Excavating Beside a Marked HV Cable
You are digging a 1.2 m deep drainage trench with a 360° excavator and a 600 mm trenching bucket. The site drawings and a CAT scan show an 11 kV cable running parallel to the trench line, with the marked centreline 1.5 m from the trench edge. The drawings are dated 2019 and are marked 'indicative'. The cable is in a sand surround with a warning tile at approximately 600 mm depth. The trench is in firm clay with a band of made ground and brick rubble at 0.8–1.0 m. The client's permit requires hand-digging within 500 mm of any buried service, and a cable avoidance tool (CAT) is available but has not been used today. The trench must be 40 m long by end of shift.
1. Set out the sequence of actions before the bucket enters the ground, and explain the reasoning behind each.
Read the worked answer
- Treat the drawings as indicative only - a 2019 'indicative' drawing is not a reliable positional record; the 1.5 m offset cannot be assumed. Assume the cable may be closer.
- Use the CAT and genny today - sweep the full 40 m line and mark the detected position with paint/pegs, then sweep again at right angles to confirm. Record the result. A CAT is a detection aid, not proof of absence.
- Reconcile CAT result with drawings - if they disagree, the CAT result and a physical trial hole govern; escalate to the permit issuer before digging.
- Locate by hand first - dig a hand-excavated inspection hole (safe digging practice, insulated hand tools, no sharp implements) at the point where the cable is expected, to confirm depth and alignment.
- Mark the 500 mm exclusion zone and brief the banksman and any plant operators.
- Confirm the permit and isolation status - is the cable live? If it cannot be proven dead, the permit must define the controls (e.g. no mechanical excavation within 500 mm, cable strike precautions, emergency contact).
Reasoning: The made ground and brick rubble will give false CAT readings and can deflect a bucket; the sand surround and warning tile are the only physical confirmation of the cable. The 500 mm hand-dig rule is the control that prevents a strike, and it only works if the cable's actual position is known - which it currently is not.
2. At 0.9 m the bucket strikes brick rubble and the operator believes he has found the warning tile. What are the next actions and what must not be done?
Read the worked answer
Stop the machine. Do not continue to dig or 'feel' with the bucket.
- Do not assume the tile means the cable is where the drawing says - the tile marks the cable's route, which may run diagonally or loop.
- Do not use the bucket to clear around the tile.
- Do switch off the engine, dismount, and hand-expose the area around the tile with insulated hand tools, working from the side, not directly above.
- Do confirm the cable's position and depth relative to the trench line; if it is within 500 mm of the trench edge, the remainder of that section is hand-dig only.
- Do record the finding, photograph it, and inform the supervisor/permit issuer - the permit and drawings need updating before further machine work.
- Do check the CAT again over the exposed section to calibrate its reading against the known position.
Why: A warning tile is a positive indicator, not a guarantee of alignment; the made ground means the cable may have been displaced during earlier works. The only safe way to establish position is controlled hand excavation with the machine stopped and the area controlled.