In most cases, the plumber could do the proper job, but since he thought what he did is a good installation, it is obvious that he cannot do it. In that case, you should have a licensed electrician. The connection between the wire and the extension has to be in a junction box, and all the exposed wire has to be in metal conduit or metal flexible cable, with a separate green ground wire. As an aside, if the hose wires are aluminum, then the connections have to be made with special connectors to prevent failure and possibly a fire.