17
®
LEARNING THE BACKGROUND FLAME
FIGURE 10.
Examples of Flame Detection Thresholds (Adjusted for Learned Background Flame))
IEC 61508-Proof Testing
According to section 7.4.3.2.2 f) of IEC 61508-2 proof tests shall be undertaken to reveal dangerous faults
which are undetected by diagnostic tests. This means that it is necessary to specify how dangerous unde-
tected faults which have been noted during the Failure Modes, Effects, and Diagnostic Analysis can be
detected during proof testing.
The suggested proof test consists of a functional test of the scanner to test the flame relay(s) plus a power
cycle to test the fault relay, see Table 5. This test will detect ~ 25% of possible DU failures of the device.
Table 5:
Proof Testing
Step
Action
1.
Bypass the safety function and take appropriate action to avoid a false trip.
2.
Remove the flame or interrupt the path between the flame and scanner. Confirm
flame switch operation. Restore the flame or flame path.
3.
Remove power to the flame scanner. Confirm fault relay operation. Restore power
to the scanner.
4.
Remove the bypass and otherwise restore normal operation.
READY FLAME
FLAME
STRENGTH
BACKGROUND
= OFF
= ON
= FLASHING
100
90
80
70
60
50
40
30
20
10
0
90
%
O
F
A
V
E
R
A
G
E
A
M
P
L
IT
U
D
E
S
T
O
R
E
D
F
O
R
T
A
R
G
E
T
F
L
A
M
E
RELAY ON
RELAY OFF
TARGET
FLAME ON
BAND
}
70
LEARNED
BACKGROUND
LEVEL
ADJUSTMENT
