CSCS Electrical Safety 1 — Questions and Answers
Question 1: What voltage supply is required for portable electrical equipment used on construction sites under BS 7671?
- 230V via standard socket
- 110V via centre-tapped earth transformer (Correct answer)
- 415V three-phase supply
- 12V extra-low voltage only
Correct answer: 110V via centre-tapped earth transformer
BS 7671 and HSE guidance require 110V supply via a centre-tapped earth transformer for portable tools on construction sites. This limits the maximum voltage to earth to 55V, significantly reducing the risk of fatal electric shock.
Question 2: What is the primary purpose of a Residual Current Device (RCD) on a construction site?
- To prevent overloading of circuits
- To detect earth leakage current and disconnect the supply rapidly (Correct answer)
- To regulate voltage fluctuations
- To monitor energy consumption
Correct answer: To detect earth leakage current and disconnect the supply rapidly
An RCD detects an imbalance between live and neutral conductors caused by earth leakage current and disconnects the supply within milliseconds. Under the Electricity at Work Regulations 1989, RCDs provide additional protection against electric shock but are not a substitute for reduced voltage systems.
Question 3: What is the safe exclusion zone distance for work near 400kV overhead power lines as specified by HSE guidance?
- 3 metres
- 6 metres
- 15 metres (Correct answer)
- 9 metres
Correct answer: 15 metres
HSE guidance note GS6 specifies a minimum safe distance of 15 metres from 400kV overhead power lines. For lower voltages, smaller distances apply — for example, 6 metres for lines up to 33kV. These distances account for potential electrical arcing and the swing of cables in wind.
Question 4: How often must portable electrical equipment on a construction site undergo PAT testing as a general guideline?
- Every 12 months
- Every 3 months (Correct answer)
- Every 6 months
- Only when visibly damaged
Correct answer: Every 3 months
HSE and IET guidance recommend that portable appliances on construction sites are formally PAT tested every 3 months due to the harsh working environment. Additionally, users should carry out visual checks before each use. The frequency is more stringent than for office environments due to the higher risk of damage.
Question 5: Under the Electricity at Work Regulations 1989, who has a duty to ensure electrical systems are safe?
- Only qualified electricians
- The HSE inspector
- The duty holder — employers, self-employed persons, and employees (Correct answer)
- Only the client who commissioned the work
Correct answer: The duty holder — employers, self-employed persons, and employees
The Electricity at Work Regulations 1989 place duties on employers, self-employed persons, and employees (collectively 'duty holders') to ensure electrical systems are constructed, maintained, and used so as to prevent danger. This means everyone from site managers to operatives has a legal responsibility.
Question 6: What is the correct procedure before carrying out electrical isolation of equipment on a construction site?
- Simply unplug the equipment and begin work
- Switch off, isolate, lock off, prove dead using a voltage tester, and attach warning notices (Correct answer)
- Ask a colleague to hold the switch in the off position
- Turn off the main supply to the entire site
Correct answer: Switch off, isolate, lock off, prove dead using a voltage tester, and attach warning notices
Safe isolation follows a strict sequence: switch off, isolate from all sources, lock off with a personal padlock, prove dead using a GS38-compliant voltage tester (tested before and after on a known live source), and attach warning notices. This procedure is mandated under the Electricity at Work Regulations 1989.
CSCS Electrical Safety 1
What voltage supply is required for portable electrical equipment used on construction sites under BS 7671?