CSCS Buried & Underground Services Questions
Cable avoidance tools, permits and safe digging practice.
These questions cover buried & underground servicesfor the CITB Health, Safety and Environment (HS&E) test, the exam UK construction workers pass to get a CSCS card.
Buried & Underground Services: practice questions (untimed, instant feedback)
- 50 questions · pass mark 90%
- Untimed practice: you see the correct answer immediately after each question.
- Some questions need more than one answer, and these are clearly marked.
All 50 Buried & Underground Services questions and answers
These are the same 50 questions as the practice test above, written out with the correct answer and the reasoning behind it. Read them through when you want to revise rather than test yourself.
1. HSG47 says a safe system of work near underground services has three basic elements. What are they?
Answer: Planning the work, detecting and marking the services, and safe digging practices
- A) Planning the work, detecting and marking the services, and safe digging practices (correct answer)
- B) Booking a permit, telling the client in writing, and putting up barriers around it
- C) Reading the plans, wearing the right gloves, and keeping the excavation pumped dry
- D) Calling the utility, agreeing a price for the work, and recording the depths found
Why: HSG47 paragraph 22 lists the three elements as planning the work, detecting, identifying and marking underground services, and safe excavation or safe digging practices. Paragraph 23 adds that they complement each other and all three are essential, so doing one well does not let you skip another.
2. You are about to break ground on a site where the plans show no underground services at all. What should you assume?
Answer: That services are present, because plans and records are never proof of their absence
- A) That a locator sweep is unnecessary, as the desktop study has already been completed
- B) That services are present, because plans and records are never proof of their absence (correct answer)
- C) That the ground is clear, because the utility companies keep accurate service records
- D) That only shallow drainage will be found, since deeper services are always recorded
Why: HSG47 paragraph 3 says underground services are widespread and to assume they are present unless you have been shown otherwise. Paragraph 41 goes further: the routes of older services in particular may never have been recorded, so the absence of records should never be taken as proof that the area is free of underground services.
3. Emergency work has to start immediately and there is no time to obtain service plans. How must the work be carried out?
Answer: As though there are underground services present in the area being worked on
- A) Without a risk assessment, because the emergency exempts the job from planning
- B) By any method the supervisor judges quickest, provided the crew are told to hurry
- C) As though there are underground services present in the area being worked on (correct answer)
- D) Using mechanical excavation only, so that the crew spend less time in the trench
Why: HSG47 paragraph 30 says that where it is not possible to obtain information, as may be the case for emergency work, the work must be carried out as though there are underground services in the area. Paragraph 73 confirms emergency work still requires planning and assessment of the risks, and paragraph 74 says the urgency is not an excuse for cutting corners.
4. What does HSG47 say about relying on service plans alone before starting work?
Answer: Plans are not sufficient on their own and are the basis for a thorough site survey
- A) Plans are sufficient if they were issued by the owner within the last twelve months
- B) Plans are sufficient provided they are printed in colour and drawn to a stated scale
- C) Plans are sufficient where the site is a green field rather than a congested street
- D) Plans are not sufficient on their own and are the basis for a thorough site survey (correct answer)
Why: HSG47 paragraph 39 states plainly that plans alone are not sufficient to identify and locate services before starting work, and that they provide basic information on which to base a thorough site survey. Paragraph 41 lists why: plans may not be drawn accurately to scale, kerb lines move, surfaces are re-graded, and cables get moved without the owner knowing.
5. A cable is terminated in the ground with a seal. It is sometimes called pot-ended or bottle-ended. How should it be treated?
Answer: As live, because such cables must not be assumed to be abandoned or disused
- A) As live, because such cables must not be assumed to be abandoned or disused (correct answer)
- B) As dead, since a sealed termination shows the cable has been taken out of use
- C) As a spare duct that can be cut back to make room for the new service run
- D) As telecoms, because power cables are never terminated and sealed underground
Why: HSG47 paragraph 89 says pot-ended or bottle-ended cables should be treated as live and should not be assumed to be abandoned or disused. It adds that they can be difficult to detect with locators even when live, which is exactly why the assumption has to run the safe way.
6. Why will a standard cable locator usually fail to find a plastic gas or water pipe?
Answer: Locators need a metallic path, so plastic needs a tracer wire, sonde or RFID marker
- A) Locators need the pipe to be pressurised, and plastic mains run at very low pressure
- B) Locators need a metallic path, so plastic needs a tracer wire, sonde or RFID marker (correct answer)
- C) Locators are tuned to mains frequency, so they tend to respond to live power cables
- D) Locators cannot work through a hard surface, so plastic pipes under roads are missed
Why: HSG47 paragraph 91 says locators, with the possible exception of ground-probing radar, do not detect plastic pipes or other non-metallic services unless a metallic tracer wire has been laid with the pipe, a signal transmitter or tracing rod is inserted into it, or RFID markers have been affixed. Paragraph 92 warns that tracer wires may not be continuous.
7. What is the purpose of digging trial holes?
Answer: To positively identify a service and its depth, and let its status be checked
- A) To test whether the ground is firm enough to take the weight of the excavator
- B) To give the excavator driver a starting point so the trench line stays straight
- C) To positively identify a service and its depth, and let its status be checked (correct answer)
- D) To provide somewhere for surface water to drain to during the rest of the dig
Why: HSG47 paragraph 96 says to use trial holes to positively identify a service and its depth, and that exposing a service safely in this way allows its status to be checked and may make it easier to apply a tracing signal. Paragraph 76 lists physical identification through trial holes as the most thorough of the three levels of survey.
8. You need to mark the line of a located service on a grassed area. What should you use?
Answer: Wooden pegs, kept to one side of the service rather than driven in above it
- A) Steel pins driven firmly into the line of the service so they are not knocked out
- B) Long steel spikes at each end, with string run between them to show the route
- C) Nothing physical, because ground markings will be lost before the dig starts
- D) Wooden pegs, kept to one side of the service rather than driven in above it (correct answer)
Why: HSG47 paragraph 97 says to mark the line with waterproof crayon, chalk or paint on paved surfaces, or with wooden pegs in grassed or unsurfaced areas, preferably to one side of the service. It adds that steel pins, spikes or long pegs which could damage services laid at shallow depth should not be used. Paragraph 127 repeats the point for stakes and posts.
9. An excavator strikes a live cable. What should the driver do?
Answer: Stay in the cab, because climbing down risks electrocution at that moment
- A) Stay in the cab, because climbing down risks electrocution at that moment (correct answer)
- B) Climb down at once and move well clear of the machine and the excavation
- C) Reverse the machine off the cable before shutting down and leaving the cab
- D) Lower the bucket to earth the machine, then step off and call the supervisor
Why: HSG47 paragraph 101 instructs drivers to stay in the cab if a cable is struck, because if they climb down they may be electrocuted. The area should be isolated and secured, and nobody should go into the excavation or approach the excavator or the cable until the cable owner has made the damaged cable safe.
10. How does the danger from damaging a gas pipe compare with damaging an electricity cable?
Answer: An excavator is worse for gas, while a hand-held power tool is worse for cables
- A) The two are equivalent, so the same separation distance is used for either service
- B) An excavator is worse for gas, while a hand-held power tool is worse for cables (correct answer)
- C) An excavator is worse for both, because it applies far more force than a hand tool
- D) A hand-held power tool is worse for both, since the operator is close to the damage
Why: HSG47 paragraph 102 says the danger created by damaging a gas pipe with an excavator is much greater than if the damage is done with a hand-held power tool, and that the opposite is true for work near electricity cables. Paragraph 143 makes the same point about cables: powered hand tools used close to live cables represent the greatest risk of injury.
11. Where practicable, how far from the indicated line of a buried service may hand-held power tools be used to break a hard surface?
Answer: No closer than 500 mm from the indicated line of the service
- A) No closer than 2 m from the indicated line of the service
- B) No closer than 50 mm from the indicated line of the service
- C) No closer than 500 mm from the indicated line of the service (correct answer)
- D) No closer than 150 mm from the indicated line of the service run
Why: HSG47 paragraph 108 says that where practicable, only use hand-held power tools 500 mm or more away from the indicated line of a service buried in or below a hard surface. Having done so, the service should be positively located by careful hand digging under the hard surface.
12. In what circumstances may the safety margin for power tools around a buried cable be reduced?
Answer: Only where congestion or obstructions make it impractical and the line is confirmed
- A) Whenever the excavation is being supervised by a person holding a valid CSCS card
- B) Whenever the ground is soft enough for the cable to be freed using hand tools alone
- C) Whenever the service owner has been telephoned and told that work is about to start
- D) Only where congestion or obstructions make it impractical and the line is confirmed (correct answer)
Why: HSG47 paragraph 109 allows the 500 mm margin to be reduced where congestion of services makes it impracticable, or where surface obstructions limit the space available, but only if the line of the cable has been positively identified by plans, confirmed by a locator, and additional precautions are used to prevent damage.
13. When excavating near a service whose position is known, where should you dig?
Answer: Alongside the service rather than directly above it, wherever that is possible
- A) Alongside the service rather than directly above it, wherever that is possible (correct answer)
- B) Directly above the service, so that its line usually stays visible as you go down
- C) From directly above but with a wider bucket, to spread the load across the run
- D) At right angles across the service, so that its depth can be measured accurately
Why: HSG47 paragraph 106 says to make every effort to excavate alongside the service rather than directly above it, and to avoid using hand-held power tools over a service unless it has already been exposed and is at a safe depth of at least 300 mm below the bottom of the hard surface material, or physical precautions have been taken to prevent the tool striking it.
14. HSG47 recommends a particular technique for the final exposure of a service. What is it?
Answer: Horizontal digging, because the force applied to hand tools is controlled better
- A) Mechanical excavation on the slowest setting, kept under constant supervision
- B) Horizontal digging, because the force applied to hand tools is controlled better (correct answer)
- C) Vertical digging, because it gives the shortest route straight down to the top of the run
- D) Vacuum excavation only, because hand tools must never be used near any service
Why: HSG47 paragraph 110 recommends final exposure of the service by horizontal digging, because the force applied to hand tools can be controlled more effectively that way. The same paragraph says to use insulated tools when hand digging near electric cables.
15. How should a spade or shovel be used when hand digging close to a buried service?
Answer: Eased in with gentle foot pressure, never thrown or spiked into the ground
- A) Thrown into the face of the dig to break the ground up before it is cleared
- B) Used with a pick first in every case, to loosen the material before lifting it
- C) Eased in with gentle foot pressure, never thrown or spiked into the ground (correct answer)
- D) Driven in hard with the heel of the boot so it cuts cleanly through the fill
Why: HSG47 paragraph 110 says spades and shovels, preferably those with curved edges, should be used rather than other tools, and should not be thrown or spiked into the ground but eased in with gentle foot pressure. Picks, pins or forks may be used with care to free lumps of stone and break hard layers, but picks should not be used in soft clay or other soft soils near underground services.
16. A service has been exposed in an excavation and workers need to climb out. What does HSG47 say?
Answer: The service must never be used as a handhold or foothold to climb out of the dig
- A) The service may be stood on if it is a cast iron main and has been proved dead
- B) The service may be used as a step provided the crew keep their weight on the heel
- C) The service may be used to climb on once it has been supported with timber packing
- D) The service must never be used as a handhold or foothold to climb out of the dig (correct answer)
Why: HSG47 paragraph 115 says that once exposed, services may need to be supported and should never be used as handholds or footholds for climbing out of excavations. Paragraph 128 covers the proper answer to access: a safe means of access into the excavation, such as a secured ladder.
17. What status should you assume for any underground service you encounter?
Answer: Always live, until it is disconnected and proven safe at the point of work
- A) Always live, until it is disconnected and proven safe at the point of work (correct answer)
- B) Dead, if the plans supplied by the owner show it as a disused run
- C) Dead, if a locator sweep of the area returns no signal from the run
- D) Live, only where the service is an electricity cable rather than gas
Why: HSG47 paragraph 118 says to assume all services are live until disconnected and proven safe at the point of work, and to obtain written confirmation of disconnection from the owner or operator before removing a redundant service. Paragraph 125 adds that status can change: a cable may become live or a pipe pressurised, so check the status when work near the service begins.
18. Why does HSG47 warn against working bare-chested near buried electricity cables or gas pipes?
Answer: Burns are made more severe, and even ordinary work clothing reduces them
- A) The employer cannot check that the right protective clothing is being worn
- B) Burns are made more severe, and even ordinary work clothing reduces them (correct answer)
- C) Loose skin contact makes an electric shock more likely to stop the heart
- D) Bare skin picks up more contamination from the soil in a service trench
Why: HSG47 paragraph 119 says burns are the main injuries resulting from damage to live electrical cables or from fire or explosion following a gas leak, and that in many cases burns are made more severe by the injured person working bare-chested. Even ordinary work clothing can greatly reduce the severity, though man-made fibres such as nylon may melt and stick to the skin.
19. A worker is issued with flame-retardant clothing for excavation near live cables. What does HSG47 say this achieves?
Answer: It reduces the severity of injury but is not a substitute for a safe system of work
- A) It allows hand-held power tools to be used directly above the line of the cable
- B) It permits the safety margin to be reduced without any additional precautions
- C) It reduces the severity of injury but is not a substitute for a safe system of work (correct answer)
- D) It removes the need for a locator sweep, since the main injury risk is now covered
Why: HSG47 paragraph 120 says employers should consider whether work near electricity cables justifies clothing designed to protect against electric arc, or flame-retardant clothing. Paragraph 121 states in bold that wearing protective clothing is not a substitute for a safe system of work. Protective clothing sits at the bottom of the hierarchy, not the top.
20. You uncover an unidentified service covered in black plastic. How should you treat it?
Answer: As a live electricity cable, until its identity has been positively confirmed
- A) As a telecommunications duct, since black plastic is the standard telecoms colour
- B) As a water pipe, because water services are the most common black plastic run
- C) As disused, because current services are colour coded to the national system
- D) As a live electricity cable, until its identity has been positively confirmed (correct answer)
Why: HSG47 paragraph 122 says water pipes, electricity cables and telecommunication cables may all be covered in black plastic, and if any black plastic service is found, assume it is a live electricity cable. The same paragraph says to treat any uncovered iron or steel pipe as a gas pipe, and where there is any doubt about identity, treat the service as an electricity cable or gas pipe until proved otherwise.
21. An excavation has exposed warning tiles and marker tape above a service. What should happen when backfilling?
Answer: They always go back in position above the service they refer to
- A) They always go back in position above the service they refer to (correct answer)
- B) They are removed, since the service has now been recorded on the drawings
- C) They are re-used lower down to protect the new service being installed
- D) They are used to guard the open excavation until the surface is reinstated
Why: HSG47 paragraph 130 says to put warning tiles and tape back in their original position above the services unless visual examination after exposure showed this to be incorrect, in which case replace them above the service to which they refer. It also says not to use warning tape for any other purpose, such as guarding an excavation, and not to discard it in the excavation.
22. What does HSG47 say about encasing underground services in concrete?
Answer: Services should never be buried or encased in concrete, as breaking it out is dangerous
- A) Services may be encased in concrete where the ground is soft and support is needed
- B) Services should never be buried or encased in concrete, as breaking it out is dangerous (correct answer)
- C) Services should be encased in concrete wherever they cross beneath a busy roadway
- D) Services may be encased in concrete provided the position is recorded on the plans
Why: The box at HSG47 paragraph 134 says services should not be buried or encased in concrete as excavating close to them is dangerous, and that using mechanical means to break up concrete can damage cables or pipes, which if live will likely mean anyone present is injured. Paragraph 140 calls cables encased in concrete bad practice, while admitting it is not uncommon.
23. At roughly what depth are most underground electricity cables laid, and what follows from that?
Answer: Between 450 mm and 1 m, but depths must never be assumed as cables can be shallower
- A) At least 1.2 m in all cases, so shallow digging near cables carries no real risk
- B) At a depth set by the voltage, so an 11 kV cable is always deeper than 1 m down
- C) Between 450 mm and 1 m, but depths must never be assumed as cables can be shallower (correct answer)
- D) Between 450 mm and 1 m, so anything found above 450 mm will be a telecoms duct
Why: HSG47 paragraph 137 says most underground cables are laid in trenches between 450 mm and 1 m deep, with some high-voltage cables deeper, and then states in bold: never assume depths, you may find cables at shallower depths.
24. How close may a mechanical excavator work to a gas pipe?
Answer: No closer than 500 mm, and the transporter may advise a greater distance
- A) No closer than 100 mm, provided a banksman watches the bucket at all times
- B) No closer than 5 m, which is the figure used whatever the gas main pressure
- C) There is no fixed distance, so long as the driver holds a plant operator card
- D) No closer than 500 mm, and the transporter may advise a greater distance (correct answer)
Why: HSG47 paragraph 150 says gas pipes may have projections such as valve housings, siphons and stand pipes that are not shown on the plans, and to allow for this, do not use mechanical excavators within 500 mm of a gas pipe. It adds that the gas transporter may advise greater safety distances depending on the pipeline pressure.
25. You suspect a gas leak after damaging a pipe in an excavation. What should you do?
Answer: Evacuate the area, call 0800 111 999, and stop all ignition sources within 5 m
- A) Evacuate the area, call 0800 111 999, and stop all ignition sources within 5 m (correct answer)
- B) Attempt a temporary repair with tape, then telephone the gas transporter to report
- C) Backfill the excavation immediately so the escaping gas is contained underground
- D) Leave the excavation open and wait at the edge for the gas transporter to arrive
Why: HSG47 paragraph 152 says that if a gas leak is suspected, do not attempt repairs. Evacuate everyone from the immediate vicinity of the escape, warn occupants of any affected building to leave, inform the gas distribution network operator by telephoning the National Gas Emergency number 0800 111 999, and prohibit smoking and extinguish naked flames and other ignition sources within at least 5 m of the leak.
26. HSG47 lists work not to be undertaken without consulting the gas pipeline owner or operator. Within what distance of a gas pipe does that apply to piling or vertical boring?
Answer: Within 15 m of any gas pipe, which is the figure HSG47 gives for piling work
- A) Within 3 m of any gas pipe, which matches the footprint of a typical piling rig
- B) Within 15 m of any gas pipe, which is the figure HSG47 gives for piling work (correct answer)
- C) Within 1 m of any gas pipe, on the basis that only very close work can affect it
- D) Within 30 m of any gas pipe, which is the figure HSG47 gives for using explosives
Why: HSG47 paragraph 154 lists work not to be undertaken without consulting the pipeline owner or operator: the use of explosives within 30 m of any gas pipe, piling or vertical boring within 15 m, excavation work within 10 m of any above-ground gas installation, building a structure over or around a gas pipe, and work that reduces the cover over a pipe. The question asks for the threshold rather than for an example, because any distance shorter than 15 m is also within 15 m.
27. A service shown on the plan cannot be found with a locator. What should you do?
Answer: Seek assistance or advice from the service owner before going any further
- A) Record it as removed and update the drawings once the dig is finished
- B) Dig a trial hole with a breaker to confirm the ground really is clear
- C) Seek assistance or advice from the service owner before going any further (correct answer)
- D) Treat the plan as out of date and carry on with mechanical excavation
Why: HSG47 paragraph 93 says that if a service recorded on a plan cannot be located, seek appropriate assistance or advice from the service owner. Failing to find something the record says is there means the survey is incomplete, not that the record is wrong.
28. Why does HSG47 tell you to use a signal generator with a locator wherever possible?
Answer: It significantly increases the accuracy of locating and tracing the service
- A) It allows the locator to be used by someone who has not been trained on it
- B) It measures the depth of the service without any need to dig a trial hole
- C) It works through concrete, which ordinary detection methods cannot do
- D) It significantly increases the accuracy of locating and tracing the service (correct answer)
Why: HSG47 paragraph 85 says in bold that use of signal generators will significantly increase the accuracy of the service location. Paragraph 90 adds that a locator may not distinguish between cables or pipes running close together and may show them as a single signal, and that if two are one above the other the lower one may not be detected, so use signal generators whenever possible.
29. What is a hum detector unable to find?
Answer: A cable carrying little or no current, such as a street lighting cable by day
- A) A cable carrying little or no current, such as a street lighting cable by day (correct answer)
- B) Any cable operating above 11 kV, because the field is too strong to read
- C) Any cable that has been laid inside an earthenware or plastic duct or pipe
- D) Any cable buried deeper than 450 mm below a paved or concrete surface
Why: HSG47 paragraph 85 says hum detectors receive the magnetic field radiated by electricity cables carrying a current, and do not respond to cables where there is little or no current flowing, such as service connection cables to unoccupied premises or street lighting cables in the daytime, nor to direct current cables, some well-balanced high-voltage cables, or pot-ended cables.
30. How often should a locator be used once excavation has started?
Answer: Frequently and repeatedly, since location gets more accurate as cover is removed
- A) Only if the ground conditions change or a service is found unexpectedly
- B) Frequently and repeatedly, since location gets more accurate as cover is removed (correct answer)
- C) Once at the start of each shift, which is what the inspection regime requires
- D) Once only, before the first cut, so the readings are not confused by the dig
Why: HSG47 paragraph 114 says in bold to make frequent and repeated use of locators during the course of the work, because service location is likely to become more accurate as cover is removed. Treating the locator sweep as a one-off at the start of the job is the mistake this paragraph exists to prevent.
31. Who has a duty to make reasonable enquiries about underground services and pass what they find to the designer and contractor?
Answer: The client, and if unable or unwilling, they must give the contractor time to do it
- A) The service owner, who alone holds the records of what is buried on the site
- B) Nobody has that duty, because the contractor is responsible for the whole site
- C) The client, and if unable or unwilling, they must give the contractor time to do it (correct answer)
- D) The excavator driver, who is closest to the ground being broken on the day
Why: HSG47 paragraph 48 says clients have a duty to make reasonable enquiries about underground services and pass relevant information to the designers and contractors. Paragraph 49 says in bold that a client who is unable or unwilling to obtain this information must allow the contractor sufficient time and resource to do so instead.
32. What does HSG47 expect a designer to do about underground services?
Answer: Reduce or design out the risks, then tell those doing the work what risks remain
- A) Leave the question to the principal contractor, who controls the site once work starts
- B) Mark the services on the ground before the contractor is appointed to the project
- C) Obtain the plans and then take no further part, as design is complete before the dig
- D) Reduce or design out the risks, then tell those doing the work what risks remain (correct answer)
Why: HSG47 paragraph 51 says designers have a duty to reduce or design out the risks arising from damage to underground services, and having reduced them so far as is reasonably practicable, to provide information about the risks that remain to those doing the work, in most cases on the working drawings. Paragraph 53 notes re-siting services away from the work is often a reasonably practicable way of avoiding the risk.
33. When a mechanical excavator is working near known services, what should the person assisting the driver do?
Answer: Watch from where they can see into the dig and stay outside the arm's radius
- A) Watch from where they can see into the dig and stay outside the arm's radius (correct answer)
- B) Stand in the excavation so they can see the bucket meet the ground directly
- C) Sit in the cab beside the driver so that instructions can be given by voice
- D) Work at the far end of the trench so the two of them cover more ground
Why: HSG47 paragraph 100 says another person should assist the excavator driver from a position where they can safely see into the excavation and warn the driver of any services or other obstacles, and that this person should remain outside the operating radius of the excavator arm and bucket.
34. Why does HSG47 say services should be traced through the full extent of the work area?
Answer: Because cables often have kinks or loops and pipes have bends not shown on plans
- A) Because tracing only the ends is enough to establish the depth in the middle
- B) Because cables often have kinks or loops and pipes have bends not shown on plans (correct answer)
- C) Because the locator battery drains and readings drift over the length of a shift
- D) Because the service owner charges for each separate section of plan requested
Why: HSG47 paragraph 95 says services should be traced through the full extent of the work area as they may not run in straight lines: cables will often have kinks or loops and pipes may have joints or bends that are not shown on service drawings. Paragraph 41 makes the related point that services marked as straight lines may in practice snake.
35. Roughly what depth of cover do gas mains normally have when laid in a roadway?
Answer: About 750 mm, and HSG47 still warns that a pipe can turn up nearer the surface
- A) At least 2 m in all cases, because of the traffic loading on a carriageway
- B) The same as the water main beside it, since both are laid in a shared trench
- C) About 750 mm, and HSG47 still warns that a pipe can turn up nearer the surface (correct answer)
- D) About 200 mm, which is why gas mains are the shallowest service in a road
Why: HSG47 paragraph 146 gives the normal depth of cover for gas mains laid in a roadway as about 750 mm, about 600 mm in a footway, about 450 mm for service connections in both, about 375 mm on private property, and at least 900 mm for high-pressure transmission pipelines. It then says in bold: never assume depths, pipes may be found at shallower depths.
36. How should plastic gas service pipes be located before mechanical excavation begins?
Answer: By hand digging, typically a trial trench near the kerb or in the footway
- A) By ground-probing radar, normally the only method that will find plastic pipe
- B) By reading the plan, because service connections are always marked on it
- C) By pressure testing the main, which shows where the connections branch off
- D) By hand digging, typically a trial trench near the kerb or in the footway (correct answer)
Why: HSG47 paragraph 149 says to locate PE gas pipes by hand digging before mechanical excavation begins, and that this is particularly important for service connection pipes, which will not be marked on plans. It suggests digging a trial trench along the road near the kerb or in the footway where the service connection pipes are likely to be at their shallowest.
37. What does HSG47 say about the accuracy of markers, tiles and tapes found above a service?
Answer: They may have been disturbed and should not be relied on to give the position
- A) They may have been disturbed and should not be relied on to give the position (correct answer)
- B) They are laid to a fixed offset, so the service is always directly beneath them
- C) They are surveyed in when installed, so they are the most reliable indicator
- D) They are only ever used above gas pipes, so their presence identifies the service
Why: HSG47 paragraph 148 says markers such as tiles, coloured plastic markers and marker posts may have been disturbed and should not be relied on as an accurate indicator of position. Paragraph 145 makes the same point about tiles, slabs or coloured marker tape above cables: any such protection may have been moved.
38. Under the Electricity at Work Regulations 1989, when may work be carried out on or near a live uninsulated conductor?
Answer: Only where it is unreasonable for it to be dead, reasonable to work live, and safe
- A) Whenever insulated tools and arc-rated protective clothing have been provided
- B) Only where it is unreasonable for it to be dead, reasonable to work live, and safe (correct answer)
- C) Whenever the work is urgent and the supervisor has recorded that in the site diary
- D) Whenever the worker holds a valid competence card for the type of work involved
Why: Regulation 14 of the Electricity at Work Regulations 1989 permits work on or near a live uninsulated conductor only where all three conditions are met: it is unreasonable in all the circumstances for it to be dead, it is reasonable in all the circumstances to be at work on or near it while live, and suitable precautions including where necessary suitable protective equipment are taken to prevent injury. All three, not any one.
39. A new pipe is being laid by a trenchless method such as directional drilling or pipe bursting. What minimum clearance from existing services does HSG47 give as a general guide?
Answer: 250 mm, or one and a half times the diameter of the pipe being laid, whichever is greater
- A) 1 m in every case, measured from the outside wall of the nearest of the existing services
- B) No general guide is given, because clearances are set by the ground conditions alone
- C) 250 mm, or one and a half times the diameter of the pipe being laid, whichever is greater (correct answer)
- D) 50 mm, or half the diameter of the pipe being laid, whichever of the two proves to be larger
Why: HSG47 paragraph 169 says that as a general guide, to avoid damage and to allow sufficient clearance for maintenance, the minimum clearance between adjacent services should be either 250 mm or one and a half times the diameter of the pipe being laid, whichever is the greater, and that for electricity cables clearances for maintenance work should be approximately 300 mm. Note where the figure sits: paragraphs 167 to 172 are the trenchless methods section, so this is the trenchless guide rather than a general open-cut rule. Paragraph 170 adds that clearances may need to be varied for the technique, the ground conditions and the requirements of the owners of adjacent services.
40. While digging you uncover a run of coloured marker tape. What does it mean?
Answer: A buried service is likely to be close by, so switch to careful hand digging
- A) The ground below is contaminated and respiratory protective equipment is needed
- B) The trench has reached the depth at which it must be shored or battered back
- C) The excavation has crossed a boundary and the landowner must be told at once
- D) A buried service is likely to be close by, so switch to careful hand digging (correct answer)
Why: HSG47 paragraph 145 lists coloured plastic marker tape among the layers laid above cables, and paragraph 148 lists marker tiles and coloured plastic markers above gas services, so tape means a service is below you. Paragraph 130 requires it to go back in its original position above the service when backfilling. Both paragraphs also warn that such markers may have been disturbed and are not a reliable guide to exact position, so finding tape means switching to careful hand digging, not measuring down from the tape.
41. What does HSG47 say about the level of survey needed on a congested urban site?
Answer: A more detailed survey than a brown or green field site is likely to be required
- A) A more detailed survey than a brown or green field site is likely to be required (correct answer)
- B) A standard survey usually applies everywhere, so the level rarely has to be judged
- C) No survey is needed where the local authority holds records for the whole street
- D) A survey is only needed where the work involves piling rather than trenching
Why: HSG47 paragraph 77 says the level of survey depends on the nature of the work site, and that some congested urban locations will require a more detailed survey than some brown and green field sites. The decision should be informed by an assessment of how likely services are to be present, and the designer should make it at the planning stage with the contractor and surveyor.
42. Who should carry out the detection survey of a work area?
Answer: Individuals with sufficient experience and technical knowledge of the equipment
- A) The client's representative, so that the results are independent of the contractor
- B) Individuals with sufficient experience and technical knowledge of the equipment (correct answer)
- C) Whoever happens to be available on the day, provided they read the equipment manual first
- D) The excavator driver, because they know where the machine will be working
Why: HSG47 paragraph 26 says detecting underground services requires information from those who own them and that individuals with sufficient experience and technical knowledge should carry out a comprehensive survey using appropriate tools. Paragraph 79 adds they need to understand the limitations of the equipment, the effect of ground conditions on results, and the limitations of the plans supplied.
43. What does HSG47 warn can follow from false readings or signals during detection?
Answer: Inaccurate information in the plan of work and a false sense of security
- A) A delay to the programme, which is the main reason surveys are skipped
- B) A requirement to repeat the desktop study before excavation can restart
- C) Inaccurate information in the plan of work and a false sense of security (correct answer)
- D) Damage to the locator, which then has to be recalibrated by the supplier before reuse
Why: HSG47 paragraph 84 says anyone selecting detection tools and survey methods must understand the range of methods and tools and their limitations, and in particular be aware of the potential for false readings or signals in certain techniques, as these may lead to inaccurate information being included in the plan of work and, in turn, lead to a false sense of security.
44. Two services run one directly above the other. What is the risk when using a locator?
Answer: The lower one may not be detected, as the pair can read as a single signal
- A) The locator may read double the true depth and place both services too deep
- B) The upper one will be missed, because the stronger lower signal masks it
- C) The locator will simply refuse to give a reading at all until they are separated
- D) The lower one may not be detected, as the pair can read as a single signal (correct answer)
Why: HSG47 paragraph 90 says a locator may not be able to distinguish between cables or pipes running close together and may represent them as a single signal, and that if two are sited one above the other the lower one may not be detected. That is one of the reasons the same paragraph tells you to use signal generators whenever possible.
45. Which underground services are commonly difficult to locate with a detecting device?
Answer: Telecommunication and railway signalling cables, unless tagged or in traced ducts
- A) Telecommunication and railway signalling cables, unless tagged or in traced ducts (correct answer)
- B) High-voltage electricity cables, because their insulation blocks the signal
- C) Cast iron water mains, because the metal is too thick to carry a tracing signal
- D) Sewers, because they are usually laid deeper than a locator can reach down to
Why: HSG47 paragraph 94 says many telecommunication and railway signalling cables cannot be located by detecting devices unless metal components such as a metal sheath are connected to earth, they have been tagged with location markers, or they are laid in ducts where tracing rods can be used. Paragraph 35 adds that telecoms companies may prefer to visit site to locate their cables themselves.
46. An 11 kV circuit is laid as separate single-phase cables. What should you allow for?
Answer: They can be spread up to 600 mm across, particularly near the cable joints
- A) They carry no current between joints, so a hum detector will not respond
- B) They can be spread up to 600 mm across, particularly near the cable joints (correct answer)
- C) They are always bundled together, so finding one means finding the circuit
- D) They are laid at least 2 m apart, so a single sweep will not find them all
Why: HSG47 paragraph 139 says a cable is positively located only when it has been safely exposed, and that even then digging should proceed with care as there may be other cables and services adjacent or lower down. It adds that some lines of 11 kV or greater can be laid out as separate single-phase cables spread up to 600 mm across, particularly near cable joints.
47. When backfilling an excavation that exposed a gas pipe, when may power compaction be used?
Answer: Only after 200 mm of selected fine fill has been placed and suitably compacted
- A) Only once the whole trench has been filled to the level of the road surface
- B) At any point, since compaction beneath the pipe is what matters for support
- C) Only after 200 mm of selected fine fill has been placed and suitably compacted (correct answer)
- D) Immediately, provided the fill is free of stones, bricks and any lumps of concrete
Why: HSG47 paragraph 133 says backfill adjacent to gas plant should be suitable fine material or sand containing no stones, bricks or lumps of concrete, that there should be no power compaction until 200 mm cover of selected fine fill has been suitably compacted, and that concrete backfill must not be used within 300 mm of a gas pipe.
48. The plans turn out to be wrong: a service is found well away from its recorded position. What should happen?
Answer: Tell the owner, preferably before backfilling, so they can amend their records
- A) Note it on the site copy of the drawing, which is normally filed at project end
- B) Nothing, because the plans were supplied as a guide and carry a disclaimer
- C) Move the service back to the recorded line shown on the plan before the excavation is filled in
- D) Tell the owner, preferably before backfilling, so they can amend their records (correct answer)
Why: HSG47 paragraph 135 says that if the plans have proved inaccurate, for example a service is found well away from its recorded position, or if the work changes the path or depth of a service, inform the service owners or operators, preferably before the excavation is backfilled, and they should amend their records accordingly.
49. Hot work with a naked flame is planned within 10 m of exposed gas plant. What does HSG47 require?
Answer: Ask the gas transporter to check the atmosphere before work and keep monitoring
- A) Ask the gas transporter to check the atmosphere before work and keep monitoring (correct answer)
- B) Wet the ground around the gas plant down and post a fire watcher for the shift
- C) Nothing extra, provided the gas plant has been proved sound and is not leaking
- D) Move the gas plant clear of the work area before the hot work is allowed to start
Why: HSG47 paragraph 155 says that if you are to carry out welding or other hot work involving naked flames within 10 m of exposed gas plant, ask the gas transporter to check the atmosphere before work begins and continue monitoring during the work. It also warns to take care that no damage occurs to plastic gas pipes or to the protective coatings on other gas pipes.
50. What does HSG47 say a permit system for hazardous work near services should involve?
Answer: Written authorisation by a responsible person, naming the work and the precautions
- A) A copy of the service owner's plan issued to each member of the excavation gang
- B) Written authorisation by a responsible person, naming the work and the precautions (correct answer)
- C) A verbal briefing given at the start of the shift, recorded afterwards in the site diary
- D) A signature from the operatives confirming they have read the service drawings
Why: The summary at the head of the HSG47 safe excavation chapter says a permit system may be appropriate for particularly hazardous work, involving written authorisation by a responsible person identifying the work to be done and the precautions to be taken, and that a permit system needs suitable supervision and monitoring to ensure the conditions of a permit are complied with.
What this topic covers in the CSCS test
Almost every site has gas mains, electricity cables, water pipes, sewers or telecoms ducts running under it. Striking one can kill through arc flash, fire or explosion. HSE guidance HSG47 sets out how to avoid that, and most test questions on this topic come straight from it.
- The three parts of a safe system of work: plan, locate and identify, then excavate safely
- Service plans from the owners, and why plans alone are never proof
- Cable locators (CAT and signal generator) and trial holes
- Safe digging: hand tools, digging alongside rather than above, the 500 mm rule
- Identifying an exposed service, and what to assume when you cannot be sure
- Marker tape, warning tiles and marker posts, and putting them back after backfilling
- What to do when a service is struck, including why the driver stays in the cab
Key facts worth memorising
- Three elements of a safe system
- HSG47 breaks the job into planning the work, locating and identifying buried services, then safe excavation, in that order.
- Assume everything is live
- HSG47 says to assume all services are live until disconnected and proven safe at the point of work.
- The 500 mm margin
- Where practicable, hand-held power tools should only be used 500 mm or more away from the indicated line of a service buried in or below a hard surface.
- Dig alongside, not above
- Excavate alongside the service rather than directly above it, expose it finally by horizontal digging, and use insulated tools when hand digging near electric cables.
- Black plastic means electricity
- Assume any black plastic service is a live electricity cable, treat any iron or steel pipe as a gas pipe, and if in any doubt treat a service as an electricity cable or gas pipe.
- Plans are a starting point
- HSE states plainly that plans alone are not sufficient, because they are not always to scale, depths change when ground is re-graded, and connections are often unmarked.
What people get wrong
The colour of the pipe or duct tells you what the service is.
A national colour coding system exists for modern utility apparatus, but HSG47 warns that older and non-utility services may not follow it, colours look different under poor light, and a duct could hold any service.
The plan shows nothing there, so it is safe to dig.
Plans give you the basis for a survey, not permission to dig. No surface indication of a service does not mean there is no service.
Once you have scanned with a CAT you can carry on digging.
HSG47 says to use locators frequently and repeatedly during the work, because location gets more accurate as cover is removed.
If the machine strikes a cable, get down and look at the damage.
Drivers stay in the cab, because climbing down can electrocute them. Isolate and secure the area, and let nobody near until the owner has made the cable safe.
Answering these questions in the test
These questions reward the slow, cautious answer. If an option involves pressing on, guessing a service from its colour or trusting a drawing, it is wrong. Look for the answer that stops, checks with a locator, digs by hand or asks the supervisor. Marker tape is a specific trap: it means a service lies just below and you carry on with extra care, not that the ground is contaminated or the trench needs shoring.
Sources
Written from the following. Where a figure could not be verified against one of these, it has been left out rather than approximated. Our sources and methodology explains how we date and re-check them. Spotted something wrong? Report an error and we will fix it.
Topics that come up alongside buried & underground services
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