CSCS Electrical Safety 4 — Questions and Answers
Question 1: What does the abbreviation 'CTE' stand for in relation to 110V site transformers?
- Central Transformer Exchange
- Centre-Tapped Earth (Correct answer)
- Construction Temporary Electrics
- Current Transfer Equipment
Correct answer: Centre-Tapped Earth
CTE stands for Centre-Tapped Earth. A 110V CTE transformer takes a 230V supply and steps it down to 110V, with the secondary winding centre-tapped to earth. This means the maximum voltage to earth is only 55V, which is considered a non-lethal level and provides enhanced safety on construction sites as required by BS 7671.
Question 2: What user checks should be carried out on a portable electrical tool before each use on a construction site?
- No checks are needed if the tool has a current PAT sticker
- Visual inspection of the plug, cable, and casing for damage, and checking the PAT test date is current (Correct answer)
- Only check that the on/off switch works
- Checks are only required weekly, not before each use
Correct answer: Visual inspection of the plug, cable, and casing for damage, and checking the PAT test date is current
Before each use, operatives must visually inspect the plug (no cracks, pins straight), cable (no cuts, kinks, or exposed conductors), casing (no damage), and confirm the PAT test label is current. This is required under the Electricity at Work Regulations 1989 and PUWER 1998. A current PAT sticker does not guarantee the tool has not been damaged since the last test.
Question 3: What action should be taken if a 110V tool gives the user a tingling sensation during use?
- Continue using it as 110V cannot cause harm
- Stop using it immediately, disconnect it, label it as defective, and report it to the supervisor (Correct answer)
- Switch to using it with wet gloves for insulation
- Increase the voltage to overcome the fault
Correct answer: Stop using it immediately, disconnect it, label it as defective, and report it to the supervisor
Any tingling sensation indicates an earth fault or insulation breakdown. The tool must be stopped immediately, disconnected, labelled as defective, and reported. Although 110V CTE limits voltage to earth to 55V, this can still cause injury particularly in wet conditions. Under the Electricity at Work Regulations 1989, defective equipment must be withdrawn from service.
Question 4: What is the purpose of earthing and bonding in a temporary electrical installation on a construction site?
- To improve the speed of electrical supply
- To provide a low-resistance path to earth so that protective devices operate quickly in the event of a fault (Correct answer)
- To reduce electricity bills
- To increase the voltage available to tools
Correct answer: To provide a low-resistance path to earth so that protective devices operate quickly in the event of a fault
Earthing and bonding provide a low-resistance path for fault current to flow to earth, enabling protective devices (fuses, MCBs, RCDs) to operate quickly and disconnect the supply. BS 7671 requires all exposed and extraneous conductive parts to be earthed and bonded to prevent them becoming live during a fault, which could cause electric shock.
Question 5: Under the Electricity at Work Regulations 1989, what standard must a voltage tester meet for safe isolation procedures?
- Any multimeter is acceptable
- It must comply with HSE Guidance Note GS38 (Correct answer)
- It must be a non-contact voltage indicator only
- There is no specific standard for voltage testers
Correct answer: It must comply with HSE Guidance Note GS38
HSE Guidance Note GS38 specifies the requirements for test instruments used during safe isolation. Voltage testers must have fused test leads, finger guards, insulated probes with limited exposed tips, and be proved on a known live source before and after testing. Non-contact voltage indicators (volt sticks) are useful aids but must not be relied upon as the sole means of proving dead.
Question 6: What must be displayed at the main intake position of a temporary construction site electrical installation?
- A list of all workers on site
- A schematic diagram of the installation, emergency contact numbers, and isolation procedures (Correct answer)
- Only the electricity supplier's logo
- Nothing — signage is optional for temporary installations
Correct answer: A schematic diagram of the installation, emergency contact numbers, and isolation procedures
BS 7671 requires that a schematic diagram or schedule of the installation is displayed at the origin of the supply. Emergency contact numbers for the DNO and site electrician, together with clear isolation procedures, must also be prominently displayed. This ensures that in an emergency, anyone can identify how to isolate the supply safely.
CSCS Electrical Safety 4
What does the abbreviation 'CTE' stand for in relation to 110V site transformers?