a supply pipe that branches off from the feed pipe at a point above the top of the water heater, or some other device to prevent the water from draining down from the water heater if there is a failure at the source of water supply;

在热水器顶部以上位置的供水管分出的支管或其他器件，以防止供水来源中断时，水从热水器倒流；

http://www.legislation.gov.hk/eng/index.htm 22.09.2011

http://www.legislation.gov.hk/eng/index.htm 22.09.2011