Active vs Passive Locating - Understanding Utility Detection

In the world of utility detection and underground mapping, accurate locating is critical for safety, compliance, and efficiency. Whether you're identifying the location of buried cables, pipes, or other underground utilities, understanding the two main types of underground utility locating methods, active and passive, is essential.

Each method serves a distinct purpose and, when used correctly, they complement each other to ensure precise and reliable locating results of your underground utilities. In this article, we'll explore the key differences between active locating and passive locating, when to use each method, and best practices to follow to prevent unwanted damage to underground infrastructure.

What is Underground Utility Detection?

Underground utility detection is the process of locating and identifying buried infrastructure - such as water, sewer, gas, electric, and communication lines before digging or construction work begins. Using specialized equipment like electromagnetic locators and ground-penetrating radar, technicians can map the position and depth of buried utilities to prevent accidental utility strikes. This practice helps protect workers, avoid costly repairs, reduce service interruptions, and ensure projects are completed safely and efficiently.

Understanding Active vs Passive Locating

What is Active Locating?

Active locating involves applying a specific signal into a utility line using a transmitter. This signal is then detected with a receiver. Because the operator is introducing the signal, it offers better control and clarity over the locating process.

Common uses:

• Tracing known utility lines
• Identifying a specific utility among multiple conductors
• Working in congested utility corridors

What is Passive Locating?

Passive locating detects naturally occurring or pre-existing signals on a utility. These could be signals from power cables (50/60 Hz), telecommunications lines, or radio frequency currents already present in the ground.

Common uses:

• Initial scans of unknown areas

• Verifying that no active signals were missed

• Quick double check without access to endpoints

Best Practices for Active Utility Locating - Transmitter & Receiver

1. Use the Correct Frequency

1. • Low frequencies (e.g., 8kHz) travel longer distances and stay on the utility better but are harder to pick up.

   • High frequencies (e.g., 33kHz – 82Hz) are easier to detect but can jump to adjacent utilities and cause ghost signals. Also, good for inductive tracing (without conductive attachment wires).

   • Choose based on distance, utility type, and ground conditions.

2. Ensure Proper Grounding

   • A poor ground connection weakens the signal and can cause false readings.

   • The radio signal must "flow" in a loop to be detected by the receiver.

   • A metal earth ground provides a focal point for the signal to travel back to the signal source ~ closing the circuit (loop).

   • Place the ground stake far enough from the utility at a 90 degree angle and in moist soil if possible.

   • Ground stake should be placed at right angles to the conductor.

   • The larger the metallic surface, the stronger the signal loop.

3. More Clues to the Probable Path of Your Conductor (“reading the street”) - Note Positions of:

   • Water - Valves, curb stops, meters, hose bibs

   • Gas - Meters, electrical test points

   • Power - Transformer boxes, switch gear, risers

   • Telephone - Pedestal boxes, risers, manhole, terminals, protector

4. Methods to Improve Your Ground - Create larger surface area to attract signal

   • Large ground rod - Be careful not to penetrate another line.

   • Wet ground-water/moisture improves conduction.

5. Verify Signal Strength and Direction

   • Always check for consistent signal strength and follow the line's direction.

   • If the signal fades or behaves unpredictably, reassess your connection and frequency. Relocate transmitter and/or ground connection.

6. Use Different Methods (Direct Connect, Induction, or Clamp)

   • Direct connection (clip) offers the strongest signal strength, the most accurate locate method, but requires physical access. Check the continuity of ground clips on occasion.

   • Induction (transmitter) is useful when access is limited, but it's less precise and has a lower signal strength over shorter distances.

   • Inductive Coupling Clamp-on methods are non-invasive and good for live lines.

7. Possible Direct Connection Points

   • Water - Hydrant, Meter, Curb Stop, Valves & Hose Bib, Valve keys

   • Gas - Meter, Valve, Trace Wire, Test Leads

   • Power - Neutral phase, Transformer Case, Pole Ground, House Ground, Meter Box, & Conductor (if disconnected)

   • Telephone - Sheath, & Vacant Cable Pair (on tip side)

8. Double-Check with a Receiver Sweep

   • Even after active locating, do a sweep of the area with your receiver to ensure continuity and confirm no unexpected signals.

Best Practices for Passive Utility Locating - Receiver Only (Verification Only)

1. Perform Passive Scans Before Digging or Activating

   • Use passive mode to detect any unknown or live utilities not previously identified.

   • Ideal for early site surveys or quick hazard checks.

2. Understand Signal Types

   • Use Power mode to detect signals from energized electrical lines.

   • Use Radio mode for detecting re-radiated signals, often from metallic pipes and cables.

   • Use CPS or CATV modes (if available) for communication lines.

3. Drawbacks or Why Not to Rely on Passive Alone

   • Passive locating can detect signals, but it doesn’t tell you what they are.

   • Lack of any signal on a buried conductor when the power source is off.

   • No control over frequency or intensity of the signal.

   • Non-specific frequency (RF).

   • Signal path may be unrelated to the signal source (60Hz on water pipes).

   • Always follow up with active locating or physical verification where possible.

4. Beware of Signal Bleed and Interference

   • Nearby power sources, electric fences, or radio towers can cause interference.

   • Move away from known sources of distortion if you detect erratic signals.

   • If using a multi-frequency locator, change the frequency or lower the output.

5. Use in Conjunction with Mapping Tools

   • Combining passive locating with digital mapping, GPS, or GIS systems enhances documentation and accuracy.

When to Use Active vs Passive Utility Locators

Key Takeaways for Safer Utility Locating

Successful public utility or private utility locating depends not only on having the right equipment, but on applying the right technique at the right time. Active and passive locating each has its own place. Knowing when and how to use them will significantly reduce the risk of service disruptions and prevent damaged of electrical lines and other buried utilities.

Remember:

• Start passive, verify active
• Use the right frequency
• Don’t assume—confirm

By following these best practices, you’ll improve your accuracy, efficiency, and safety on every job site.

If you need help choosing pipe or cable locators, contact your Core & Main Supply account manager for expert advice on products, accessories, and availability.

Visit supply.coreandmain.com or call 888-885-2673 for more information.