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03/15 MN03407004Z-DE/EN
Safety analysis
The following functional safety characteristics were
determined for overload relays ZB12 and ZB32:
For low demand operating mode with a 1oo1 architecture,
consisting of type A subsystems and a hardware fault
tolerance (HFT) of 0 (see Table 3 in EN 61508 Part 1 and
Table 2 in EN 61508 Part 2) for the overload relays at an
ambient temperature of 55 °C:
• Motor protection (overload)
– Safety integrity level
SIL 1
– Safe Failure Fraction (SFF)
92 %
– Failure rate for undetected safe failures
-9
468 x 10
/h
– Failure rate for detected safe failures
-9
1,092 x 10
/h
– Failure rate for undetected dangerous failures
-9
169 x 10
/h
– Failure rate for detected dangerous failures
-9
394 x 10
/h
Average probability of a dangerous failure when the safety
function is on demand, using an interval of 36 months for
repeat test:
1st demand level F 1/year (low demand mode):
-3
PFD
: 2.2 x 10
h
avg
The mean lifespan between two failures (MTBF): 54 years
For the safety-related parts of control systems as per
EN ISO 13849, the following data was determined at an
ambient temperature of 55 °C:
Safety analysis
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sd
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dd
45
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