Lecture 4 - Failures and Failure Modes

Question 1

Define failure

Answer 1

• loss of ability to perform as required
  – Note 1 to entry: A failure of an item is an event that results in a fault of that item: see fault (3.22).
  – Note 2 to entry: A failure of an item is an event, as distinct from a fault of an item, which is a state [source: ISO/TR 12489:2013].
  – Note 3 to entry: This concept as defined does not apply to items consisting of software only.
• When defining failure, the failure criteria should be unambiguous
• Failure should be related to a measurable parameter or a clear indication (hence the need to define performance levels for the Function)
• Recognise there can be subjective variations in assessing failures but we should try and minimise this.

Question 2

Define fault

Answer 2

• inability to perform as required, due to an internal state.
• Note 1 to entry: A fault of an item results from a failure, either of the item itself, or from a deficiency in an earlier stage of the life cycle, such as specification, design, manufacture or maintenance. A latent fault is a fault that has not become apparent.
• Note 2 to entry: A fault is often a result of a failure of the item itself but the state can exist without a failure (see ISO 20815:2008, 3.1.14).

Question 3

What are the definitiosn associated with failures?

Answer 3

• Failure mode: manner in which a failure occurs
• Failure cause: set of circumstances that leads to failure
• Failure effect: consequence of a failure mode in terms of the operation, function or status of the item
• Failure mechanism: process that leads to failure
• Common mode failures: failures of different items characterized by the same failure mode

Question 4

What is a failure mode?

Answer 4

• Failure mode – manner in which a failure occurs
• Can be categorized as
  – Desired function is not obtained (e.g. failure to start)
  – Specified function lost or outside of operational limits (e.g. spurious stop, high output)
  – Failure indication is observed but there is not immediate or critical impact on the equipment-unit function (e.g. initial wear)

Examples:
Abnormal instrument reading
External leakage
Erratic output
Faulty output frequency
Internal leakage
Loss of redundancy
Vibration

Question 5

What are errors?

Answer 5

• Discrepancy between a computed, observed or measured value or condition and the true, specified or theoretically correct value or condition
• Note 1 to entry: An error within a system can be caused by failure of one or more of its components, or by the activation of a systematic fault.
• Note 2 to entry: An error can be caused by a faulty item, e.g. a computing error made by faulty computer equipment.
• Note 3 to entry: In this International Standard, error is also specifically used for software and human errors.