OBR Construction — Cable Avoidance Tool & Signal Generator
~30 minTheory
10Questions
75%Pass Mark
Practical + TheoryRequired
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Practical Session Required Before Theory Assessment
This course requires completion of a supervised practical session with your OBR Training Manager before your certificate can be issued. The theory module below can be completed at any time, but your full certificate will only be issued once your training manager has confirmed your practical competency in the admin dashboard.
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! Why This Training Matters
Striking a buried underground cable is one of the most dangerous incidents that can occur on a construction site. In the UK, around 60,000 cable strikes occur every year — causing electrocutions, explosions, fatalities, and major disruption. The majority are preventable.
A Cable Avoidance Tool (CAT) and Signal Generator (Genny) are the primary tools used to locate buried services before excavation, driving stakes, or any intrusive ground works. Used correctly, they dramatically reduce the risk of a cable strike. Used incorrectly or not at all, they provide no protection.
Legal requirement: Under HSG47 (Avoiding Danger from Underground Services) and CDM 2015, underground services must be located before any excavation work begins. Failure to do so is a criminal offence. The HSE actively investigates cable strikes and prosecutes contractors.
1 Understanding Underground Services
Before using a CAT and Genny, it is essential to understand what types of services may be present underground and the risks each presents.
Red — Electricity
High and low voltage cables. Strike risk: electrocution, arc flash, explosion. Most dangerous buried service. Cables can be at any depth — often shallower than expected due to ground movement.
Yellow — Gas
Natural gas and other fuel gases. Strike risk: explosion, fire, asphyxiation. Gas can accumulate in excavations. Even a small leak is extremely dangerous.
Blue — Water (Potable)
Mains water supply. Strike risk: flooding, ground instability, service disruption. High-pressure mains can cause significant damage and injury.
Black — Sewerage
Foul and surface water sewers. Strike risk: contamination, flooding, harmful gases (hydrogen sulphide). Sewage contains biological hazards.
Green — Telecoms / Data
Fibre optic and copper telecoms cables. Strike risk: service disruption, potential for high voltage in some cable types, compensation claims. Fibre optic cables can release harmful light energy when broken.
Services are not always where plans show them. Ground movement, previous works, and inaccurate records mean services are frequently found at unexpected depths and positions. Plans are a guide — not a guarantee.
2 The CAT and Genny Explained
The CAT (Cable Avoidance Tool) and Genny (Signal Generator) work together as a system to detect buried services. Understanding what each does — and their limitations — is critical to using them safely.
The CAT — Cable Avoidance Tool
A handheld receiver that detects electromagnetic signals emitted by buried cables and pipes. It has three detection modes:
Power mode — detects signals from live electricity cables (50Hz). Works passively — no Genny needed.
Radio mode — detects radio frequency signals that naturally conduct along buried metallic services. Works passively.
Genny mode — detects the signal applied by the Genny to a specific service. Most accurate mode.
The Genny — Signal Generator
A portable signal generator that applies an electromagnetic signal to a service, making it easier for the CAT to detect it accurately. Used to:
Locate non-live or non-metallic services
Trace a specific service route
Improve accuracy in areas with multiple services
Detect plastic pipes containing a trace wire
The Genny can be applied directly to an access point (manhole, valve) or inductively over the ground.
Critical limitations:
The CAT cannot detect non-metallic pipes (e.g. plastic water or gas pipes) without a Genny and trace wire
Signal can be masked by reinforced concrete, other cables, or interference
The CAT shows where a signal is strongest — not necessarily where the cable is
A clear CAT reading does NOT mean the area is free of services — use all three modes and always excavate carefully
3 Pre-Use Checks and Equipment Setup
Before using a CAT and Genny on site, a series of checks must be carried out. Using faulty or incorrectly set up equipment is as dangerous as not using it at all.
CAT pre-use checks:
Check batteries are fully charged — low batteries reduce detection range significantly
Carry out a self-test as per the manufacturer instructions — all indicators should function correctly
Check the instrument has a current calibration certificate — CATs must be calibrated annually by an accredited centre
Check for physical damage — a cracked or damaged CAT may give unreliable readings
Test all three detection modes (Power, Radio, Genny) before use
Genny pre-use checks:
Check batteries are fully charged
Check cables and clamps for damage
Confirm the output frequency matches the CAT being used
Test signal output before applying to service
Calibration: CATs must be calibrated at least annually by an accredited calibration centre. An out-of-calibration CAT may underread or miss signals entirely. Always check the calibration date on the instrument label before use. Do not use an out-of-calibration CAT — remove it from service and report it to your supervisor.
Service plans first: Always obtain service drawings from the utility provider and mark up known service positions on the ground before beginning any CAT scan. The CAT supplements plans — it does not replace them.
4 Safe Scanning Technique
Correct scanning technique is as important as having the right equipment. Poor technique leads to missed services and cable strikes.
The correct scanning sequence — always use all three modes:
Power mode first — scan the full area in power mode to detect any live electricity cables
Radio mode second — scan the full area in radio mode to detect any services carrying radio frequency signals
Genny mode third — apply the Genny to known access points and scan in Genny mode to trace specific services
Scanning technique:
Hold the CAT vertically and sweep it slowly side to side in overlapping passes — do not rush
Walk at a steady pace of approximately 1 metre per second
Scan in two perpendicular directions (north-south and east-west) to maximise detection
Mark detected services on the ground using appropriate marking paint or pins
Pay particular attention to areas where the signal strength peaks — this indicates the likely position of the service
Reduce sensitivity settings when close to a detected service to improve accuracy of positioning
Marking Detected Services
Mark all detected services on the ground using HSS colour-coded marking spray or flags — matching the utility colour code (red for electric, yellow for gas, etc.). Mark both the centreline and the estimated extents of the service.
Safe Excavation Near Services
Once services are marked, excavate with hand tools only within 500mm of any detected service. Never use a mechanical excavator within 500mm of a marked service. Hand dig carefully and expose the service before any further machine excavation alongside it.
Never assume a clear reading means no services are present. Always hand dig trial holes to confirm service positions before any machine excavation. Services may be present that the CAT cannot detect.
5 If You Strike a Cable or Service
Despite all precautions, cable strikes can still occur. Knowing the correct response can save lives. The wrong response — such as staying in the excavation to investigate — can be fatal.
If you strike an electricity cable:
Do NOT touch the cable — assume it is live at all times
Evacuate the area immediately — all personnel must move at least 15 metres away
Do not use mobile phones near the strike — use a phone away from the area
Call 999 and the electricity network operator emergency line
Do not re-enter the area until the network operator confirms the cable is safe
Report to your supervisor — the incident must be investigated and reported to the HSE
If you strike a gas pipe:
Evacuate the area immediately — do not use any ignition sources
Do not operate any electrical switches
Call 999 and the National Gas Emergency number: 0800 111 999
Keep people away from the area until the gas network operator confirms it is safe
Reporting requirements: All cable and service strikes must be reported to your supervisor immediately, recorded in the site accident/incident book, and reported to the relevant utility provider. Serious strikes must be reported to the HSE under RIDDOR. The CAT and Genny must be retained for inspection as evidence.
6 Key Summary
DO
Obtain service plans before scanning
Check calibration date before every use
Use all three CAT modes — Power, Radio, Genny
Scan in two perpendicular directions
Mark all detected services before excavating
Hand dig within 500mm of any detected service
Evacuate immediately if a cable is struck
Report all strikes to your supervisor
DON'T
Start excavation without scanning first
Use an out-of-calibration CAT
Rely on only one detection mode
Assume a clear reading means no services
Use a machine within 500mm of a detected service
Touch a struck cable — assume live
Re-enter an area after a strike without clearance
Ignore service plans — use them alongside the CAT
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Reminder: Practical Session Required
Passing this theory assessment does not complete your CAT and Genny training. Your certificate will only be issued once your supervised practical session has been confirmed by your OBR Training Manager. Make sure you have booked or completed your practical session.
? Knowledge Assessment
Answer all 10 questions. You must score 75% or above (8/10) to pass. Your certificate will be issued once your practical session has also been confirmed.
Question 1 of 10
Approximately how many cable strikes occur in the UK every year?
Question 2 of 10
What colour are gas pipes marked in the HSS underground service colour coding system?
Question 3 of 10
How often must a CAT be calibrated by an accredited calibration centre?
Question 4 of 10
In which order should the three CAT detection modes be used when scanning an area?
Question 5 of 10
The CAT gives a clear reading with no signal detected. What does this mean?
Question 6 of 10
Within what distance of a detected underground service must hand tools only be used for excavation?
Question 7 of 10
You strike an electricity cable during excavation. What is your immediate first action?
Question 8 of 10
What is the purpose of the Genny (Signal Generator)?
Question 9 of 10
Why must you scan in two perpendicular directions when using the CAT?
Question 10 of 10
What is the emergency number for a gas escape?
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Theory Assessment Passed — Practical Sign-Off Pending
Well done — you have passed the theory assessment. Your record has been submitted to OBR Construction.
Your certificate will be issued once your Training Manager has confirmed your practical competency in the CAT and Genny.
Please speak to your Training Manager to arrange or confirm your practical session. Quote your reference number below.
Once your practical has been signed off, your certificate will be available from the OBR Training Admin Dashboard.
Recording your completion...
Certificate of Completion
CAT and Genny
This is to certify that
CAT and Genny Training
Has successfully completed the OBR Construction CAT and Genny Training programme, comprising both a supervised practical competency assessment and an online theory module, in accordance with HSG47 (Avoiding Danger from Underground Services) and CDM 2015.
