In some 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 United Kingdom, plumbers refer to waste water as 'bad water'.
This is 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. In other systems, such as fountains or swimming pools where waste fluid is recirculated, the drain is the input to the recirculating pumping machine. In the last case, there is an obvious safety issue, because many people do not expect to encounter more than the head of water above the drain when touching a drain.
For example, in a recent case, a lifeguard went to the bottom of a pool to retrieve something and his hand got stuck because of the extra pressure from the pump.When there is a recirculating pump, the risk of suction consists of the head of water, plus the suction of the pump (up to a maximum of 1 atmosphere). Źródło: https://en.wikipedia.org/wiki/Drain_(plumbing)
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