The Double Check Detector Assembly (DCDA) is an outgrowth of the double check valve and is primarily utilized in fire line installations. Its purpose is to protect the potable water supply line from possible contamination or pollution from the fire system, back-pressure from fire line booster pumps, stagnant "black water" that sits in fire lines over extended periods of time, the addition of non-potable water through outside fire district connections (FDCs), and the detection of any water use in the fire line due to fire line leakage or deliberate water theft. It consists of two spring loaded check valves, a by-pass assembly with water meter and double check valve, and two tightly closing OS & Y gate valves.

A diagram of a double check detector assembly is provided below: