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APPENDIX A

From 2010 Uptime Institute

6

Tier I Assumptions:

Two 12-hour scheduled outages for maintenance

1.2 failures per year, 4 hours per failure

Tier 1 Availability:

2x12 + 1.2*4 = 28.8 hours down per year => (8760-

28.8)/8760 = 99.67 % availability.

Tier 2 Assumptions:

One 18-hour scheduled outage per year

1 failure per year, 4 hours per failure

Tier 2 Availability:

18 + 4 = 22 hours down per year => (8760-22)/8760

= 99.75% availability

Tier 3 Assumptions:

No planned outages

1 failure per 2.5 years, 4 hours per failure

Tier 3 Availability:

4 / 2.5 = 1.6 hours down per year => (8760-1.6)/8760

= 99.98% availability.

Tier 4 Assumptions:

No planned outages

1 failure per 5 years, 4 hours per failure

Tier 4 Availability:

4 / 5 = 0.8 hours per year downtime => (8760-

0.8)/8760 = 99.99% availability

APPENDIX B: RELIABILITY AND

AVAILABILITY

In some industries, losses due to facility downtime

are proportional to downtime:

Lost $ = Rate * Downtime

In data centers and many other critical facilities, there

are substantial losses incurred during the first instant of

downtime. For example, A banking facility might

require 16 hours to restore normal operations after a

major data center outage disrupts ATM operations.

The losses in terms of business, customer satisfaction,

regulatory effects, etc., all happen in the first instant;

even if power is restored in 1 second, it takes 16 hours

to restore the full ATM network.

In these facilities, losses due to facility downtime

include an initial loss term, represented as D:

Lost $ = D + Rate * Time

The amount of loss probably increases with duration of

the outage

7

, but the penalty D is always incurred.

If a planner wants to estimate the total losses for a

facility over its operating lifetime, and the facility

does not have large initial losses, the calculation is:

Total Loss =

(Rate * downtime) = Rate *

downtime

This is true because Rate is constant. The symbol

downtime means “the sum of all downtime.”

If this planner has a valid figure for availability of

the system, they can calculate the expected

downtime over a given interval:

downtime = (1 – Availability) * mission_time

If availability is 99%, and mission time is 10 years, then

downtime = (1 – 99%) * 10 * 8760 hours = 876 hours.

The calculation is different if the facility suffers a

significant loss for each event, regardless of its

duration.

Total Loss =

(D + Rate * downtime) = n*D +

Rate*

downtime

Where n is the number of outages expected over the

mission time.

Availability cannot be used to predict

the number of events. Reliability must be used to

determine the frequency of outages.

1 In some facilities it is acceptable to interrupt cooling for short periods.

2 “Reliability of Data Centers by Tier Classification”, Arno, R., Friedl, A., Gross, P., Schuerger, R.J., Industry Applications, IEEE Transactions on

(Volume:48 , Issue: 2 ), March-April 2012, pp. 777 - 783

3 “Reliability of Example Mechanical Systems for Data Center Cooling Selected by Tier Classification”, Arno, R., Githu, G., Gross, P., Schuerger, R.,

Wilson, S., Industry Applications Society Annual Meeting (IAS), 2010 IEEE

4 See Appendix A

5 “IEEE Recommended Practice for Determining the Reliability of 7x24 Continuous Power Systems in Industrial and Commercial Facilities”, IEEE

Std 3006.7-2013

6 Insert citation

7 The rate of loss need not be linear with time, for example food or drug spoilage might not occur for some time after an outage begins. The analysis of

availability versus reliability still applies.

Steve Fairfax is the President at MTechnology, Inc. He can be reached at

Fairfax@mtechnology.net