For contractors and engineers who use utility locating services regularly, understanding the difference between active and passive signal modes is essential to getting the most out of a locate — and knowing when to ask for more.

Passive Mode: Listening for What’s Already There

In passive mode, the locator’s receiver operates without any transmitter. It detects signals already present on buried infrastructure — primarily power frequency (50 or 60 Hz) signals that exist naturally on live power cables, and radio frequency (RF) signals that have coupled onto metallic pipes and cables from broadcast sources.

Passive mode is a useful first sweep of an area because it requires no setup and can reveal the presence of energized lines quickly. However, it has significant limitations. Not all utilities carry detectable passive signals. Non-metallic pipes, de-energized cables, and lines with good insulation may be completely invisible in passive mode. Relying on passive alone is one of the most common causes of missed utilities.

Active Mode: Putting a Known Signal on the Target

Active locating applies a controlled signal from a transmitter to a specific target line. This can be done in three ways:

Direct connection is the most effective method. The transmitter is connected directly to the pipe or cable (or a tracer wire), and the signal travels along the conductor with high efficiency and minimal loss. This gives the best depth accuracy and tracing distance.

Inductive clamp connection involves clamping a signal-inducing coil around a cable or pipe without breaking the circuit. This is useful where direct connection isn’t accessible but results in some signal loss compared to direct connection.

Induction (signal broadcast) places the transmitter flat on the ground above a suspected utility. The transmitter broadcasts a signal downward that couples onto any metallic conductors beneath. This is the most convenient method but also the least selective — the signal may couple onto multiple lines simultaneously, requiring skill to interpret the results correctly.

Frequency Selection Matters

Different frequencies behave differently underground. Lower frequencies (512 Hz, 8 kHz) travel further along a line and are better for long pipeline traces — but they’re also more prone to coupling onto adjacent lines, which can cause false reads. Higher frequencies (33 kHz, 65 kHz) are more easily applied using induction and are useful for difficult connections — but they attenuate faster and are less suitable for long-distance traces.

Experienced locators select frequency based on the specific challenge at hand, not just habit or default settings.

Depth Measurement

Depth estimation is one of the most misunderstood aspects of a locate. Most modern locators provide electronic depth estimation based on the signal geometry — but this reading is only accurate under specific conditions: the signal must be on the target line (not a coupled adjacent line), the line must be relatively straight, and there must be no signal distortion from nearby metallic structures.

At Materials Lab, our locating technicians are trained not just to operate the equipment, but to understand when a depth reading is reliable — and when it needs to be verified by other means.

Ready to Talk About Your Locate Requirements?

Whether you’re planning a trenching program, crossing a utility corridor, or trying to trace an unknown line on a brownfield site, Materials Lab has the technical capability and field experience to support you. Visit materialslab.ca or contact our team to discuss your scope.

 

📧 info@materialslab.ca
📞 587-594-5521

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