What is the difference between CAT (passive) mode and Genny (active) mode?

Passive mode detects electromagnetic fields produced by cables already carrying a signal — either the 50Hz field from mains voltage cables, or radio frequency fields induced in buried metallic services from natural or man-made RF sources. Passive mode requires no preparation and can detect any energised mains cable or pipe carrying a signal. Active mode (Genny mode) uses a signal generator to inject a specific frequency signal onto a cable or service, which the CAT detector then locates. Active mode is more accurate and more reliable for locating specific services because the injected signal is stronger and at a distinct frequency not present in the environment. It can also locate cables that are de-energised (no current, no passive field). Active mode requires physical access to the cable or a connected conductor to apply the Genny signal.

How accurate is the depth indication on a cable detector?

Depth indication on a CAT is based on the ratio of signal strength between a pair of antenna elements at known separation. It is an approximation — not a precision measurement. Accuracy is typically ±10–15% under ideal conditions (homogeneous soil, single isolated cable, correct detector orientation). Accuracy degrades significantly when multiple cables are close together (mutual interference), when the cable is shielded, when soil conditions are heterogeneous, or when the detector is not held in the correct orientation. Always treat depth readings as indicative only. The Highway Authorities and Utilities Committee (HAUC) code of practice and PAS 128 (specification for underground utility detection) both state that depth readings should be used for initial assessment only, with trial excavation required to confirm depth before heavy digging.

Can a cable detector find de-energised buried cables?

In passive mode, no — a cable detector relies on an electromagnetic field generated by current flowing in the cable. A de-energised cable with no current carries no field and cannot be detected passively. In active mode (Genny mode), the signal generator injects a signal onto the cable. This requires physical access to the cable termination or a connected metallic conductor. If you can connect the Genny to a metal pipe or cable tray that is electrically connected to the buried cable, the injected signal will travel along the buried service and the CAT can detect it. For completely isolated buried cables with no accessible termination, ground-penetrating radar (GPR) is the appropriate technology — but this is specialist equipment beyond the scope of a standard cable detector.

What is the peak-null method for accurate cable location?

The peak-null method uses two different antenna configurations on the CAT to accurately pinpoint the cable position. In peak mode, the detector displays maximum signal strength when the antenna is directly above the cable — the signal peaks at the cable position. In null mode, the detector uses a different antenna orientation that produces a null (minimum) signal when directly above the cable, with maximum signal at either side. The null point is narrower and more precise than the peak point, making it easier to pinpoint the exact cable position. The standard procedure is to first use peak mode to find the approximate cable direction, then switch to null mode to pinpoint the exact position by finding the minimum signal. Mark the cable line from multiple null points. Modern CATs like the Radiodetection CAT4+ perform this automatically, but understanding the underlying method helps interpret readings correctly.

How do I use a Genny to trace a specific cable?

The Genny (signal generator) is applied to the cable or service you want to trace. Connection methods depend on the access available: direct connection using crocodile clips to the cable termination conductors, inductive (clamp) coupling around the cable where it is accessible, or signal coupling through a metallic pipe or cable tray electrically connected to the buried service. The Genny must be earthed — either to the installation earth, a temporary earth stake, or a metallic object with a good earth connection. The injected signal frequency should be set to a frequency that the CAT receiver is tuned to detect. On the Radiodetection CAT4+, the receiver automatically detects the Genny4 signal frequency. Tune the receiver sensitivity so the signal is detectable but not so strong that it couples onto adjacent services and creates a false trace.

Is a cable detector necessary for drilling into walls inside a building?

Yes. Buried wiring inside walls, floors, and ceilings can be located and damaged by drilling or chiselling during alterations, second fixing, or maintenance work. The risk of cable strike and electric shock or arc flash is real even in internal wall drilling. A compact cable detector (such as the Bosch GMS 120 for domestic use, or the Ridgid Scout 100 for professional use) provides a rapid scan before drilling to identify buried cables, water pipes, and other services. PAS 68 (buried services guidance for domestic work) recommends cable detection before any core drilling, chiselling, or chase cutting that penetrates a wall or floor. Cable strikes during internal building work are a common cause of electrical accidents reported to HSE under RIDDOR.

What is the difference between the Radiodetection CAT4 and CAT4+?

The CAT4 and CAT4+ are the current (4th generation) Radiodetection cable avoidance tools. The CAT4+ adds several features over the base CAT4: a strike alert that records every time the detector enters a high-signal-strength zone (providing a tamper-evident log of near-misses for safety records), Bluetooth data logging for live depth and position data, and a more sensitive receiver for lower-current cables. Both models detect the same frequency ranges in passive and active modes. The Genny4 signal generator is compatible with both. For contractors who need strike logging for safety compliance records, the CAT4+ is the appropriate choice. For general use where data logging is not required, the CAT4 offers the same core detection performance at a lower cost.

TOOLS & EQUIPMENT GUIDE

Cable Detector Buying Guide: CAT & Genny Systems for UK Electricians 2026

Everything you need to know about cable detectors — passive vs active detection, the CAT & Genny system, depth indication limitations, common survey errors, and the best cable avoidance tools for professional electrical and civil work in 2026.

Start 7-day free trial

Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians

12 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

ShareX in W

1,000+

UK electricians

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer — DP Electrical

Key Takeaways

1Cable detectors (also called Cable Avoidance Tools, or CATs) detect buried services before excavation or drilling to prevent cable strike injuries and service damage.
2There are two detection modes: passive mode detects live cables by the electromagnetic field produced by mains frequency current; active mode uses a signal generator (Genny) to inject a traceable signal onto a known cable or system.
3Depth indication is an approximation — depth figures from a CAT should be treated as indicative, not precise. Always verify with trial excavation before heavy digging near the indicated depth.
4Signal strength varies with cable depth, current level, cable shielding, soil conditions, and the orientation of the detector. Sweep in multiple directions and use the peak-null method for accurate location.
5The Radiodetection CAT4+ with Genny4, Leica DD130, and Ridgid Scout 100 are the leading professional systems for UK electricians and civil contractors in 2026.

01 · Tools & Equipment Guide

Why Cable Detection is Safety-Critical

Cable strikes are one of the most serious hazards in construction and electrical work. Striking a buried cable while drilling, chiselling, or excavating can cause electrocution, arc flash, explosion, fire, and gas release if associated services are also damaged. HSE statistics consistently identify cable strikes as a significant cause of serious injuries and fatalities in construction.

The Health and Safety Executive (HSE) guidance document HSG47 "Avoiding Danger from Underground Services" sets out the requirement to identify and locate underground services before excavation. The principle applies equally to internal building work — buried cables in walls and floors must be located before drilling or chasing.

A cable detector (Cable Avoidance Tool, or CAT) is the primary tool for locating buried services on site. Used correctly with a signal generator (Genny), it can locate mains cables, water pipes, gas pipes (where they carry a signal), and telecomms conduits before work begins. This guide covers the detection methods, the CAT & Genny system, depth indication, common errors, and the best detectors available in 2026.

Free download

Get the BS 7671 A4:2026 Cheat Sheet — free

Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.

Every regulation change summarised
New model forms (EIC + MEIWC)
Free PDF — no subscription

02 · Tools & Equipment Guide

Signal Generator Method vs Live Cable Detection

Cable detection relies on two fundamentally different principles. Understanding both is essential for reliable service location:

Passive Detection (Live Cable Mode)

Detects the electromagnetic field radiated by current flowing in a buried conductor. Mains cables carrying 50Hz alternating current produce a characteristic 50Hz electromagnetic field that the CAT receiver detects and displays. Radio frequency detection (often called "Radio" mode) detects lower-level RF fields induced in buried metallic conductors by man-made and natural radio frequency signals.

+No preparation required
+Detects any energised mains cable
–Cannot detect de-energised cables
–Multiple cables cause mutual interference
–Cannot distinguish which service is which

Active Detection (Genny Mode)

A signal generator (Genny) is connected to the target service and injects a specific frequency signal. The CAT receiver detects only this injected frequency, giving a strong, clean signal on the specific cable being traced while ignoring other services.

+Traces a specific identified service
+Works on de-energised cables
+Stronger, more reliable signal
–Requires access to connect the Genny
–Signal can couple to adjacent services

Best practice per HSG47 and PAS 128 is to use both methods: passive mode first to identify all services in the area, then active mode with the Genny to trace specific services and confirm their routes before excavation.

03 · Tools & Equipment Guide

The CAT & Genny System Explained

The CAT (Cable Avoidance Tool) and Genny (signal generator) system is the industry standard for professional cable location in the UK. The two instruments work as a matched pair: the Genny generates a signal at a specific frequency, and the CAT receiver is tuned to detect that frequency.

Genny connection methods: Direct connection (crocodile clips to cable termination conductors), inductive coupling (clamp the Genny transmitter coil around the cable where accessible), or signal plate coupling (place the Genny signal plate on the ground over a buried service to couple inductively without direct contact). The earthing connection is always required — Genny to installation earth, earth stake, or metallic earth reference.
Signal frequencies: Most Genny systems operate at 33kHz (general purpose) and 8Hz (selective, lower coupling to adjacent services). The Radiodetection CAT4+/Genny4 system uses 33kHz and 131kHz with automatic frequency detection in the receiver. Lower frequencies travel further along services with less coupling to adjacents; higher frequencies give a stronger signal over short distances.
Signal coupling to adjacent services: The injected signal will couple electromagnetically to any nearby metallic service running parallel to the target cable. This can create a false trace on an adjacent water pipe or telecomms duct. Always compare signal strength between services — the strongest signal should be on the target service. Use lower frequency (8Hz) to reduce coupling to adjacents in congested service corridors.

04 · Tools & Equipment Guide

Depth Indication and Signal Strength

Depth indication and signal strength display are two key outputs from a professional CAT. Understanding their limitations is as important as knowing how to use them:

Depth Indication

Calculated from the ratio of signal strength at two antenna heights. Provides an approximate depth under ideal conditions — single isolated cable, homogeneous ground, correct detector orientation. Error of ±10–15% under ideal conditions; much larger in practice near multiple services, rebar, or heterogeneous ground. Always treat depth as indicative — confirm by trial excavation before heavy plant work. PAS 128 quality levels define acceptable depth accuracy for survey work.

Signal Strength Bar/Indicator

Displays the relative strength of the detected signal. Signal strength increases as you move directly over the cable and decreases as you move away. Used to track the cable route by keeping to the line of maximum signal. Sensitivity should be set so the maximum on-cable reading is approximately 80% of full scale — not pegged to maximum, as this saturates the display and makes it impossible to distinguish the peak location.

Signal Sweep Technique

Walk a grid pattern over the suspected cable area in two perpendicular directions. Mark cable positions from multiple sweeps. Cross-reference peak mode markings with null mode markings (which are more precise) for accurate position. In congested areas with multiple services, the signal patterns from individual cables overlap and must be carefully interpreted. Call in a specialist buried services surveyor where services are densely packed or high-risk.

Try Elec-Mate free for 7 days

16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.

Start free trial

05 · Tools & Equipment Guide

Common Errors and How to Avoid Them

Cable detection errors lead to cable strikes. The most common errors are:

Assuming no signal means no cable: Passive mode only detects energised cables. De-energised, low-load, or well-shielded cables may give no detectable signal. Always use active Genny mode to supplement passive detection.
Incorrect sensitivity setting: Too high a sensitivity causes the signal to saturate — the detector shows maximum signal everywhere, making it impossible to locate the cable precisely. Too low a sensitivity misses weak signals from deep or low-current cables. Adjust sensitivity for each location.
Sweeping in one direction only: Cables run in unexpected directions. Always sweep in at least two perpendicular directions. A cable running parallel to your sweep direction may give only a weak, broad signal — sweeping perpendicular gives a clear cross-cable reading.
Treating depth as accurate: Depth indication is approximate. Cables may be deeper or shallower than indicated due to ground conditions, cable bends, or interference from nearby services. Use depth indication for initial assessment and confirm by careful trial excavation.
Failing to obtain service records: Always request service records from the relevant utility companies and building management before commencing work. Cable detection is a supplement to, not a substitute for, researching existing drawings and service records.

Document site conditions and services before work starts

Use Elec-Mate's AI to generate method statements and risk assessments for excavation and cable work — including buried service identification procedures…

Try it free for 7 days

06 · Tools & Equipment Guide

Best Cable Detectors for Electricians 2026

The professional cable detector market in 2026 is dominated by Radiodetection and Leica at the top end, with strong mid-range options from Ridgid and C.Scope. Here are the leading choices for UK electricians and contractors:

Radiodetection CAT4+ with Genny4

Industry Standard Professional System — ~£900 (kit)

The Radiodetection CAT (Cable Avoidance Tool) and Genny (signal generator) system is the most widely used professional cable detection system in the UK. The CAT4+ adds strike alert data logging and Bluetooth connectivity over the base CAT4. Detects power (50Hz passive), radio (RF passive), and Genny4 signal (33kHz and 131kHz active). Depth indication on active signal. Excellent sensitivity for low-current cables. The go-to recommendation for any contractor involved in excavation or buried services work.

Leica DD130 (CAT & Genny)

Best Alternative Professional System — ~£750 (kit)

Leica's DD130 is a capable professional alternative to the Radiodetection system. Passive power and radio detection plus active Genny signal. Clear display with depth indication. Robust construction suited to outdoor site conditions. Compatible with Leica's broader geospatial survey equipment ecosystem — useful for contractors who already use Leica survey instruments. Slightly lighter than the CAT4+ which some users prefer for all-day carrying on large sites.

Ridgid Scout 100

Best for Internal Building Work — ~£300

Compact cable and pipe locator designed for use inside buildings — walls, floors, and ceilings. Detects energised cables and metallic water/gas pipes. Separate transmitter applies a signal to pipes and cables for active tracing. Smaller form factor than full-size CAT systems suits internal wall scanning before drilling. A good choice for electricians who primarily do internal alterations work and do not regularly work on buried external services.

C.Scope MXL4 with SGA4

UK-Made Alternative — ~£600 (kit)

C.Scope is a UK-based cable detector manufacturer with a long track record in the utilities sector. The MXL4 provides all standard passive and active detection modes. The SGA4 signal generator pairs with the MXL4 for active service tracing. Preferred by some UK utilities contractors who appreciate the UK manufacturer support and service network. Competitive with Radiodetection on sensitivity and features at a slightly lower price point.

07 · Tools & Equipment Guide

Cable Detection in Practice: Before You Drill or Dig

Cable detection integrates into the pre-work safety routine for any job involving drilling, chasing, or excavation. Here is the systematic approach recommended by HSG47:

Obtain service records — contact the relevant utility companies and building owner to obtain existing service drawings. Check the planning drawings and building services as-built drawings for internal work.
Visual survey — identify surface features that indicate underground services: valve boxes, junction boxes, marker posts, overhead lines, and the route of service entry points into the building.
Passive CAT sweep — sweep the work area in power mode (50Hz) and radio mode to identify all detected services. Mark positions with marking spray or pins.
Active Genny tracing — connect the Genny to known accessible services and trace their routes through the work area. Mark confirmed routes.
Reconcile with records — compare detected service positions with service records. Investigate any discrepancies before proceeding.
Trial excavation — hand dig trial holes to expose and visually confirm service positions and depths before using mechanical plant.

Generate pre-work risk assessments and method statements

Elec-Mate's AI generates site-specific method statements for drilling, chasing, and excavation work — including buried services identification…

Try it free for 7 days

Frequently Asked Questions About Cable Detectors

What electricians say

Verified reviews from the UK App Store.

One App for Everything!

Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.

Fantastic app for electricians

I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.

Absolutely amazing

I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.

Trusted by electricians across the UK

Real feedback from real sparks

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer

Sole Trader · DP Electrical

“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”

Nathan Perry

Electrician · NP Electrical Services

“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”

Jake Pizey

3rd Year Apprentice · Apprentice

7-Day Free Trial — Cancel Anytime, No Hassle

Generate Method Statements and Risk Assessments for Cable Work

Join 1,000+ UK electricians using Elec-Mate for AI-generated method statements, risk assessments, and professional EIC certification. 7-day free trial, cancel anytime.

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer, DP Electrical

From £6.99/mo after trial — less than a coffee a week

or download the app

7 days free, then from £6.99/moCancel in one tap — no calls, no hassleiOS, Android & WebBS 7671 compliant

Cable Detector Buying Guide: CAT & Genny Systems for UK Electricians 2026

Key Takeaways

Why Cable Detection is Safety-Critical

Get the BS 7671 A4:2026 Cheat Sheet — free

Signal Generator Method vs Live Cable Detection

Passive Detection (Live Cable Mode)

Active Detection (Genny Mode)

The CAT & Genny System Explained

Depth Indication and Signal Strength

Depth Indication

Signal Strength Bar/Indicator

Signal Sweep Technique

Try Elec-Mate free for 7 days

Common Errors and How to Avoid Them

Document site conditions and services before work starts

Best Cable Detectors for Electricians 2026

Radiodetection CAT4+ with Genny4

Leica DD130 (CAT & Genny)

Ridgid Scout 100

C.Scope MXL4 with SGA4

Cable Detection in Practice: Before You Drill or Dig

Generate pre-work risk assessments and method statements

Frequently Asked Questions About Cable Detectors

What is the difference between CAT (passive) mode and Genny (active) mode?

How accurate is the depth indication on a cable detector?

Can a cable detector find de-energised buried cables?

What is the peak-null method for accurate cable location?

How do I use a Genny to trace a specific cable?

Is a cable detector necessary for drilling into walls inside a building?

What is the difference between the Radiodetection CAT4 and CAT4+?

Continue reading

Multimeter Guide for Electricians

Drill Guide for Electricians

Safe Isolation Procedure

EIC Certificate App

Cable Sizing Calculator

Health and Safety Training

What electricians say

One App for Everything!

Fantastic app for electricians

Absolutely amazing

Trusted by electricians across the UK

Generate Method Statements and Risk Assessments for Cable Work