What a CAT and Genny Cannot Pick Up
A standard CAT and Genny cannot detect empty plastic pipes, clay or pitch-fibre drains, fibre-optic ducts with no tracer wire, or perfectly balanced and unloaded power cables. It finds metal and live power. Anything non-metallic, or carrying no current and no applied signal, is effectively invisible to it.
This is the most important thing to understand about cable location, and the gap that catches people out. A clear sweep does not mean clear ground. It means clear of the things your CAT can detect in the modes you used. Here is exactly what slips through, and how to find it.
Why can't a CAT find plastic water pipes?
A CAT in Power or Radio mode listens for electromagnetic fields. A blue MDPE water pipe is plastic and carries water, not current, so it radiates nothing. There is simply no signal to detect. The same applies to plastic gas mains, plastic ducting and modern plastic drainage. To find them you have to introduce something detectable: a sonde traced through the pipe, or a traceable rod with a signal applied to it.
What about empty drains and sewers?
Clay, concrete and pitch-fibre drains are non-metallic, so the CAT ignores them. The professional answer is to push a sonde (a small transmitter) along the drain with a drain rod or camera, then locate the sonde from the surface with the CAT switched to the sonde frequency. This is how drainage teams pinpoint a collapse or a blockage before excavating. We cover the method in precision drain repair with sonde locating.
Can a CAT miss a live electricity cable?
Yes, and this is the dangerous one. Power mode only detects cables that are carrying an unbalanced load. Two situations defeat it:
- Unloaded cables: a street light cable in daylight, or a new connection not yet energised, may carry little or no current.
- Balanced cables: in some three-phase or twin cables the fields cancel out, leaving almost nothing to detect.
The fix is the Genny. Apply a 33kHz signal directly or with a clamp and the cable becomes traceable regardless of load. This is precisely why best practice is never to rely on Power mode alone.
Does fibre-optic cable show up?
Pure fibre carries light, not electricity, so it is invisible to a CAT unless it was installed with a metallic tracer wire or in a duct you can trace. Many modern fibre and telecoms ducts do include a tracer specifically so they can be located later. If there is no tracer, you are back to sonde-and-rod or ground penetrating radar.
So how do you find the things a CAT misses?
| Buried service | How to locate it |
|---|---|
| Plastic water/gas pipe | Sonde or traceable rod fed through the pipe |
| Clay or plastic drain | Sonde pushed with rod or drain camera |
| Unloaded/balanced power cable | Genny applied signal (direct or clamp) |
| Fibre/telecoms duct | Tracer wire, or duct rod, or GPR |
| Anything non-metallic, no access | Ground penetrating radar |
The safe takeaway
Treat your CAT as one tool in a layered approach. Sweep in all modes, always apply the Genny to confirm, use a sonde for non-metallic pipes and drains, and reach for GPR when there is no signal to chase. Read a silent CAT as a prompt to investigate further, never as permission to dig. For the full method, see our 2026 ultimate guide to cable avoidance, or talk to us about the right cable avoidance kit for your work.