May - Library

acute failures; repair costs for these failures are frequently more
expensive. Not only are direct costs higher (repairs may re q u i re more
labour hours, specialist services or the costly expediting of spares),
but the opportunity cost of lost production per failure is higher. With
this in mind, we are now ready to answer the question as to which
f a i l u res should be prioritizing in order to best improve business
performance.
5. Downtime Priorities and the Business
Cycle
The discussion so far has assumed that the electrical failure s
causing unplanned downtime of the shovel fleet are (i) evident to the
operators and (ii) do not cause significant safety or enviro n m e n t a l
h a z a rd. Hidden and potentially hazardous failure modes should be
a d d ressed and can be identified by applying Reliability-Centre d
Maintenance (RCM) decision logic such as that developed by
Moubray (1997).
Assuming that the hidden and hazardous failure modes have
a l ready been filtered from the shovel electrical downtime data,
maintenance priorities will be determined solely on the basis of the
economic consequences of failures and their associated repair costs.
The economic consequence of a failure includes the opportunity cost
of lost production, the extension of fixed costs such as operator
salaries which must be paid irrespective of equipment downtime, the
cost of maintaining an increased number of spares as a result of the
failure and the cost of maintaining redundant equipment capacity in
order to mitigate the effects of lost production.
In some industries, the economic consequences of plant or
equipment downtime for critical equipment far outweigh repair and
maintenance costs. For example, the mining industry is subject to
highly cyclical commodity prices. With copper prices currently at fiveyear
highs, the opportunity cost of lost production will significantly
exceed the direct cost of repair and maintenance. In such
c i rcumstances, it is desirable to prioritize production, hence
equipment availability and reliability, over repair costs.
H o w e v e r, when copper prices fall, the cost of production becomes
m o re significant. Thus in commodity price troughs, controlling and
reducing maintenance and repair costs will be as important as
maintaining equipment availability and reliability.
F i g u res 9 and 10 illustrate these two scenarios. Figure 9
c o rresponds to a scenario where the opportunity cost of lost
production far exceeds shovel repair and maintenance costs. Figure
10 corresponds to the scenario of a commodity price squeeze causing
production cost concern.
For ease of construction, the availability limit has been defined as
the line of constant downtime equal to the product of the two
t h reshold limits calculated in equations (5) and (6). Note that the
availability limit effectively separates the acute and chronic quadrants
into two areas: acute A and B and chronic A and B respectively. The
expression for the availability limit is:
ni X MTTRi = D where 0 < ni < Limitn (9)
Q
ni = Limitn where ni > Limitn in the case of Figure 9 and:
MTTRi = LimitMTTR ni X MTTRi = D
Q
where 0 < ni < Limitn where ni > Limitn (10)
in the case of Figure 10. The resulting graphs have been christened
“jack-knife” diagrams because of the inverted V shape of the limits.
Because the analysis of unplanned shovel downtime due to
electrical failures was undertaken during a time of very low copper
prices, the jack-knife limits shown in Figure 10 were used to establish
downtime priorities (see Table 2).
It is valid to question the decision not to include all of the events
affecting shovel reliability in the prioritized list. In effect, this means
adding those events classified as “Chronic type B” to priority list.
Code Description Quantity Duration % Time Av. Time
ACUTE & CHRONIC FAILURES
1 Electrical inspections 30 1015 13.0 33.8
10 Overload relay 23 685 8.8 29.8
Sub total 21.8 63.3
ACUTE FAILURES
2 Damaged feeder cable 15 785 10.1 52.3
7 Auxiliary motors 13 600 7.7 46.2
12 Earth Faults 7 575 7.4 82.1
8 Main motors 12 555 7.1 46.3
15 Air compressor 8 355 4.6 44.4
17 Over current faults 6 220 2.8 36.7
16 Operator controls 5 155 2.0 31.0
Sub Total 41.7 339
CHRONIC FAILURES - TYPE A
11 Motor over temperature 36 745 9.6 20.7
3 Change of substation or shovel move 27 690 8.8 25.6
Sub total 18.4 46.3
CHRONIC FAILURES - TYPE B
Downtime Priorities, Jack-knife Diagrams, Business Cycle
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5 Power cuts to substations 21 395 5.1 18.8
9 Shovel lights 26 240 3.1 9.2
Sub total 8.2 28
Table 2: Electrical maintenance problems prioritised according to Jack-Knife Principles.
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