Types of Cable locators

Hum Devices: these are receiving instruments which detect the magnetic field radiated by live electricity cables. They work only when current is flowing through the cables so, for example, will not detect street lighting cables during the daytime.
Radio Frequency Detectors: these are receiving instruments that respond to low-frequency radio signals, which may be picked up and re-emitted by cables and long metallic pipes. The results can be confused by other metallic objects which may re-radiate the signal.
Transmitter-Receivers: small portable transmitters or signal generators can be connected to a cable or pipe, so that the signal is induced into it ; the receiver can then detect this signal. These cable locators can provide useful information in difficult situations but demand more skill of the operator than other types.
Metal Detectors: conventional metal detectors can be useful in locating flat metal covers and joint boxes, but may well miss round cables and pipes.
Cable locators should be used strictly in line with manufacturers’ instructions and should be checked regularly and properly maintained for the better results.

Disclaimer: As obtained from the Internet