1 ZB12-…/XTOB…BC1 and ZB32-…/XTOB…CC1 overload relays
1.4 Safety analysis
34
ZB12-…, ZB32-…
11/16 MN03407004Z-DE/EN
www.eaton.com
1.4 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:
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): PFD
avg
: 2.2 x 10
-3
h
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:
Motor protection (overload)
Safety integrity level
SIL1
Safe Failure Fraction (SFF)
92 %
Failure rate for undetected safe failures (
λ
su
)
468 x 10
-9
/h
Failure rate for detected safe failures (
λ
sd
)
1.092 x 10
-9
/h
Failure rate for undetected dangerous failures (
λ
du
)
169 x 10
-9
/h
Failure rate for detected dangerous failures (
λ
dd
)
394 x 10
-9
/h
Size
Value
Category
1
Performance Level (PL)
1)
c
MTTF
d
after 3 years
2)
54 years
1) The performance level is the result of the risk
assessment based on the percentage of risk mitigation
achieved with the control system‘s safety-related parts.
2) MTTF
d
= Mean time to dangerous failure
