Most utility locating is done with the use of electromagnetic locating equipment. This equipment consists of two parts, a transmitter and a receiver. In short, we use the transmitter to place a “signal” onto the line then go hunt down that signal with the receiver. As long as the line is metallic and continuous we can locate it with electromagnetic equipment. In theory.

So now the long version. The signal that we place on the line is actually an electromagnetic frequency. You can think of it as radio waves that are tuned to a specific frequency that follow along the path of the line. Because they are tuned to a specific frequency, we can use this signal to separate our target line out from all the rest of the utilities in the ground. Furthermore, we can change the frequency to control the signal so that it picks up more or fewer lines.

Connecting to a utility.
We can “hook up” onto utilities in a few different ways; the two most common ways are directly connecting to a target conductor or clamping around the line depending on accessibility.

There are many different ways that we can get that signal onto the line. We can connect directly to the metal wire within the line, we can use a clamp to surround the wire with signal, or we can lay the transmitter over the line and induce signal onto it (usually only as a last resort). Each method has it’s pros and cons. The first two methods are best but require us to actually be able to touch the wire at an access point. The last method puts signal onto everything in the ground and is imprecise but sometimes the only option. Access to a connection point is often our biggest obstacle.

Once we get signal onto a line, our next challenge is controlling that signal. In a perfect world the signal would stay on our target line from start to finish. In reality, we have to work around the whole issue of common bonding. Common bonds are points at which multiple utilities are grounded together. The most common example is in your house. Every house is grounded to protect it and it’s occupants from lightning and electrical surges. Most are actually grounded by both a ground rod and by the water service entering the house. From there, all the other utilities usually use the house’s ground (some by being bonded to the cold water pipes). So in effect, once our signal gets to the house, it’s then free to spread out over all the other lines. While in most circumstances we can’t completely isolate one line from everything else, we can usually isolate it enough.

Most of the time we want to keep our signal from bleeding off onto other lines so that we know which lines are which. Just knowing that there’s a line in the area isn’t enough, we need to know exactly what it is so that we can make sure that all of the utilities are accounted for. Sometimes we want the signal to bleed off onto other lines. Usually this is part of a “safety sweep” for unknown lines. If we don’t know that a line exists, we can’t always find it. So during a safety sweep, we might connect onto a utility that we know is commonly bonded and then go look for bleed off signals. Sometimes we find old abandoned utilities, lines placed for redundancy, and sometimes lines that we’ve tried to locate other ways. Occasionally we’ll try to use bleed off to locate lines for which we don’t have an access point.

So as you can see, although the theory and science behind locating is fairly straightforward, the devil is in the details. Here are a couple ways that you can help ensure a thorough and accurate locate:

  • Provide any information you might have about the site. Any kind of print or record is helpful even though they are rarely accurate. Information on what might have been on the site in the past, who did the work, and how old buildings and services are help the locator understand what they might find.
  • Premark/define the area well. Having a well defined work area allows the locator to concentrate on what’s important without getting side-tracked by difficult lines that aren’t in the work area. Sometimes the locator will have to find things that are well outside the work area in order to be certain that they are not in the work area. It can occasionally come down to a process of elimination.
  • Understand that often nothing is certain until the locator has checked all the lines in an area. One utility can sometimes mimic another quite convincingly until we can see the whole picture.
  • Make sure that there is access to any possible connection point. If there’s a telephone line to a separate garage, the locator will probably need access into the garage. For commercial sites, the locator will likely need access to the communications and utility rooms and want to lay eyes on anything that goes through the foundation.