The reference level is defined as 10 metres head of pressure at the boundary stop tap with a flow of nine litres per minute. There are exceptions to this to allow for circumstances beyond a company's control. For example, pressure problems can be caused by a customer's own plumbing. It is impractical to measure the pressure and flow at the boundary of every customer's property, so companies are allowed to report against an alternative reference level, normally 15 metres head of pressure in the distribution main supplying the property. This is a sufficiently high pressure, even allowing for the connection from the water main to the property boundary.
Companies keep registers that identify the properties at risk of receiving low pressure.
The number of properties at risk of low pressure has fallen year on year since privatisation. Companies maintain registers to identify properties affected by supply interruptions. Restrictions on use of water DG4 measures the percentage of the population served by the company that has experienced restrictions in using water. Companies are required to report the percentage of population affected by any of the above water restrictions. There are two measures covered by DG5.