ReliabilityThe probability that an item will perform a required function under stated conditions for a stated period of time. has a formal definition but is generally taken as an umbrella term that can cover a number of topics. In contracts or standards the critical 'reliability' parameters are usually defined in terms of either a maximum failure rateThe number of failures of an item per unit time. May be observed failure rate (e.g. from 'returns' data); assessed failure rate (from tests) or extrapolated (from tests at higher stress levels). or availabilityThe proportion of time that an item is capable of operating to specification within a large time interval. target.
Reliability targets may be purely contractual but for systems with safety aspects the targets can be mandatory or set in standards.
Contractual reliability targets can be set rationally by comparing the cost of downtime or failures and the cost of obtaining decreased unavailability or failure rates. Alternatively reliability targets may be set by competitive pressures.
An example of mandatory reliability targets is given below. This table is consistent with the proposals in EN50126, EN50128 and EN50129 and is a recommendation for the Safety requirements for electronic systems to be used on a public utilities (railways in this case).
|
SIL |
Low Demand Systems |
High Demand Systems |
Description |
|---|---|---|---|
|
4 |
10-5 to <10-4 |
10-9 to <10-8 |
Safety-critical |
|
3 |
10-4 to <10-3 |
10-8 to <10-7 |
Safety-critical |
|
2 |
10-3 to <10-2 |
10-7 to <10-6 |
Safety-related |
|
1 |
10-2 to <10-1 |
10-6 to <10-5 |
Safety-related |
|
0 |
No target |
No target |
No safety aspects |
SILA measure of the safety consequences resulting from failure of a product, system or sub-system. Ranges from 0 (no safety consequences) to 4 (safety critical). Assessed by taking into account the likely hood of failures causing injuries and the likely severity of injuries sustained. is the Safety Integrity Level and is set by a review activity that considers the consequence of failuresThe end of the period ot time when an item can perfrom its required function or maintain a specified level of performance. Failures can be categorised by cause and type. of the system.
Low demand refers to systems that are only required occasionally (once per year) e.g. a telephone that is used to report the failure of another item of equipment.
High demand refers to systems that are required continuously or most of the time e.g. the signalling system of a railway.
Note: that the type of reliability target is related to what the system is used for.
To show compliance with requirements it must be possible to make rational reliability predictions. This must be part of the design review process as well as being part of product acceptance deliverables.
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