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.