A drain is the primary vessel or conduit for unwanted water or waste liquids to flow away, either to a more useful area, funnelled into a receptacle, or run into sewers or stormwater mains as waste discharge to be released or processed. In most systems, the drain is for discharge of waste fluids, such as the drain in a sink in which the water is drained when it is no longer needed. In the UK, plumbers refer to waste water as "bad water", under the premise that the water they are moving from one area to another via the use of a drain is not needed and can be removed from the area, like a "bad apple" being removed from a fruit bowl. Design, installation, and maintenance Drain design and installation parameters are engineered to ensure drain functionality for its intended purpose, allowing for variances in volume, matter to be conveyed and maintenance requirements. Drain installation takes into account principles related to gravity, vacuum, grade, health hazards from biological agents, and resistance to functional failure. Also incorporated in drain design are requirements to allow drain maintenance and repair of a blocked drain, such as cleanout access. Safety Swimming pools and fountains In systems such as swimming pools or fountains where waste fluid is recirculated, the drain is the input to the recirculating pumping machine. These fixtures can be very dangerous because people do not expect to encounter more than the head pressure of water above the drain when contacting a drain. When there is a recirculating pump, the force of suction consists of the head of water, plus the suction of the pump (up to a maximum of 1 atmosphere). Fatalities have occurred around drains as a result of suction entrapment (also known as pool suction-drain injury). In these situations, a portion of the body, hair, or clothing may become stuck against the drain and may become impossible to release, resulting in drowning. For example in 1994, Cristin Fitzpatrick drowned in a Variety Village swimming pool when her hair became entangled in a water funnel. Properly designed drains in swimming pools and spas mitigate this hazard by either adding multiple drains, or increasing the surface area of the drain opening with the use of many holes or safety covers. Since drain suction increases rapidly when a portion of the drain flow is blocked, having two drains or a larger drain