Environmental Engineering Reference
In-Depth Information
It is now straightforward to show that R /1,000 at the various warranty intervals
may be easily calculated and converted to FIT or MTBF as follows:
R
1, 000 ¼ð
l t Þ 1, 000 j t ¼ T
R
1, 000 ¼ t ð FIT Þ 10 6
ð 6 : 29 Þ
1 10 9
MTBF
ð 1 Þ FIT ¼
The expression in (6.29) shows that t = T is the prescribed number of operating
hours for a particular warranty interval. The warranty interval operating hours are
listed in Table 6.7.
Table 6.7 Definition of warranty versus operating time
Warranty interval
12/12
3/36
10/150
Operating time (h)
400
1,200
5,000
Rearranging (6.29) and using the operating hours for 10/150 warranty interval,
it is easy to show the equivalent R /1,000 for a given FIT level as
1, 000 10 = 150 ¼ 5, 000FIT 10 6
R
ð 6 : 30 Þ
According to (6.30), a failure level of 1 FIT equates to 0.005 R /1,000 at 10/150
warranty interval. In a typical hybrid propulsion system, a major component, a
controller, for example, may have 30 R /1,000 at 3/36 warranty interval. Table 6.8
summarizes this failure rate along with a comparison of the various reliability
metrics for 1-FIT level (comparison values are in bold for that row).
Table 6.8 Comparison of automotive warranty metrics
FIT
MTBF = 1/ ll
R /1,000| 12/12
R /1,000| 3/36
R /1,000| 10/150
1 10 9
1 10 9
1 *
0.0004
0.0012
0.005
25 10 6
25,000
40,000
10
30 *
125
*The values in bold are comparison values for that row.
A failure level of 1 FIT is very stringent as compared to the failures in R /1,000
for 3/36 warranty interval (see Table 6.8). Here, the values differ by 25,000 (= 30/
0.0012), as would be expected because that the failure rates, l are in this ratio. So,
an effort to require that safety critical systems achieve integer FIT levels of
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