CSCS Buried & Underground Services Questions
Cable avoidance tools, permits and safe digging practice.
These questions cover buried & underground services for the CITB Health, Safety and Environment (HS&E) test, the exam UK construction workers pass to get a CSCS card.
Buried & Underground Services: practice quiz
- 20 questions · 30 minutes · pass mark 90%
- Answers and explanations are shown at the end, with every question to review.
- A fresh 20 questions every time, drawn from all 114 in this topic's bank. Includes 2 select-two questions and 1 picture question, as in the real test.
- About 90 seconds a question, against about 54 in the real test, so there is time to think.
10 sample Buried & Underground Services questions and answers
These 10 come from the 114 questions the practice quiz above draws from, 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. 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.
3. 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.
4. 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.
5. 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.
6. A water main is damaged during digging. What injury risk does HSG47 describe from the escaping water?
Answer: A high pressure jet that can also throw out stones from the ground
- A) Scalding, because water mains carry water close to boiling point
- B) Poisoning, because mains water is treated with strong chemicals
- C) None, because water pipes cannot cause injury when damaged
- D) A high pressure jet that can also throw out stones from the ground (correct answer)
Why: HSG47 paragraph 16 says a jet of water from a main can have enough pressure to injure a person and may contain stones or other hard objects ejected from the ground around the pipe.
7. Service location information has been gathered for a job. What does HSG47 say should be done with it?
Answer: Copy it onto the working drawings for those doing the work
- A) Give it to the client only, as workers do not need to see it
- B) Throw it away once the locator survey has been carried out
- C) Copy it onto the working drawings for those doing the work (correct answer)
- D) Keep it at head office so that it cannot be lost on site
Why: HSG47 paragraph 43 says service location information should be copied onto the working drawings for the guidance of those carrying out the work.
8. Why will RFID markers often not help when locating older services, according to HSG47?
Answer: RFID is mostly found on new services, so older ones are not tagged
- A) RFID cannot be read by any detecting device used in the UK
- B) RFID is mostly found on new services, so older ones are not tagged (correct answer)
- C) RFID markers wear out after a year and stop giving any signal
- D) RFID only works on services deeper than about two metres
Why: HSG47 paragraph 85 says RFID is increasingly used to tag new services, so it will often be found on new services and will not necessarily assist with older ones.
9. You uncover an iron or steel pipe and cannot tell whether it carries water or gas. How does HSG47 say you should treat it?
Answer: As if it were a gas pipe
- A) As if it were a gas pipe (correct answer)
- B) As if it were a water pipe
- C) As a disused pipe
- D) As a drainage pipe
Why: HSG47 paragraph 122 says iron and steel water pipes and gas pipelines may look very similar, so any such pipe uncovered should be treated as if it were a gas pipe.
10. Moling or pipe bursting is done too near other services. What extra problem does HSG47 describe?
Answer: Displaced soil can damage the other services or get into them
- A) The mole will be pulled towards metal pipes by magnetism
- B) The new pipe will freeze where it passes the old services
- C) Pipe bursting makes the old services carry a higher voltage
- D) Displaced soil can damage the other services or get into them (correct answer)
Why: HSG47 paragraph 168 says that if moling or pipe bursting is done too near other services or ducts, displaced soil may damage or enter them.
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
Ready for the real thing?
When you are scoring 90% or better here, take a CSCS mock test for Operatives under exam conditions, then how to book the CSCS test. Still under 90%? Try a CSCS practice quiz on another topic.