1. Scope of watermist systems in BS 8458
BS 8458 has a clearly defined scope:
‘recommendations for the design, installation, water supplies, commissioning, maintenance and testing of watermist systems with automatic nozzles installed in residential and domestic occupancies’.
It defines automatic nozzles as:
‘Automatic watermist nozzles operate at a pre-determined temperature to discharge water over a known area below. The flow of water thus initiated causes the sounding of an alarm. Only those watermist nozzles operate which are individually heated above their operating temperature by the heat from the fire.’
The standard was produced with the commonly used mechanical automatic nozzles in mind.
Automatic nozzles with wet pipes are one of the ways to suppress the fire but not the only one. Open nozzles with dry pipes and detector actuated pumps are common in industrial applications and therefore included in the BS 8489 standard. It is only because they are not common in residential that they are not included in its scope. Regardless, as long as the objective of fire performance is not compromised, deviations can be justified.
We are working with the relevant BSi working group to expand the scope of the standard to include electronically controlled nozzles. Standards are meant to be expressed in terms of performance rather than design or descriptive characteristics to not limit anticompetitive effects or impede innovation. However, unfortunately, this was not considered when the standard was first created.
Return to the Prescriptive non-applicable clauses.