There are roughly four million kilometres of buried pipes and cables under the UK, and striking one can kill. Most people involved in excavation know they should scan before they dig. Fewer are clear on what the scan actually proves — and that gap is where accidents happen.
This article covers where cable avoidance equipment fits within HSE guidance HSG47, what a CAT can and cannot detect, and how CAT and Genny training differs from the NRSWA Location and Avoidance unit — a distinction that catches people out when a client asks for one and they hold the other.
The single most important point on this page: a clear scan is not proof that the ground is clear. A CAT detects electromagnetic signals. Services that emit none — plastic water pipes and non-metallic ducts without a tracer wire, in particular — can be entirely invisible to it.
No signal means no signal was detected. It does not mean there is nothing there.
HSG47: the three elements of safe excavation
HSG47, Avoiding danger from underground services, is HSE’s principal guidance for anyone planning, managing or carrying out work that breaks ground. It is guidance rather than law, supporting compliance with the Health and Safety at Work etc. Act 1974 and CDM 2015 — but it describes what HSE considers good practice, which is what you will be measured against if something goes wrong.
It sets out three elements of a safe system of work. Cable avoidance equipment sits in the middle one.
Plan the work
Obtain up-to-date service plans from the utility owners before anyone arrives on site. Establish who is responsible for what. Plans are the starting point — they are frequently out of date, incomplete or drawn to the wrong depth, which is exactly why HSG47 says plans alone are not sufficient.
Locate and identify
This is where the CAT and Genny come in. Locating equipment is used to trace service routes on site and mark them up, confirming or correcting what the plans showed. HSG47 is explicit that anyone using this equipment must be competent to do so.
Excavate safely
Trial holes to confirm actual positions, hand digging within proximity zones, careful use of mechanical plant, and different precautions for electricity, gas, water and telecoms. Scanning does not replace safe digging technique; it informs it.
All three, every time. The most common failure is treating the scan as the whole job — no plans obtained beforehand, or the machine brought in as soon as the marks are down. Each element catches what the others miss, which is the entire point of having three.
What the equipment actually does
CAT — Cable Avoidance Tool
A receiver. It detects electromagnetic signals radiating from buried services, and works in three modes:
Power — picks up the 50Hz field from live electricity cables. Finds nothing on a cable carrying no load.
Radio — detects re-radiated radio signals travelling along long metallic services. Useful for pipes and unloaded cables, but unreliable on short runs.
Genny — picks up a signal you have deliberately applied. The most precise mode, and the reason the two tools are used together.
Genny — Signal Generator
A transmitter. It applies a known signal to a service so the CAT can trace it precisely, either by direct connection, by clamping around a cable, or by induction from the surface.
Using the Genny transforms a rough indication into a traceable route, which is why scanning in power and radio modes alone is only a partial survey.
Some signal generators can also be applied along pipes, which helps in situations where passive modes have not worked.
What a CAT will not find
This is the part that formal training covers and casual familiarity does not. A CAT has real limitations, and knowing them is most of the competence.
- Plastic pipes with no tracer wire. Modern water and gas distribution uses a great deal of plastic. With nothing metallic to carry a signal and no tracer wire fitted, there is nothing for the CAT to detect.
- Unloaded electricity cables in power mode. A cable carrying no current radiates no 50Hz field. It is still perfectly capable of injuring someone when energised or struck.
- Fibre optic ducts without a metallic element.
- Deep services beyond the equipment’s effective depth range.
- Services running close together, where signals merge and one route masks another.
- Anything the operator walks past because the sweep pattern was wrong, the sensitivity was set badly, or the daily function check was skipped.
Which is why trial holes and hand digging are not optional extras. The scan tells you where services probably are; careful excavation is what confirms it.
Keeping the equipment trustworthy
A locator that has drifted out of calibration gives confident readings that are simply wrong, and nothing about it looks broken. Two habits matter:
- Daily function check before use, following the manufacturer’s procedure — confirming the CAT responds to the Genny at a known distance is the usual method.
- Periodic calibration to the manufacturer’s schedule, commonly annual, with certificates kept. Clients and principal contractors increasingly ask to see them.
CAT and Genny training or the NRSWA LA unit?
These get confused constantly, and holding the wrong one is an awkward discovery on a Monday morning.
| CAT and Genny training | NRSWA Unit LA | |
|---|---|---|
| What it is | Equipment competence — using locating tools correctly | A regulated unit within the street works scheme: Location and avoidance of underground apparatus |
| What you get | A training certificate evidencing competence with the equipment | A unit recorded on your SWQR card, valid five years |
| When you need it | Any excavation where you scan before digging — sites, private land, utilities work | Working on the public highway. LA is a key certificate: you cannot add other street works units without it |
| Book | CAT and Genny Training | Location and Avoidance of Underground Apparatus |
In short: if you are digging in the public highway, you need the LA unit and it goes on your SWQR card. If you are excavating anywhere else, CAT and Genny training is what evidences your competence with the equipment. Plenty of people working across both need both. Our guide to NRSWA training sets out how the LA unit fits with the rest of the street works scheme.
Book cable avoidance training
Evidence your competence with locating equipment before the next excavation, and keep your scan-before-dig procedure defensible.
Common questions
Is CAT and Genny training a legal requirement?
No specific course is named in law. What applies is the general duty to ensure employees are competent, and HSG47’s expectation that anyone using cable locating equipment is competent to do so. Training is the normal way of evidencing that, and its absence is difficult to defend after a strike.
How often should it be refreshed?
No statutory interval applies. Many organisations work to a two or three year cycle, and refresh sooner when new equipment is introduced, after a near miss or strike, or where someone scans only occasionally and is out of practice.
The scan came back clear. Can we dig with the machine?
No. A clear scan reduces uncertainty; it does not remove it. Plastic pipes without tracer wire, unloaded cables and deep services can all go undetected. HSG47’s third element still applies — trial holes to confirm positions, and hand digging within proximity zones before mechanical excavation.
Can CAT and Genny training be done online?
The knowledge element works well online — how the equipment functions, the modes, the limitations, where it sits in HSG47. Operating the equipment is a practical skill, so pair the e-learning with hands-on practice using your own kit, ideally supervised by someone experienced. Some clients specify a practical assessment, so check the requirement before booking.
Where can I get HSG47?
Free from HSE. The current version is the third edition, and it is worth reading in full if you manage excavation work — it is short, practical, and it is the standard you will be measured against. Download HSG47 from HSE.
Related training
Excavation and underground services
Construction and utilities
Related safety topics
This page is general guidance and does not replace HSE guidance HSG47 or your own risk assessment.