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Probability of Failure (PoF)
The likelihood, based on realistic forecasts, that an asset will reach functional failure ("F") at a point in time (usually within in a particular calendar year), which is expressed along a probability distribution.

The probability that an asset will fail during a particular age interval, given that it survives to enter that age.

The probability that performance will be maintained into the future



Measurement
PoF is established by deterioration models wherein there are two primary methods for graphically representing the PoF along a curve:

Further refinement of the survivor curves is derived from the following analysis.
Another group of curves that are helpful for establishing the PoF were developed by Nolan Heap, as follows:


Expressions
PoF can be expressed in a variety of linguistic scales (low, medium, high) or numerical scales (1,2,3,...) relative to a particular year.

Listed below is an example of a five-tiered linguistic scale:
  • Impossible (Extremely unlikely)
  • Remote
  • Possible
  • Probable (Immediate)
  • Highly likely
For example:
  • It is highly likely that our roof will need to be replaced within the next two years
  • It is extremely unlikely that we will need to replace our dry type transformer for at least another 20 years.

Formulas
 In order to establish PoF, we need several pieces of information, including: typical service life, consumed life and remaining life, PF interval, FMEA. Some of this information is empirical and some is statistical in nature.

Listed below are some of the key elements to establish the PoF for a single asset:
  • Service Life
  • Consumed Life (measured from the tope of a performance curve).
  • Remaining Useful Life (measured to the bottom of a performance curve or to functional failure or to complete failure).

Application
The PoF can be applied in a variety of useful ways to the manageement of assets:
  • The PoF is used to establish the horizontal (x) axis of a criticality matrix.
  • The aggregated probability of failure (PoF) of all the assets within a building is expressed as the proximity load, which can be used for resource allocations decisions. 


Mitigation Factors 
The PoF is directly affected by various factors, including the following:
  • The quality of maintenance applied to the preservation of the assets.
  • Exposure or protection of the assets from the elements and other loadings.
  • Durability of the materials used in the production of the assets.
  • Effective age of the assets relative to their chronological age.

Management Principles
The PoF is utilized in development of the following asset management principles, tools and techniques:

Risk-based decision making is at the heart of asset management and this requires mindful consideration of the relationship between the probability of failure (PoF) and the consequences of failure (CoF). The complexities of these correlations can sometimes be captured on a risk matrix.
Fig. Risk-based decision making is at the heart of asset management and this requires mindful consideration of the relationship between the probability of failure (PoF) and the consequences of failure (CoF). The complexities of these correlations can sometimes be captured on a risk matrix.



Some of the key statistical elements of an asset survivor curve
Fig. Some of the key statistical elements of an asset survivor curve.


The whole life of assets can be divided into life stages
Fig. The whole life of assets can be divided into life stages - early life, mid-life and late-life, with different maintenance strategies at each life stage.



Probability of Failure (PoF) expressed as survivor curves with skewness.
Fig. Probability of Failure (PoF) expressed as survivor curves with either positive or negative skewness.


Probability of Failure (PoF) expressed as a degradation curve (performance curve) relative to the points of Potential Failure ("P") and Functional Failure ("F").
Fig. Probability of Failure (PoF) expressed as a degradation curve (performance curve) relative to the points of Potential Failure ("P") and Functional Failure ("F").


The relationship between Consequences of Failure (CoF) represented on the vertical (y-axis) and Probability of Failure (PoF) on the horizontal (x-axis) of a criticality/risk matrix.
Fig. PoF represented on the horizontal (x-axis) of a criticality/risk matrix.


I. Care is oblivious to the extraordinary events that can totally wipe out his assets and upset the delicate order of things, such as force majeure, acts of God
Fig. I. Care is oblivious to the extraordinary events that can totally wipe out his assets and upset the delicate order of things, such as force majeure and acts of God.



Relationship between Probability of Failure (PoF) and the horizontal axis of a risk matrix.
Fig. Relationship between Probability of Failure (PoF) and the horizontal axis of a risk matrix.

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