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For reliability calculations a number of fixed definitions and relevant symbols are used internationally. For the sake of completeness a few commonly used definitions are set out briefly.
Input data
The reliability input data for all components are:
•Failure frequency: the mean number of occurrences that the component fails.
•Repair duration: the mean duration in minutes that component repair or replacement takes.
•Maintenance frequency: the mean number of occurrences that the component is in maintenance.
•Maintenance duration: the mean duration in minutes that component maintenance takes.
•Maintenance cut-off duration: the mean duration in minutes that it takes to abort component maintenance in case of an emergency.
•No operation probability: Probability that a protection device refuses to trip or a circuit breaker refuses to switch off the fault.
Results
After a calculation the following results are available:
•F, Outage Frequency: the mean number of component outages per year due to a failure in the network
•D, Outage Duration: the mean outage duration of the component per outage due to a failure in the network
•P, Outage Duration per year: the mean outage duration of the component per year due to a failure in the network
•P, Outage Probability: the outage probability of the component per year due to a failure in the network (converted from outage duration)
•ENS, Energy Not Served: the mean amount of energy per year that an element can not consume due to a failure in the network
•P*BC: large customers outage duration: product of P and the number of large customers
•P*SC: small customers outage duration: product of P and the number of small customers
Non-availability
The term non-availability is defined as the time a component is not scheduled out of service. The scheduled out of service duration is not taken into account. The maintenance time is taken into account in combinantion with failures during maintenance.
Group
A group is a set of components that is protected by one protection device or limited by network openings. Failure of one component in the group always leads to outage of all other components in the whole group. See also: Group.
Single mode failure
Each failure is regarded as one event. During the repair time no other failures may occur in the model (exception common cause failures and protection refusing). A component failure usually leads to a group outage by an always selective protection.
Protection failures
There are two modes of protection failures:
•failure in a circuit breaker (short circuit)
•a power switch does not switch off a fault on a network component (stuck breaker).
If a power switch fails to open, its function is taken over by another power switch or fuse. In that case there is no selective tripping.
Common cause failure
A common-cause-fault, a double fault with a common cause, results in the simultaneous failure of two or more network components. A common-cause-fault is regarded as one event that is characterised by a failure frequency where the two or more components fail simultaneously. No new fault occurs until the failed components have been repaired.
Preventive maintenance
If case of maintenance the whole group is out of service. All switching devices will be taken into maintenance during this outage. Therefore it is not necessary to specify maintenance data for switching components.
See also: