CAT Power and Radio Modes: What Passive Scanning Misses on Underground Cables

Passive Modes Find What Happens to Be There

Power mode listens for the field around a loaded cable. Radio mode listens for long wave radio re-radiating off long buried conductors. Neither puts anything onto the line, which means both depend entirely on conditions you do not control, and both go quiet when those conditions change. That is not a defect. It is what passive means, and the whole session is about the consequences of forgetting it.

What the Instrument Is Actually Detecting

Alternating current changes direction fifty times a second, and that alternation generates a magnetic field around the conductor. The receiver has aerials, usually a top and a bottom one, reading that field and comparing what each of them sees. The detail worth holding on to is that magnetic fields are not exclusive to buried cables. They come off buildings, substations and all manner of things around you, which is part of why a passive response is not the same as a located service.

The Networks Changed, and the Assumption Did Not

Modern power cable does not behave the way the training folklore assumes. Balanced loads and modern construction reduce the external field that power mode depends on, so a live cable can be genuinely present and genuinely quiet. High voltage is the sharpest example: people expect high voltage to mean a big signal, and the balancing effect on HV often means very nearly the opposite.

The Same Ground, Two Hours Apart

One of the clearest demonstrations in the session is the same piece of ground scanned at different times of day. Early, with load on the network, power mode responds. Later, with the load gone, the same cable in the same position gives far less. Nothing underground changed between the two scans. Only the electricity did. Any survey resting on passive modes is therefore resting on the time of day it happened to be carried out.

Which Is Why the Genny Is the Point

None of this is a criticism of the receiver, and the session says so directly. It is an argument about sequence. If the passive modes are the survey, the survey is at the mercy of load, weather and timing. If the Genny is the survey and the passive modes are the sweep that supports it, you are working with a signal you put there deliberately, at a frequency you chose, on a service you identified. That is a different job, done with the same equipment.

A Passive Response Can Be in the Wrong Place

Even when passive modes do respond, the position they give you is not guaranteed. Where the field around a cable is distorted, the receiver reads the angle it is presented with rather than the truth, and the two aerials no longer see an equal, symmetrical field. The example used in the session is an eleven kilovolt cable read on power mode: from the surface the instrument indicates one position, and once you are down in the hole with it, it indicates another. Neither reading is the cable. Only one of them would have been marked on the road.

The Fix Is the Same Fix

The remedy for all of this is not a better passive mode, it is applying a signal. Connect a transmitter to the cable, or induce onto it if there is no safe connection, and you know which conductor the field is coming from, at a frequency you selected, on a service you identified. That is why this session ends where it does: passive scanning is a useful sweep and a poor survey, and the difference between the two is the transmitter.