3.5 Underground Services Avoidance (HSG47): Cable Avoidance Tools (CAT) & Genny Operation

Striking buried underground services—such as high-voltage electricity cables, high-pressure gas mains, water mains, or telecommunication lines—is one of the most severe risks in civil engineering and groundworks. Direct contact with live electric cables causes fatal burns and electrocution, while rupturing gas pipes leads to catastrophic fires and explosions.

To prevent utility strikes, Site Managers must strictly implement the HSE guidance publication HSG47 (Avoiding Danger from Underground Services).

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The Three-Stage HSG47 Safe System of Work

HSG47 mandates a sequential three-stage methodology that must be executed prior to breaking ground on any site:

+-------------------------------------------------------------------------+
| STAGE 1: PLANNING                                                       |
| Obtain current utility plans from all gas, electric, water, & telecom   |
| providers. Review record drawings on site before work begins.           |
+-------------------------------------------------------------------------+
                                    |
                                    v
+-------------------------------------------------------------------------+
| STAGE 2: LOCATING                                                       |
| Sweep the excavation area using a Cable Avoidance Tool (CAT) and        |
| Signal Generator (Genny) operated by competent, trained personnel.      |
+-------------------------------------------------------------------------+
                                    |
                                    v
+-------------------------------------------------------------------------+
| STAGE 3: SAFE DIGGING                                                   |
| Hand dig trial holes using insulated tools (BS 8020) or vacuum         |
| excavation. Prohibit power tools within 0.5m (500mm) of services.        |
+-------------------------------------------------------------------------+

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Stage 1: Planning and Utility Service Plans

Before any excavation commences, Site Managers must request and obtain up-to-date statutory service plans from all utility owners operating in the geographic area (electricity Network Operators, gas networks, water authorities, telecommunications companies, and cable TV operators).

Critical Limitations of Utility Plans:

• Indicative Only: Utility drawings show the approximate position of mains lines, but rarely show service connections to individual buildings.
• Depth Variations: Depth of cover changes over time due to road re-surfacing, regrading, or soil erosion. Never assume a cable is at a 'standard' depth.
• Inaccuracies: Older utility plans may contain historical alignment errors.

Plans must be present on site and referred to by supervisors and excavator operators throughout groundworks.

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Stage 2: Locating Services Using CAT and Genny

Site personnel operating locating equipment must hold valid training (e.g., EUSR or CITB CAT & Genny certification). Operatives must sweep the ground in a structured grid pattern.

Operational Modes of the Cable Avoidance Tool (CAT):

1. Power Mode (P):
   Detects electromagnetic signals radiated by live, loaded AC electricity cables carrying 50Hz current.
   Limitation: Power mode will not detect un-loaded live cables (e.g., streetlights during daytime), dead cables, DC cables, or non-metallic pipes.

2. Radio Mode (R):
   Detects low-frequency radio signals naturally present in the atmosphere that re-radiate along long, continuous metallic pipes and cables.
   Limitation: Radio mode will not detect plastic water/gas pipes or short, isolated metallic lines.

3. Generator Mode (G / Genny Mode):
   Detects a specific high-frequency signal emitted by the Signal Generator (Genny).

Why the Genny is Essential:

Using a CAT on its own in Power or Radio mode misses a vast proportion of underground utilities. The Genny must be used in conjunction with the CAT to apply a detectable signal onto target services via three connection methods:

• Direct Connection: Applying crocodile clips directly to a valve, street lamp stile, or exposed metallic pipe.
• Induction Clamp: Clamping a specialized signal jaw around a specific pipe or cable.
• Surface Induction: Placing the Genny on the ground surface above estimated service routes to induce a signal into buried lines beneath.

Depth Estimation: Modern CAT and Genny units operating in Generator Mode can measure the depth of cover down to buried services.

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Stage 3: Safe Digging Practices

Once buried services are traced and marked on the ground surface using color-coded spray paint or chalk, safe digging procedures must be enforced:

Safe Digging Exclusion Zone (0.5 Meter Rule):

• Mechanical excavators, power tools, hydraulic breakers, and mechanical augers are strictly prohibited within 0.5 meters (500mm) of marked live underground services.
• Within this 0.5m exclusion zone, ground must be excavated carefully by hand digging trial holes or using non-destructive vacuum excavation (suction excavation).

Hand Digging Standards:

• Hand digging must be carried out using non-conductive insulated hand tools conforming to BS 8020 (fully tested to 10,000V and rated for live working up to 1,000V).
• Digging tools must be inserted parallel to the service line; never use sharp picks, forks, or drive spades vertically into the ground above marked cables.
• Exposed utility pipes and cables must be physically supported using slings or timber bridging to prevent sagging and joint fracture.

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National Utility Color Coding Standards (NJUG Guidance)

Under National Joint Utilities Group (NJUG) standards, underground pipes and ducting follow standardized color schemes:

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Emergency Response Protocols for Utility Strikes

Site Managers must establish clear emergency procedures in the event of an accidental utility strike:

Gas Main Strike:

1. Evacuate the site upwind immediately.
2. Eliminate all potential ignition sources (shut down plant engines, prohibit smoking, do not switch electrical equipment on or off).
3. Inform the National Gas Emergency Service (0800 111 999) and Site Manager immediately.
4. Never attempt to plug or cap a leaking gas pipe.

Electricity Cable Strike:

1. Treat the cable and surrounding ground as LIVE.
2. If operating an excavator, the driver should stay inside the cab. If forced to leave due to fire, jump clear landing with both feet together—never touch the machine and ground simultaneously.
3. Cordon off an exclusion zone of at least 10 meters around the vehicle.
4. Contact the Distribution Network Operator (DNO) via 105 to isolate electrical power.