For current to flow, and signal to be generated, the target utility must be continuously metallic down it's entire length. PVC, XLPE, AFC and clay are non-metallic, and therefore not conductive. Also, insulated joints or gaskets can stop or impede the current's flow, depending on the frequency being transmitted. (See the article on Frequency for more information on this topic.)

Like water flowing downhill, the locating signal (current) will follow the circuit that offers the lowest resistance path to ground. If an alternate conductor offers a lower resistance path than your target utility, the majority of the signal will flow down the alternate conductor instead of the one you want to locate. A common example for this is when the target utility shares a common bond (ground) with another metallic conductor that has an easier path to ground.

If the target conductor is insulated, the insulation must be intact. Broken insulation can allow the signal to leak off the line at that spot. This can limit your ability to locate past that point, and in some cases the signal may jump onto an adjacent non-insulated metallic conductor. If this happens, you could mark the wrong utility.