Glossary – Arrow Valves Knowledge Hub

Glossary

Plain-language definitions of the terms used across this Knowledge Hub — from air gaps and BA devices to WRAS, AIM and HTM 04-01.

A

B

C

D

E

F

G

H

I

J

K

L

A

WRAS

Water Regulations Advisory Scheme. Approves fittings and publishes AIM documents used by water suppliers.

Wholesome water

Water supplied for drinking, washing, cooking and food production. Fluid Category 1.

Water Fittings Regulations 1999

The Water Supply (Water Fittings) Regulations 1999. Primary UK legislation governing backflow prevention on wholesome supplies.

Upstream

The wholesome-supply side of a backflow prevention device.

Type B air gap

Air gap with weir overflow (AB). Common on break tanks.

Type A air gap

Unrestricted air gap (AA). Highest form of backflow protection. Mandatory for Fluid Category 5.

Spillover level

The level at which water would overflow from a receiving vessel. Used to measure air gap height.

Schedule 2

The technical schedule of the Water Supply (Water Fittings) Regulations 1999. Contains Regulation 15 (backflow) and the fluid category table.

Reduced pressure zone

The chamber between the two checks of an RPZ, held at a pressure lower than upstream so any leak discharges rather than passing.

Reduced pressure zone (RPZ) valve

A mechanical backflow prevention assembly that uses two check valves with a differential-pressure relief valve to provide Category 4 protection against both back-pressure and back-siphonage.

Regulation 15

Schedule 2 Regulation 15 of the Water Supply (Water Fittings) Regulations 1999. Requires every wholesome-water fitting to be protected against backflow.

Point of use protection

Backflow device fitted at the individual fitting rather than at the zone or whole-site boundary.

Isolation valve

Service valve fitted immediately upstream (and often downstream) of a backflow prevention assembly to permit testing and replacement.

HU tap

Hose-union tap. See Hose union tap.

HTM 04-01

NHS Health Technical Memorandum covering water systems in healthcare premises. Category 5 assumed for many clinical washdown points.

Hose union tap

Bib tap or wall hydrant fitted with a hose union. Default Fluid Category 3; Category 4 or 5 if the hose can reach a hazardous source.

Fluid Category 4

A fluid that represents a significant health hazard because of the concentration of toxic substances, including chemical or radioactive.

Fluid category

Classification of a fluid according to the hazard it represents to health if it entered the wholesome supply. Defined in Schedule 2 Table.

Filling loop

Temporary or permanent connection used to fill a closed heating or chilled-water system from the wholesome supply.

EA device

EN 1717 designation for a controllable check valve (single).

ED device

EN 1717 designation for a verifiable double check valve assembly.

Downstream

The side of a backflow device facing the potential contamination source.

Double check valve

Two independently acting check valves in series (ED). Permitted for Fluid Category 3 only; not for heating filling loops on non-house systems.

DC pipe interrupter

Device with atmospheric ports that break siphonage. Limited to low-hazard, unrestricted-discharge applications.