Load cell KxxD-(D)
5
Functional safety
From a safety point of view is the KxxD-(D) load cell an individual safety component
connected to an overall safety control unit. Each individual amplifier strain gauge
bridge output shall be connected to separate control system input(s) as view in figure
below, see also application examples.
For double channel system, cross monitoring of measurement signals is assumed to be
implemented in the overall measuring instrument. The hardware reliability figures for
the double channel load cell are valid when the overall measuring instrument is
fulfilling correct monitoring of the load cell(s).
KxxD load cell safety parameters
According to EN ISO 13849-1:
-
Category = 1
-
MTTFd = 48 year
-
Maximum performance level, PL = c
KxxD-D load cell safety parameters
1. According to EN ISO 13849-1:
-
Category = 3
-
MTTF
d
= 48 year
-
Maximum performance level, PL = d
The achievable performance level assumes that the double channel load cell shall be
connected to an overall measuring instrument in a category 3 structure with a
diagnostic coverage, DC
≥
90%. (Value of DC level is according to Annex E, table E.1).
The comparison tolerance must be selected with respect to the specified element safety
function.
2. According to EN 61508 with DC Low
3. According to EN 61508 with DC High
-
HFT = 1
-
SFF = 80%
-
PFH = 1.16*10
-8
-
λ
s
= 1,20*10
-6
-
λ
dd
= 7,21*10
-7
-
λ
du
= 4,80*10
-7
-
Maximum SIL = 2
-
HFT = 1
-
SFF = 99.5%
-
PFH = 2,41*10
-10
-
λ
s
= 1,20*10
-6
-
λ
dd
= 1,19*10
-6
-
λ
du
= 1,20*10
-8
-
Maximum SIL = 2
The overall measurement system (control) must implement the following diagnostic
technique: “Input comparison/voting (1oo2, 2oo3 or better redundancy)” with
DC = Low (60%) or DC = High (99%) according to IEC 61508-2 Table A.13.
The comparison tolerance must be selected with respect to the specified element safety
function.
KxxD
Control
output
Control
mV/V
output 1
Control
Control
output
KxxD-D
Load
Load
mV/V
output 2
mV/V
output