ECS Electrical Safety Unit

Ohm's law: current = voltage ÷ resistance, so 230 ÷ 46 = 5 A. Option d comes from multiplying instead of dividing.

Source: Ohm's law (basic electrical science) (I = V / R)

Touching the casualty while they are in contact with the supply can make you a second casualty, so break the circuit first by switching off and isolating, then call 999 and give first aid such as CPR if needed. Pulling them by the arm is the most tempting choice but puts you in the current path. HSE guidance on electrical safety and first aid.

Source: HSE guidance on electrical safety and first aid (Electric shock: break contact with the supply before touching the casualty)

Warning signs are triangular, with a yellow background, black border and black symbol, such as the lightning flash for electrical danger. Blue circles are mandatory signs (you must do something), red circles with a bar are prohibition signs and green rectangles show safe conditions. Health and Safety (Safety Signs and Signals) Regulations 1996.

Source: Health and Safety (Safety Signs and Signals) Regulations 1996 (Schedule 1, warning signs)

Current passing through the body can burn muscles, nerves and blood vessels along its path, so the damage inside can be far worse than the small entry or exit marks on the skin. Anyone who has had a significant electric shock should be checked by a medical professional. HSE guidance on electrical injuries.

Source: HSE Electrical injuries (Burns along the length of the current flow)

Regulation 14 allows live work only when it is unreasonable for the conductor to be dead, reasonable to work live, and suitable precautions are taken. Regulation 12 covers means of cutting off supply and isolation; 16 covers competence; 4 covers systems, work activities and protective equipment.

Source: Electricity at Work Regulations 1989 (Regulations 4, 12, 14, 16)

BS 7671 is a British Standard, not law in itself, but HSE guidance says installations that follow it are likely to meet the relevant parts of the Electricity at Work Regulations 1989. The EAWR is the statutory instrument and was not replaced. BS 7671 applies to all kinds of premises. HSE HSR25, guidance on the EAWR.

Source: HSE HSR25 The Electricity at Work Regulations 1989: guidance on regulations (Status of BS 7671 in relation to the EAWR); BS 7671:2018+A4:2026 (Part 1, scope and status (paraphrased))

A tester that fails silently would show 'dead' on a live circuit. Proving it on a known live source or proving unit before and after the test shows it was working when it read dead.

Source: HSE GS38 and HSG85 (Proving dead / safe isolation procedure)

Locking off with a personal lock, and keeping the key, secures the isolation so nobody else can re-energise the circuit while you are working on it. A caution notice should be fixed as well. Locking off does not prove the circuit is dead: you must still prove dead with a working voltage indicator. HSE HSG85, secure isolation.

Source: HSE HSG85 Electricity at work: Safe working practices (Secure isolation: locking off and caution notices)

A two-pole approved voltage indicator with GS38 probes and leads, proved on a proving unit or known live source before and after use, gives a reliable result. Non-contact pens are useful for a quick check but are not reliable enough to prove dead. A resistance range can be damaged by, and may not detect, a live supply. HSE GS38 and HSG85.

Source: HSE GS38 Electrical test equipment for use on low voltage electrical systems (Voltage indicators for proving dead); HSE HSG85 Electricity at work: Safe working practices (Proving dead)

Since the harmonised colours were introduced, L1 is brown, L2 black and L3 grey. Blue is neutral and green-and-yellow is the protective conductor. Red, yellow and blue are the old colours, still found in older installations.

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))

In the harmonised colours, line is brown, neutral is blue and the protective conductor is green-and-yellow. Black was the neutral colour in the old colour code, which is why it is the tempting wrong answer; in new wiring black is a line colour (L2). BS 7671, Chapter 51, identification of conductors.

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))

On a 400 V three-phase, four-wire supply, 400 V is measured between any two line conductors and 230 V between any line and neutral. 400 V is the tempting wrong answer because it is the figure quoted for the supply as a whole. Basic electrical science: three-phase supplies.

Source: Basic electrical science (Three-phase line and phase voltages)

Before the harmonised colours, the three line conductors were red (L1), yellow (L2) and blue (L3), with black for neutral. Brown, black and grey are the harmonised line colours used in new work. Blue was a line colour in the old code but is neutral now, so never rely on colour alone: identify the conductor and prove dead. BS 7671, Chapter 51 (paraphrased).

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))

Green-and-yellow is used only to identify protective conductors, such as circuit protective conductors and earthing and bonding conductors, and must not be used for anything else. Although the neutral is connected to earth at the supply, it is a live conductor and is identified blue. BS 7671, Chapter 51 (paraphrased).

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))

Extra-low voltage is normally not exceeding 50 V a.c. (or 120 V ripple-free d.c.). Above that, up to 1000 V a.c., is low voltage. 120 V is the d.c. ELV limit, a common mix-up.

Source: BS 7671:2018+A4:2026 (Part 2, definitions: voltage bands (paraphrased))

Low voltage is above extra-low voltage (50 V a.c.) and up to 1000 V a.c. The figures 120 V and 1500 V are the d.c. limits for ELV and LV, a common mix-up with the a.c. values. Normal 230 V and 400 V supplies are therefore low voltage. BS 7671, Part 2, definitions.

Source: BS 7671:2018+A4:2026 (Part 2, definitions: voltage bands (paraphrased))

PV panels and batteries produce d.c. The inverter converts it to a.c. at 50 Hz so it can feed the installation and the grid. Note that the d.c. side can stay live in daylight even when the a.c. side is isolated.

Source: Basic electrical science (Inverters (d.c. to a.c.))

A transformer uses magnetic coupling between two windings to change an a.c. voltage; a step-down transformer gives a lower output voltage, for example 230 V to 110 V or to ELV. It only works on a.c. Converting a.c. to d.c. is done by a rectifier, and storing energy is the job of a battery. Basic electrical science: transformers.

Source: Basic electrical science (Transformers)

Centre-tapping the 110 V winding to earth means each line is only 55 V from earth, which greatly reduces the shock risk from portable tools on site. 110 V is the voltage between the two lines, which is the tempting wrong answer. BS 7671, reduced low voltage systems (Regulation 411.8).

Source: BS 7671:2018+A4:2026 (Regulation 411.8, reduced low voltage systems (single value cited))

An RCBO (residual current circuit-breaker with overcurrent protection) does the job of an MCB, protecting against overload and short circuit, and of an RCD, detecting earth leakage, in one unit for one circuit. It does not change the voltage or isolate the whole installation. Basic electrical knowledge: protective devices.

Source: Basic electrical knowledge (RCBOs (combined overcurrent and residual current protection))

Regulation 3 of the Management of Health and Safety at Work Regulations 1999 requires a suitable and sufficient risk assessment. The EAWR is the tempting answer, but it sets electrical safety duties rather than the general risk assessment duty. BS 7671 and Approved Document P cover installations, not risk assessment. MHSWR 1999, regulation 3.

Source: Management of Health and Safety at Work Regulations 1999 (Regulation 3, risk assessment)

A method statement describes the safe sequence of work and how the controls from the risk assessment will be put in place, such as isolation points, equipment and who does what. A list of hazards and risk ratings is the risk assessment, which the method statement builds on. HSE guidance on managing risk.

Source: HSE guidance on managing risk (Risk assessments and method statements)

The hierarchy starts with removing the hazard altogether; for electrical work that usually means isolating and working dead. Next come substitution, engineering controls and administrative controls such as signs and training. PPE is the last line of defence because it only protects the wearer and can fail. HSE guidance on controlling risks.

Source: HSE: Risk assessment, steps needed to manage risk (Control the risks)

Section 2 places a general duty on employers to ensure, so far as is reasonably practicable, the health, safety and welfare at work of their employees. This includes safe plant and systems of work and the information, instruction, training and supervision people need. Transport, medical checks and pay rises are not part of this duty. HSWA 1974, section 2.

Source: Health and Safety at Work etc. Act 1974 (Section 2, general duties of employers to their employees)

Insulation resistance is a dead test: a high d.c. test voltage is applied between conductors, so the circuit must be isolated. Earth fault loop impedance, RCD and prospective fault current tests need the supply connected, so they come after the dead tests.

Source: BS 7671:2018+A4:2026 (Part 6, initial verification sequence (paraphrased))

Inspection comes before testing and is normally done with the relevant part of the installation disconnected from the supply. It finds visible defects, such as damage, wrong cable or missing covers, before any test voltage or supply is applied. Doing it after live tests would expose people to defects that could have been seen. BS 7671, Part 6 (paraphrased).

Source: BS 7671:2018+A4:2026 (Part 6, Regulation 642.1, inspection before testing (paraphrased))

Polarity testing checks that single-pole devices such as switches, fuses and circuit-breakers are connected in the line conductor, and that terminals such as socket and lampholder contacts are correctly connected. A switch wrongly placed in the neutral would leave the equipment live when switched off. BS 7671, Part 6 (paraphrased).

Source: BS 7671:2018+A4:2026 (Part 6, polarity (paraphrased))

Dead tests are only safe on a circuit that has been securely isolated, locked off and proved dead with a proven voltage indicator. Just switching off a breaker leaves it open to someone switching it back on. Loop impedance is a live test and comes after the dead tests. HSE HSG85, safe isolation; EAWR 1989, regulation 13.

Source: HSE HSG85 Electricity at work: Safe working practices (Safe isolation and proving dead); Electricity at Work Regulations 1989 (Regulation 13, precautions for work on equipment made dead)

A ring final circuit relies on its line, neutral and protective conductors each forming a complete loop back to the board. If a ring is broken, it becomes two long radial legs and the cable may be overloaded even though everything seems to work. The test also shows up interconnections that make the readings uneven. BS 7671, Part 6, continuity of ring final circuit conductors (paraphrased).

Source: BS 7671:2018+A4:2026 (Part 6, continuity of ring final circuit conductors (paraphrased))

R1 is the resistance of the circuit's line conductor and R2 that of its circuit protective conductor, measured from the board to the far end of the circuit. Together they form the circuit's own part of the earth fault path, and a sensible reading confirms the protective conductor is continuous. The neutral is not part of it. BS 7671, Part 6 (paraphrased).

Source: BS 7671:2018+A4:2026 (Part 6, continuity of protective conductors (paraphrased))