Page 404 - Offshore Electrical Engineering Manual

Circuit Breaker Illustration 391

Note that the adequacy of load and fault rating is assumed, as the purpose of this
analysis is not to question the system design calculations but is concerned with the
effects on the system of a particular mode of failure.

CIRCUIT BREAKER MODES OF FAILURE

There are two modes of failure associated with the above-mentioned first function,
namely,

(a) circuit breaker fails to open when isolation/disconnection is required,
(b) circuit breaker fails to close when power supply is required.
For the fault clearing function, there is one failure mode:
(c) circuit breaker fails to interrupt fault current.
If required this could be split into
(a) fails to interrupt make fault,

(b) fails to interrupt break fault.

Lastly, there are faults associated with the breakdown of insulation within the
circuit breaker cubicle:
(d) interphase fault in circuit breaker cubicle,
(e) earth fault in circuit breaker cubicle.

Some of these faults may be due to the failure of the trip/close battery supply or
a protection relay. Depending on the level of detail required, however, these compo-
nents can be included as part of the circuit breaker ‘block’ or if necessary a new block
diagram may be drawn showing individual blocks for each component in the ‘circuit
breaking system’.
Having identified the failure modes, the next stage is to describe the failure
effects. This is usually done at three arbitrary levels:

(a) effect on the system
(b) effect on adjacent equipment
(c) effect on the offshore installation

If the system is well designed, there should be no effects on the installation and
only one or two effects on adjacent equipment, usually associated with fire. All these
effects should have some compensating provision or remedial measure available and
this should be listed as shown in Fig. 8.1.1, column 9. Where no such provision or
remedy exists, column 10 should be used to propose a solution which prevents such
an effect from occurring or alleviates the problem. At the end of the study, all such
proposals should be listed.
If a significant number of proposals appear in column 10, it will probably be nec-
essary to rank the associated effects in the order of criticality. This is usually done by
drawing a criticality matrix as shown in Fig. 8.1.3 (extracted from MIL-STD-1629A:
ISO 9000, and IEC 61508 methodologies are other and more modern approaches