Underground Services Toolbox Talk

The jobs that hit services are rarely the ones you would expect

Trenching gets the permit, the drawings and the locator sweep. The fence post does not. Neither does the road pin, the ground anchor, the tree pit, the stake driven in to hold a hoarding, or the breaker going through a slab to see what is underneath. Those jobs feel too small to warrant the paperwork, which is precisely why they are the ones that find the cable.

HSE is blunt about what happens next. Damaged cables kill and injure people through electric shock, electrical arcs causing an explosion, and flames, and the burns reach hands, face and body even when protective clothing is being worn. You do not have to touch anything. In HSE’s own case material, one worker was killed striking an 11,000 volt cable, and another suffered severe burns from a cable at 415 volts, the same order of voltage as a three-phase supply to a domestic street.

Depth is not the protection people assume it is

HSG47 gives typical burial depths, and they are shallower than most crews guess. Electricity cables usually sit between 450 mm and 1 m down. Gas mains are around 750 mm under a roadway and 600 mm under a footway, with service connections to individual properties shallower again at roughly 450 mm, and around 375 mm once they are on private ground. Water mains and sewers are generally 900 mm or more. Every one of those figures is a typical, not a guarantee, and ground that has been opened and reinstated before can be shallower than any of them.

What the law requires

Underground services have their own regulation in CDM 2015, and the fourth paragraph of it is one of the more direct sentences in construction law. It does not ask you to intend to take precautions. It says the work must not be carried out until they have been taken.

• No work until the steps are taken. Construction work liable to create a risk to health or safety from an underground service, or from damage to or disturbance of it, must not be carried out unless suitable and sufficient steps have been taken to prevent the risk so far as is reasonably practicable. CDM 2015, reg. 25(4).
• Locate, check and indicate. Where necessary to prevent danger, energy distribution installations must be suitably located, periodically checked and clearly indicated. CDM 2015, reg. 25(1).
• Work near a system must not create danger. Every work activity, including work near an electrical system, must be carried out so as not to give rise to danger so far as is reasonably practicable. Electricity at Work Regulations 1989, reg. 4(3).
• Competence is a legal test, not a judgement call. Nobody may be engaged in work where technical knowledge or experience is needed to prevent danger unless they have it, or are supervised to a degree appropriate to the work. Electricity at Work Regulations 1989, reg. 16.

The point people miss about pre-construction information

Getting hold of the service drawings is not the digging crew’s problem to solve on the morning of the dig. HSE is clear that clients and principal designers are expected to provide pre-construction information about underground services, and CDM 2015 puts that duty upstream of the people holding the tools. If a crew is standing over unproven ground with no drawings, the failure happened weeks earlier.

Source: The Construction (Design and Management) Regulations 2015; The Electricity at Work Regulations 1989; HSE, Avoiding danger from underground services (HSG47); HSE, Electricity, underground cables.

Where the danger actually comes from

• Electric shock, arc and flame. A struck cable can kill outright, and the arc flash burns hands and face through protective clothing. The injury does not need contact.
• Gas. A struck main fills a trench or a basement faster than people can climb out of it, and any ignition source within a few metres is enough.
• You do not need a trench. Fence and sign posts, ground anchors, road pins, tree planting, stakes and breaking out a slab. These are the jobs where nobody thinks to ask for a drawing.
• Drawings are approximate. Services snake, get relaid, get abandoned in place and go unrecorded. High and low voltage cables are often on separate drawings, and symbols differ between utilities.
• Shallower than you expect. Electricity typically 450 mm to a metre, gas around 750 mm under a road and 600 mm under a footway, service connections shallower again. Ground opened before can be shallower than any of it.
• Colour tells you less than you think. Older services predate the colour conventions, and in a wet hole under a work light everything is much the same colour anyway.
• A locator used once. A sweep on Monday proves nothing about the ground you reach on Thursday, and devices miss plastic pipes and cables carrying no current.
• The service nobody owns. Private cables and pipes across a site, feeds to lighting, gates, hoardings and cabins, on nobody’s utility drawing and usually live.
• Water and sewers. Generally 900 mm or deeper, and a burst main floods the hole and undermines the sides within minutes.
• Damage that gets buried again. A nicked sheath or a crushed pipe that nobody reports does not fail today. It fails later, with somebody else standing over it.

Controls to brief your crew on

• Get the drawings before the plant arrives. Every utility in the area, plus the highways authority, Network Rail or the MoD where they are involved, and the site’s own private services.
• Treat every drawing as approximate. Use it to know what to expect and where to look, never as a position you can dig to.
• Sweep with a locator, and keep sweeping. A cable avoidance tool with a signal generator, used by someone trained on that device and its limits, in calibration, and re-swept as the dig advances.
• Prove it with a trial hole. Detection tells you something is there. A trial hole dug by hand tells you what it is and how deep.
• Mark it on the ground and on the drawing. Marks that survive the shift and the weather, and a drawing the next crew can actually read.
• Dig alongside, not above. Hand tools near the marks, insulated tools near cables, and final exposure by digging horizontally where you control the force.
• Assume it is live. Any unidentified cable is a live cable until the owner says otherwise, and that confirmation comes from the owner rather than from how it looks.
• If something turns up that is not on the drawing, stop. Not work round it carefully. Stop, and get the drawing and the sweep put right before anyone digs on.
• Know the strike response before you need it. Cable: the operator stays in the cab, everyone else clears the area, and nobody approaches the machine until the owner makes it safe. Gas: get everyone out, no smoking or ignition sources, and call the National Gas Emergency line on 0800 111 999. Water: tell the water company.
• Report every bit of damage, however slight. Never attempt a repair. Tell the owner, and put warning tape and tiles back where they were before anything is backfilled.

Source: HSE, Avoiding danger from underground services (HSG47); HSE, Excavation and underground services; CDM 2015, reg. 25; Electricity at Work Regulations 1989, regs. 4 and 14.

Who is competent to do this work

• Locator operators need training on the specific device, on what it cannot detect, and on using a signal generator, not just on switching it on. HSE is explicit that operators must understand the limitations of the equipment they are using.
• Whoever signs the permit to dig must be competent to judge whether the drawings, the sweep and the trial holes actually cover the ground being opened, and confident enough to refuse when they do not.
• Machine operators must be trained and authorised for the plant, and briefed on where the marks are and how close the machine may work to them.
• Work on or near live conductors is not general construction work. It needs the technical knowledge or the supervision the Electricity at Work Regulations require, and in most cases it needs the service made dead first.

Paperwork that should be in place

• Service drawings from every owner in the area, current, readable and on site with the crew who are digging.
• A permit to dig signed against the locator sweep and the trial holes, not against the drawing alone.
• The locator sweep record, showing what was swept, by whom and when.
• Calibration and function check records for cable avoidance tools and signal generators.
• Training and authorisation records for locator operators, machine operators and whoever signs the permit.
• Damage reports for any service struck, nicked or crushed, with confirmation that the owner was told.

Five questions to get the crew talking

1. Which utilities have given us drawings for this ground, and has anyone here actually seen them?
2. Who swept this area with the locator, and when was it last done?
3. If your bucket exposed a black cable in the next hour, what would you do in the first ten seconds?
4. Do you know the gas emergency number without looking it up?
5. What private services does this site have that are on nobody’s utility drawing?

How to run this toolbox talk

1. Bring the drawings to the huddle. Not the folder in the office. The actual sheets for the ground these people are opening today, so they can see what is marked and what is not.
2. Do the sweep in front of them. Five minutes with the locator while everyone watches is worth more than any amount of talking about it.
3. Rehearse the strike response out loud. Operator stays in the cab, everyone else back, nobody touches the machine. Say the gas number until people can repeat it.
4. Name the small jobs. Ask who is putting in a post, a pin or a stake this week, and agree there and then how that ground gets proved.
5. Pair it with the excavations toolbox talk, which covers holding the ground up once the hole is open.
6. Record who attended on the free toolbox talk sign-in sheet.

Frequently asked questions

How deep are underground cables and pipes in the UK?

HSG47 gives typical depths: electricity cables between 450 mm and 1 m, gas mains around 750 mm under a roadway and 600 mm under a footway, gas service connections about 450 mm and around 375 mm on private ground, and water mains and sewers generally 900 mm or more. High pressure gas transmission pipelines are laid at a minimum of 900 mm. All of these are typical figures rather than guarantees, and ground that has been dug and reinstated before can be considerably shallower.

Do service drawings show the exact position of a cable?

No. HSE is explicit that plans give approximate positions only, and that they should guide the search rather than replace physical detection. Services snake around obstructions, get relaid, get abandoned in place, and sometimes were never recorded at all. High and low voltage cables often appear on separate drawings and symbols vary between utilities. Use the drawing to know what to expect; use the locator and a trial hole to know where it is.

Is a CAT and Genny enough on its own?

No. A locator is one of four elements HSE describes, alongside planning the work, using cable plans and safe digging practice, and it is meant to be used frequently and repeatedly through the job rather than once at the start. Detection devices also have real limitations: they can miss plastic pipes and cables that are not carrying current. That is why the trial hole exists. Detection tells you something is there, and only exposing it by hand tells you what it is and how deep.

What should you do if you hit an underground electricity cable?

Stop work. If a machine has struck the cable, the operator stays in the cab, because the machine may be at a different potential to the ground. Everyone else clears the area and nobody approaches the machine or the excavation. Contact the cable owner immediately and keep the area isolated and secured until they have made it safe. Report the damage even if it looks slight, and never attempt a repair or backfill over it.

What should you do if you strike a gas pipe?

Do not attempt any repair. Evacuate everyone from the immediate vicinity, prohibit smoking and any other ignition source within at least five metres, and telephone the National Gas Emergency number on 0800 111 999. Report the damage to the gas distribution network operator even if it appears minor, because a crushed or scored plastic pipe can fail later.

Do you need a permit to dig?

There is no regulation that uses the phrase, but CDM 2015 regulation 25(4) prohibits work liable to create a risk from an underground service unless suitable and sufficient steps have been taken to prevent that risk. A permit to dig is how most sites demonstrate that those steps happened, and that they happened before the ground was opened rather than afterwards. What matters is that the permit is signed against the drawings, the locator sweep and the trial holes, not against the drawing on its own.

Who is responsible for providing information about buried services?

The duty sits upstream of the digging crew. HSE expects clients and principal designers to provide pre-construction information about underground services, and CDM 2015 places planning and information duties on those roles. Contractors then have to plan, manage and monitor the work so that the risk is prevented. In practice that means a crew should never be the first people in the chain to wonder whether there is a cable there.