMUR signal)
Lead breakage and short circuit are out of range and counted as safe failures.
•
Digital output (de-energized to safe
–
DTS)
Lead breakage and short circuit interrupt the energy transfer to the field and are counted
as safe failures.
•
Analog input (4 mA to 20 mA)
Lead breakage and short circuit are out of range and counted as safe failures.
•
Analog output (4 mA to 20 mA)
Lead breakage and short circuit interrupt the energy transfer to the field and are counted
as safe failures.
•
Resistance thermometer (RTD)
Measurement current = 200
µ
A (i. e. KFD2-UT2-1)
- R
3137
(Pt1000 at 600
°
C)
- R
60
(Pt100 at -100
°
C)
Wire resistance = 35
(1000 m total and 0.5 mm
2
Cu)
Lead breakage and short circuit are out of range and counted as safe failures.
•
Thermocouple (TC)
- U
80 mV (type E at 1000 K)
- U
-10 mV (type E at -270 K)
Lead breakage and short circuit lead to plausible temperature readings and were rated
dangerous undetected. Special values apply, see Table 3.2. If you are using a signal
converter with line fault detection, the values for standard 2-wire applications apply.
For the evaluation, all deviations from the input signal were rated as dangerous undetected,
if the deviations are
•
greater than the specified leakage current or
•
greater than the 3
line resistance.
The user must observe the valid range for the signals in the application and react accordingly
if this range is left.
The values given in the following table are calculated for 2-wire applications as field devices
are usually connected by more than one wire. For the calculation, add the numbers from
the respective column to the numbers given for the safety loop. They are already summarized
for the respective application.
