Technical Article

Reliability, Availability & Maintainability (RAM) Study: A Practical Guide Using ETAP 24

Every power system engineer eventually runs into the same question from a client or a design review panel: "How reliable is this system, really?" A RAM study (Reliability, Availability and Maintainability) is how we answer that question with numbers instead of gut feel. This article walks through the core concepts, the formulas behind them, and a practical step-by-step method to run a RAM study in ETAP 24.

In This Article

  1. Understanding RAM: Reliability, Availability & Maintainability
  2. Reliability vs. Availability: What's the Difference?
  3. Key RAM Terms You Should Know
  4. Quick Reference: RAM Calculation Formulas
  5. Performing a RAM Study in ETAP 24: Step by Step
  6. Typical Reliability Data When Vendor Data Isn't Available
  7. References

Understanding RAM: Reliability, Availability & Maintainability

Reliability

Per IEEE Std 493 (Gold Book), reliability is the ability of an item — under the combined effects of its reliability, maintainability, and maintenance support — to perform its required function at a stated instant, or over a stated period, of time. In simple terms, it's the probability that equipment keeps doing its job, under stated conditions, for a defined stretch of time.

Availability

Availability describes how ready a piece of equipment is for use at any given moment. It's the probability that an item is in a working state at a given instant or over a given interval, assuming the resources needed to operate it are on hand.

Maintainability

Maintainability, as defined in IEC 60050 (191-02-07), is the probability that a maintenance action on an item can be completed within a stated time, under stated conditions and using stated procedures and resources. In practice, it's a measure of how quickly a system can be brought back online after a failure.

Reliability vs. Availability: What's the Difference?

Reliability is time-dependent the longer the period you evaluate, the lower the reliability number gets, no matter how well the system is designed. Availability behaves differently: it's largely time-independent because it's expressed as a ratio of two averages. Inherent availability (Ai), for instance, is simply MTBF divided by (MTBF + MTTR). That's exactly why availability? not reliability? It is the number engineers reach for when comparing competing system designs. It's also where terms like "5-9's" (an Ai of 0.99999) come from.

Key RAM Terms You Should Know

λ

Failure rate (λ)

The average number of failures of a component or system per unit of exposure time, expressed in failures per hour (f/h) or failures per year (f/y).

Ai

Inherent availability (Ai)

The instantaneous probability that a component or system is up or down, counting only repair downtime — logistics delays and planned maintenance are excluded.

MTBF

Mean time between failures (MTBF)

The average exposure time between one failure and the next, for a repairable component.

MTTF

Mean time to failure (MTTF)

The average time between a repair (or installation) and the component's next failure. This metric is mostly used for non-repairable items, such as fuses or bulbs.

MTTR

Mean time to repair (MTTR / r)

The average time taken to replace or repair a failed component, excluding logistics time such as parts procurement or crew mobilisation.

Quick Reference: RAM Calculation Formulas

Calculated Data
Formula
Ai, inherent availability
Ai = MTBF / (MTBF + MTTR)
Ao, operational availability
Ao = MTBM / (MTBM + MDT)
λ, failure rate (f/h)
λ = Tf / Tp
λ, failure rate (f/y)
λ = Tf / (Tp / 8760)
MDT, mean downtime (h)
MDT = (Rdt + Rlt + Mdt) / Tde
MTBF, mean time between failures (h)
MTBF = Tp / Tf
MTBM, mean time between maintenance (h)
MTBM = Tp / Tde
MTTM, mean time to maintain (h)
MTTM = Mdt / Tma
MTTR, mean time to repair (h)
MTTR = r = Rdt / Tf
R(t), reliability
R(t) = e^(-λt)

Performing a RAM Study in ETAP 24: Step by Step

1
Step 1 — Build the Model Single Line Diagram (SLD)

Start by modelling the system in ETAP 24: all buses, generation sources, transformers, cables/lines, switchgear, and loads within the boundary of the study.

Example single line diagram (ETAP sample project) used to illustrate the workflow
Example single line diagram (ETAP sample project) used to illustrate the workflow
Note: The SLD shown above is ETAP's own sample project, used only to illustrate the workflow. Model your actual project SLD for a real study.

2
Step 2 — Enter Reliability Parameters

ETAP 24 adds a "Reliability" tab to every element type that participates in the RAM module. This is where the failure-rate, repair-time and switching-time data for each piece of equipment is entered.

Reliability parameters entry dialog in ETAP 24 (Utility shown)
Reliability parameters entry dialog in ETAP 24 (Utility shown)

Equipment Inputs Required for RAM Modelling in ETAP

Here is the complete set of reliability and where relevant inputs ETAP expects for each element type:

Bus

  • Active Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Nominal kV (load flow option only)

2-Winding & 3-Winding Transformer

  • Active Failure Rate
  • Passive Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Voltage Rating, Power Rating & Impedance (load flow option only)

Cable / Transmission Line

  • Length
  • Active Failure Rate
  • Passive Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Impedance, Configuration & Allowable Ampacity (load flow option only)

Impedance & Current-Limiting Reactor

  • Active Failure Rate
  • Passive Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Impedance & Rating (load flow option only)

Power Grid (Utility) & Synchronous Generator

  • Active Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Short Circuit Impedance & Rating — Utility (load flow only)
  • Rating — Generator (load flow only)

Synchronous Motor

  • Active Failure Rate
  • Repair Time
  • Replacement Time
  • Load Sector
  • Quantity (No. of Loads)
  • Rating (load flow option only)

Induction Machine

  • Active Failure Rate
  • Repair Time
  • Replacement Time
  • Load Sector
  • Quantity (No. of Loads)
  • Rating (load flow option only)

Static Load

  • Active Failure Rate
  • Repair Time
  • Replacement Time
  • Load Sector
  • Quantity (No. of Loads)
  • Rating (load flow option only)

Lumped Load

  • Active Failure Rate
  • Repair Time
  • Replacement Time
  • Load Sector
  • Quantity (No. of Loads)
  • Rating (load flow option only)

UPS

  • Active Failure Rate
  • Passive Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Load Sector
  • Rating & Loading (load flow option only)

VFD

  • Active Failure Rate
  • Passive Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Rating & Loading (load flow option only)

Charger

  • Active Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Load Sector
  • Rating & Loading (load flow option only)

Inverter

  • Active Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • Rating & Loading (load flow option only)

Breaker

  • Active Failure Rate
  • Repair Time
  • Switching Time
  • Replacement Time
  • (Governs isolation/switching logic during upstream contingencies)
Note: Use actual vendor, OEM or client data wherever it's available. Where project-specific data doesn't exist, the generic values published in IEEE Std 493-2007 (Gold Book), Chapter 10, are the accepted fallback — see the reference tables further below.

3
Step 3 —Reliability Assessment Results

Once the study runs, the one-line displays per bus and branch the failure rate (f/yr) and the average repair/outage duration (hr/yr), along with the resulting downtime contribution to each downstream load.

Reliability Assessment results displayed on the one-line diagram
Reliability Assessment results displayed on the one-line diagram

Typical Reliability Data When Vendor Data Isn't Available

The tables below summarise commonly used fallback reliability figures for major equipment categories, reformatted here for quick reference. They are intended as a starting point only — always prefer actual vendor, OEM or site data when it exists.

Generators

Category / Class
Unit-years
Failures
Failure rate (f/yr)
MTBF
MTTR
MTTM
MDT
Gas turbine generator — Packaged
750.9
399
0.53139
16485.055
21.6
2.1103
2.366
Gas turbine generator — Packaged, 750 kW–7 MW, continuous
167.9
290
1.7276
5070.6
27.39
1
1.225
Gas turbine generator — Packaged, 750 kW–7 MW, standby
583
109
0.18696
46853.7
6.18
4
4.453
Gas turbine generator — Unpackaged, 750 kW–7 MW, continuous
170.6
1
0.00586
1494384
336
5
5.146
Fuse, 0 to 5 kV — Item E17-100
371.3
0
0.00137
6377929.4
0
0
xxx
Gauge, fluid level — Item C5-100
532.2
0
0.00096
9140564.7
0
0
xxx
Heat exchanger, boiler system, steam — Item H21-100
210
6
0.02857
306624
0.5
29
28.3

Batteries

Category / Class
Unit-years
Failures
Failure rate (f/yr)
MTBF
MTTR
MTTM
MDT
Battery (all types)
10543.8
74
0.00702
1248161.4
12.11
0.149
0.217
Gel cell-sealed, strings
2333.7
47
0.02014
434961.4
2
0.1318
0.152
Lead acid, strings
3215.3
24
0.00746
1173590.3
32.13
0.1463
1.023
Nickel-cadmium, strings
4994.8
3
0.0006
14584865.3
10.33
0.1591
0.163

Inverters

Category / Class
Unit-years
Failures
Failure rate (f/yr)
MTBF
MTTR
MTTM
MDT
Inverters, all types
414.8
2
0.00482
1817016
26
5.1691
5.321

Switchgear

Category / Class
Unit-years
Failures
Failure rate (f/yr)
MTBF
MTTR
MTTM
MDT
Bare bus, ≤600 V, all cabinets, ckt bkrs not incl.
1791.3
17
0.00949
923068.2
7.29
4
4.308
Bare bus, >5 kV, all cabinets, ckt bkrs not incl.
780.2
14
0.01794
488208.8
2.27
1
1.296
Bare bus, >600 V ≤5 kV, all cabinets, ckt bkrs not incl.
667.4
2
0.003
2923296
372
10
14.27
Insulated bus, ≤600 V, all cabinets, ckt bkrs not incl.
322.7
0
0.00158
5543247.1
0
3
3.182
Insulated bus, >5 kV, all cabinets, ckt bkrs not incl.
732.5
3
0.0041
2139024
37.33
14
14.434
Insulated bus, >600 V ≤5 kV, all cabinets, ckt bkrs not incl.
264.4
1
0.00378
2316000
8
1
0.774

Transformers

Category / Class
Unit-years
Failures
Failure rate (f/yr)
MTBF
MTTR
MTTM
MDT
Dry, air cooled, ≤500 kVA
2267.4
0
0.00022
38946258.8
0
4
3.826
Dry, air cooled, >1500 kVA ≤3000 kVA
840.2
0
0.00061
14432242.4
0
4
4.206
Dry, air cooled, >500 kVA ≤1500 kVA
1221.4
0
0.00042
20979011.8
0
6
6
Dry, isolation, delta wye, <600 V
6696.1
19
0.00284
3087252.6
21.26
1
2.519
Liquid, forced air, ≤10 000 kVA
419.8
3
0.00715
1225880
248
23
23.677
Liquid, forced air, ≤5000 kVA
1821.5
23
0.01263
693748.2
3.65
1
0.976
Liquid, forced air, >10 000 kVA ≤50 000 kVA
351.7
2
0.00569
1540524
1440
22
23.203
Liquid, non-forced air, ≤3000 kVA
5407.8
6
0.00111
7895436
5
10
8.394
Liquid, non-forced air, >10 000 kVA ≤50 000 kVA
627.6
11
0.01753
499773.8
6.09
1
0.648
Liquid, non-forced air, >3000 kVA ≤10 000 kVA
190.7
1
0.00524
1670904
1
3
2.5

References

This article draws on the following standards and documentation. Figures and definitions have been paraphrased and reformatted for readability; for exact wording and complete data sets, please consult the original sources directly.

Source
What It Covers
How It Was Used Here
IEEE Std 493-2007 ("Gold Book")
IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems
Basis for the reliability/availability concepts and the typical equipment reliability data used in this article
IEC 60050-191
International Electrotechnical Vocabulary — Chapter 191: Dependability and Quality of Service
Basis for the maintainability definition used in this article
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