Guide Words

Guide words can be used to identify how activities can fail in a systematic way. Although the methodology (HAZOP) was originally designed for industrial processes, it can also be used to identify possible failure modes of other types of activity.

Method of Use

1. For a particular activity, identify a series of parameters or attributes related to activity and record them.
2. For each of the parameters or attributes, ask whether there is a way for the parameter to deviate in accordance with each of the guide words.
3. Determine the probability and likely consequences of such a deviation occurring.
4. Document the results.

Common Guide Words

Guide Word	Meaning
NO OR NOT	Complete negation of the design intent
MORE	Quantitative increase
LESS	Quantitative decrease
AS WELL AS	Qualitative modification/increase
PART OF	Qualitative modification/decrease
REVERSE	Logical opposite of the design intent
OTHER THAN	Complete substitution
EARLY	Relative to the clock time
LATE	Relative to the clock time
BEFORE	Relating to order or sequence
AFTER	Relating to order or sequence

Example

As an example, let's consider how a building's Heating, Ventilation and Air Conditioning (HVAC) system might fail:

Parameters (for this example): air temperature, power supply voltage.

• Air temperature: what happens if it is too high (MORE), or too low (LESS). From the guide word AS WELL AS we might also consider the effects of smoke or other pollutants..
• Power Supply Voltage:  the guide words might suggest we consider lightning (MORE/AS WELL AS), over-voltage (MORE), brownout (LESS), and power failure (LESS/PART OF).

Using this method of combining guide words with parameters / attributes forces us to consider the many ways any process or activity can fail.

Further Reading

More information on the HAZOP methodology and the use of guide words can be found on Wikipedia.