CSCS Electrical Safety Questions
Safe voltages, cables, tools and overhead/underground power.
Electrical Safety — practice questions (untimed, instant feedback)
- 10 questions · pass mark 90%
- Untimed practice — you see the correct answer immediately after each question.
- Some questions need more than one answer — these are clearly marked.
All 10 Electrical Safety questions and answers
These are the same 10 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. Why should you never carry batteries loose in your tool bag?
Answer: If the terminals short out they can start a fire
- A) If the terminals short out they can start a fire (correct answer)
- B) They release a poisonous gas in confined spaces
- C) You may forget to charge them
- D) The extra weight could hurt your back
Why: Loose battery terminals can short against metal objects and start a fire.
2. Why does HSE advise using cordless (battery) tools on site where you can?
Answer: They remove the electric shock risk that comes with a mains supply at the tool
- A) They remove the electric shock risk that comes with a mains supply at the tool (correct answer)
- B) They never need inspection, testing or maintenance
- C) They do not cause hand-arm vibration
- D) They can be used in wet conditions without any precautions
Why: HSE says to use cordless tools, or tools running from a 110V centre tapped to earth supply so the maximum voltage to earth does not exceed 55V. A cordless tool has no mains supply at the tool and no trailing lead to damage or trip over, which removes that shock risk. It does not remove the others: battery tools still need inspection and maintenance, and they still cause hand-arm vibration.
3. What is the recommended safe voltage for electrical equipment on a building site?
Answer: 110 volts
- A) 110 volts (correct answer)
- B) 12 volts
- C) 230 volts
- D) 24 volts
Why: 110 volts, supplied through a transformer, is the recommended safe voltage for site power tools.
4. How do you check that an RCD (residual current device) fitted to a power tool is working?
Answer: Press the test button on the RCD
- A) Press the test button on the RCD (correct answer)
- B) Run the tool flat out to see if it trips
- C) Turn the tool on and off
- D) Test it with a hand-held RCD meter
Why: Pressing the test button confirms the RCD trips as it should.
5. What two things must you do to stop an extension reel overheating while it is in use? (Choose two)
Answers: Avoid overloading it and Fully unwind the cable
- A) Avoid overloading it (correct answer)
- B) Run it at its maximum capacity
- C) Keep it near a fan
- D) Fully unwind the cable (correct answer)
- E) Wind the cable up tightly
Why: A coiled or overloaded reel overheats — fully unwind it and don't overload it.
6. What is the most important step before using a portable electrical tool?
Answer: Inspect it for damage
- A) Read the manual
- B) Inspect it for damage (correct answer)
- C) Test how fast it runs
- D) Wipe it clean
Why: A pre-use visual check for damage is the key safety step.
7. What should you do with a defective portable electrical tool?
Answer: Tell your supervisor and label it as defective
- A) Tell your supervisor and label it as defective (correct answer)
- B) Try to repair it yourself
- C) Use it carefully
- D) Throw it away on the spot
Why: Report and label a faulty tool so no one else uses it until it is repaired.
8. Why use tools with double insulation?
Answer: They give extra protection against electric shock
- A) They are cheaper
- B) They do not conduct electricity
- C) They give extra protection against electric shock (correct answer)
- D) They are lighter
Why: Double insulation adds a second layer of protection against shock.
9. Which plug colour indicates a 110-volt tool?
Answer: Yellow
- A) Red
- B) Blue
- C) Yellow (correct answer)
- D) Green
Why: 110-volt site tools use a yellow plug/connector.
10. What does a residual current device (RCD) do?
Answer: Protects the user from electric shock
- A) Boosts power efficiency
- B) Protects the user from electric shock (correct answer)
- C) Makes the tool faster
- D) Cuts the noise
Why: An RCD rapidly cuts the power to prevent a serious electric shock.
What this topic covers in the CSCS test
Electricity gives no warning. As HSE puts it, unlike most other hazards you cannot see, feel, hear or smell it. About three workers are electrocuted each year during refurbishment work on buildings, and more die in incidents involving overhead lines and buried cables. The test sticks to what an operative controls: site voltage, RCDs, and checking a tool before you plug it in.
- Why site tools run at 110V from a transformer centre tapped to earth
- The plug and connector colour convention on UK sites
- What an RCD does, its tripping current, and how to test it
- Pre-use visual checks, and how often 110V and 230V equipment is inspected and tested
- Reporting and labelling defective tools, and why makeshift repairs are banned
- Double insulation, battery tools and safe use of extension reels
- Working near overhead power lines and buried cables
Key facts worth memorising
- 110V centre tapped to earth
- HSE says use cordless tools, or tools running from a 110V centre tapped to earth supply so the maximum voltage to earth does not exceed 55V.
- Yellow means 110V
- Site plugs and connectors are yellow for 110V and blue for 230V. The colour code is set by the industrial connector standard (BS EN 60309-2), not by HSE, but it is universal on UK sites.
- An RCD should trip at no more than 30 mA
- HSE gives 30 milliamps as the maximum tripping current for protecting people, and says to press the test button before use to confirm the mechanism works.
- Check intervals differ by voltage
- On construction sites HSE suggests a user check weekly for 110V and every shift for 230V, with a combined inspection and test before first use then three monthly for 110V and monthly for 230V.
- An RCD is a second line of defence
- It detects some, but not all, faults, so a shock can still be fatal. Never bypass one, and treat every trip as a fault to find before the tool goes back into use.
- You need not touch a power line to be hurt by it
- HSE warns that a close approach to live conductors can cause a flashover, that contact can be lethal at voltages as low as 230V, and that current can flow through damp wood or plastic.
What people get wrong
The tool is on an RCD, so a damaged lead does not matter.
An RCD only catches some faults and HSE treats it as a secondary measure. The visual check comes first: an undamaged plug, no exposed internal wires, and no cable patched with tape or an unsuitable connector.
If the RCD trips, reset it and carry on.
A trip means there is a fault. HSE says check the system before using it again, and if it keeps tripping with no fault found, consult the manufacturer of the RCD.
Taping a nicked cable will do until the job is finished.
HSE says damaged cable must be replaced by a competent person and that makeshift repairs must not be made. Take the tool out of service, label it and report it so nobody else picks it up.
110V cannot hurt you.
Centre tapping to earth holds the voltage to earth at about 55V, which makes a fatal shock far less likely. It reduces the risk, it does not remove it, and damaged 110V equipment still comes out of use.
Answering these questions in the test
Four numbers cover most of these questions: 110V for site tools, 55V maximum to earth, 30 mA for an RCD trip, and yellow for the 110V plug. Otherwise the pattern is the usual one. Where one option is a check, a report or an isolation and another is a workaround, take the check. And remember the inspection intervals differ by voltage: on site, user checks are weekly for 110V but every shift for 230V.
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 electrical safety
Ready for the real thing?
When you are scoring 90% or better here, take a timed 50-question mock under exam conditions, then book your CITB HS&E test.