This invention prevents blockage that occurs in the general drainage traps of buildings. The invention consists in connecting a conduit by positioning it in the inlet part above the trap water float level (M) and in the outlet part below a line (N) thereby forming a new auxiliary trap. The invention is suitable for use in traps in buildings which are connected to the common sewage system. The invention informs and thereby prevents the obstruction of the trap by the use of an alarm which is installed in the conduit wherein the water is diverted due to gravity when the conduit is obstructed and signals any obstruction in the conduit so that it can be cleared without waiting for the consequences of such a situation to occur. Also although water flows freely through the auxiliary trap section air does not freely communicate between the inlet and outlet pipes and foul odors are avoided.