CAT and Genny basics: the three modes, and the mistakes that miss services
A short refresher on what a CAT4+ actually does, why Radio mode falls over in town, and why Power mode misses about half of what's in the ground. Not a replacement for formal training.
By GroundPin team, GroundPin
The yellow-and-black box on site is a CAT. Usually a Radiodetection CAT4+, though there are a few others about. Most crews that use one daily could describe the three modes in their sleep. But it's surprising how often false negatives happen because somebody was in the wrong mode for the job, or because Radio was picking up broadcast noise and telling the operator there was nothing there.
This is a short refresher. It isn't a substitute for actual training. If you're the person running the scan, do the Radiodetection CAT and Genny courses (or equivalent through a competent trainer) and keep your sticker in date.
The three modes
Most CATs have three detection modes. Each one works on a different principle.
Power mode. Detects the 50 Hz hum radiating from live mains cables. It only works on energised services. A de-energised feeder radiates nothing. An isolated cable end radiates nothing.
Radio mode. Detects distant VLF radio signals that couple onto long metal objects in the ground. Works on cables whether live or not, and on long metal pipes. Unreliable on short runs and in areas with a lot of nearby radio or broadcast sources.
Genny / Signal mode. Uses a separate signal generator to induce a traceable signal onto the target asset. The CAT then traces the signal. It's the most precise of the three, but it needs a signal source on the target, which means either a direct connection to a known point, a clamp around the cable, or induction from a generator sat on the ground above the asset.
Why Power mode misses more than people expect
Power mode feels reassuring. Big audio tone, clear line on the display. Easy to mistake for completeness.
It doesn't detect:
- Fibre (not energised)
- Water and gas pipes (they aren't electric)
- Dead cables, isolated feeders, temporary supplies that aren't on at the moment
- Street-lighting feeds that are off during the day
A lot of what gets struck on a typical site is in one of those categories. A Power-only sweep can read "clear" over a run of telecoms fibre and a dead three-phase feeder and you'd never know.
Use Power as one pass, not the only pass.
Why Radio mode falls over in town
Radio mode looks for re-radiated VLF signals. That works well when the ground has long metal assets and the surroundings are radio-quiet. A rural site, open ground, flat in most directions.
In central London, surrounded by broadcast masts, tram feeders, and dense overhead infrastructure, Radio mode can produce enough ambient noise to be nearly useless. Same story in most city centres and around substations.
If Radio is noisy, switch to Genny mode. Or, if you only want a quick passive pass, do Power and rely on the drawings. Don't tell yourself Radio's working fine when the CAT's chirping at every step.
Genny — where it shines
Genny mode is the one you reach for when you want to trace a specific asset.
Direct connection: if you can access a known end of the cable (after isolation — never energised), clip the Genny leads onto it and the CAT will trace the length back.
Clamp: if the cable runs through a chamber, clamp the Genny's inductive clamp around it. Signal passes through the insulation and you can trace the run from the surface.
Induction: drop the Genny on the ground above where you expect the asset to be. It induces a signal into whatever's underneath that can carry one. Less precise than clamp or direct, still more useful than Radio in town.
Genny is also the best bet for plastic gas and water mains that have a tracer wire installed with them. Most modern plastic gas mains do. Older cast-iron gas doesn't need it; it's traceable on Power or Genny directly.
The sweep pattern
Cross-hatched. Over the area you're going to dig, not just along the trench lines. At least two perpendicular passes. Mark the finds with spray, and write them up on a marked copy of the drawing in the cabin.
Two common mistakes.
Doing the sweep at the start of the day, then digging the same ground three days later. Services don't usually move, but new ones sometimes appear. A temporary supply for a neighbour's trade. A subcontractor dropping a cable in overnight. Re-sweep before the dig.
Stopping the sweep at the trench walls. Services you hit are often the ones 200mm outside the planned trench, where the spoil crumbles and the operator cuts in a bit further. Sweep wider than the dig.
When to trial hole anyway
CAT and Genny is a very good tool. It isn't perfect. If you've got:
- Multiple services stacked at different depths
- Congested urban ground with a lot of metal noise
- A dig location directly on top of a marked service line
Dig a trial hole. Hand dig or air vacuum. It's the only way to be genuinely confident at a specific point.
Where GroundPin fits
A CAT finds what's there. GroundPin records what your crews found, so the next crew on site has the finding available before they pick up the CAT. Fewer starts from scratch, fewer repeated mistakes, fewer searches that rely on somebody's memory of a job from six months ago.