Most P&C Locators come with an attachment commonly called a Clamp. This actually functions as a coupler because it is used to couple the utility line, but “clamp” is the more common word. Plug the cable of the clamp into the transmitter and then place the clamp around the utility.
This is a form of induction because it induces current onto the utility from the clamp. Induction current behaves differently than direct current, though you have to get into more involved details of what is happening to illustrate that point.
Clamps are most commonly used on cables along pole lines, and cables in manholes and handholes. Clamps are only rarely used on pipes, not because they would not work, but because access to short sections of a pipe are far less common than access to short sections of a cable. However, a clamp can certainly be used on a pipe exposed in a vacuum test hole or anywhere else.
The fourth method of using a P&C Locator is in Passive Mode. In this case only the receiver is used. In Passive Mode you can take advantage of the currents which are already present on buried utilities.
Those currents can be originating in two ways: those which are being applied to the utility by the utility owner, such as electric power on power cables; or they can be stray currents coming from a wide variety of sources. We live in a world of electricity, radio broadcasts, TV broadcasts, wi-fi, Bluetooth, CB radios, NOAA weather broadcasts, and hundreds more. Either way, there is not a single buried utility in the world which is not carrying at least some amount of stray current.
The two most commonly used Passive frequencies are Power Passive (50Hz to 60Hz), and Radio Passive (16 kHz to 19.5 kHz).
Power Passive is usually set to the 50Hz to 60Hz band in order to detect any electric power being generated in any country. The United States, Canada, and South American countries generate electricity at 60Hz, while the rest of the world generates electricity at 50Hz. Setting the instrument at 50-60 Hz covers the electric power transmissions no matter which country you are in.
The most critical issues here are that: some electric power, especially 3-phase electric power can eliminate its own field. In other words, you can be standing directly above high voltage 3-phase transmission power lines – and yet not detect anything.
Another important point here is that all coaxial cable carries electric power as well as its communication signals. This is injected into the Cable TV system at the Power Supplys and is mandatory for the operation of Cable TV. So, coaxial cable in the United States contains 60Hz power which can be detected just as easily as any true power line.
Then there is the issue that any other buried conductor can attract stray power current. This will be most common with any ferrous metal (iron or steel) pipe, and with any cable which has a steel sheath and is well insulated to retain that current (telephone cables).
So, Power Passive can detect buried power lines – but it will also be detecting buried coaxial cable, telephone cables, probably some large steel gas pipes, and other possibilities. And if that is not enough, Power Passive often does not detect the most high profile power lines – 3-phase high voltage.
Radio Passive detects dying radio waves on buried utilities. However, those radio waves are not simply from “radio broadcasts”, but from any type of broadcast in the radio wave format. This includes radio broadcasts, TV broadcasts, Bluetooth, wi-fi, cellular telephone, and right down to the truck driver using a CB radio.
These radio waves are easily attracted to very long conductors without connections. In other words, if you have two utility lines, both of them running for two blocks. One is a large steel gas pipe and the other is only a tracer wire placed along side a fiber optic cable. The gas pipe has regular connections (service attachments), but the fiber cable is only bypassing the neighborhood. Which one will be most likely to attract radio waves? The long tracer wire, not the large steel gas pipe.
The third most common Passive frequency is CPS, but that is covered in the Cathodic Protection text under MISC INFO.