Tracer Wire

An insulated copper wire buried alongside a non-metallic pipe or duct so it can be located electromagnetically later. When it is present, continuous, and accessible, plastic pipe traces as easily as metal; when it is broken or was never installed, it usually does not trace at all.

Tracer wire is the difference between a findable and an unfindable plastic service, which is why codes increasingly require it on new non-metallic installations. In practice it fails often: it gets cut during other excavation, corrodes at splices, terminates in a box nobody can find, or was omitted at install. A locator who reports that a tracer wire goes dead partway along a run is telling you something real about the wire, not about the pipe — the pipe may well continue past the break.