Electrical Inspection, Testing & Documentation – Module 6 Detailed Notes

Pre-Work Survey: Before Any Electrical Inspection or Test

• Initial objective: Identify all conditions that could influence safety, compliance with BS 7671 and the success of subsequent tests.
• Core questions the inspector must answer:
  • Which type of earthing arrangement (TN-S, TN-C-S, TT, IT) is employed?
  • What is the maximum required disconnection time according to BS 7671 for the particular circuit and earthing system?
  • Are the isolation and switching arrangements adequate, accessible and identifiable?
  • What labels or notices (e.g.
  • Main switch / isolator label,
  • Circuit schedule,
  • RCD test notice,
  • Bonding labels,
  • Warning notices for multi-earthing, PME etc.) are already present or still required?
  • Is the measured or expected earth-fault loop impedance Z_s below the maximum permitted value given in BS 7671 tables?
  • Is additional protection by RCD or RCBO mandatory (e.g. for socket-outlets ≤ 32\,\text{A}, buried cables < 50\,\text{mm} deep, special locations) or prudent because measured Z_s is borderline?
  • Does the work fall within a special location (bathroom, swimming pool, medical location, agricultural, caravan, photovoltaic, EV charging, etc.) requiring further rules?
  • What risks or hazards exist: access to live parts, moving machinery, moisture, confined space, lone working, vulnerable users, asbestos, combustible dust, working at height, etc.?
• Moral / legal duty of care: The inspector must ensure personal safety and the safety of others; failure may lead to electric shock, fire, prosecution or loss of licence.

Safe Working Procedures & Tools

• Always carry out SAFE ISOLATION before touching conductors:
  1. Identify the supply.
  2. Switch OFF.
  3. Lock off and place warning tags.
  4. Prove the instrument on a known source.
  5. Test the circuit is dead (phase–earth, phase–neutral, neutral–earth).
  6. Prove the instrument again.
• Dual working / buddy system recommended for higher-risk environments.
• Test equipment and leads must be GS38-compliant (shrouded connectors, finger guards, fused probes, CAT rating appropriate).
• Environment must not impair safety: adequate lighting, dry surface, secure footing, segregation from other trades.

Automatic Disconnection of Supply (ADS)

• Principal means of basic & fault protection in domestic and most commercial installations.
• Components:
  • Protective earthing (earthing conductor, main earthing terminal).
  • Protective equipotential bonding (main & supplementary bonding conductors).
  • Automatic disconnection by protective devices (fuses, MCBs, RCBOs, RCDs) within required time if a fault drives exposed-conductive-parts above permissible touch voltage.

Continuity of Circuit Protective Conductors (CPCs) & Bonding

• Requirement: BS 7671 Regulation 643.2 – continuity of all protective conductors shall be verified.
• Definitions used in tests:
  • Line conductor resistance = R_1.
  • Neutral conductor resistance = R_n.
  • Circuit protective conductor resistance = R_2.
• Typical conductors:
  • CPC of a twin-&-earth final-circuit cable: copper, usually 1.5\,\text{mm}^2 (or same size as lives if smaller than 16\,\text{mm}^2).
  • Main protective bonding: single-core, green-&-yellow, \ge 10\,\text{mm}^2 copper, linking MET to extraneous-conductive-parts (water, gas, oil, structural steel).
• Acceptable resistance limits for bonding conductors:
  • Main bonding: \le 0.05\,\Omega between service point and MET.
  • Supplementary bonding: \le 0.05\,\Omega between the two bonded parts.
• Adverse outcomes if continuity is absent: exposed metal may remain live during a fault, protective device may not operate → shock / fire risk.

Test Instrumentation

• Low-resistance ohmmeter (part of multifunction tester) applying typically 200\,\text{mA} test current.
• Must resolve changes of 0.01\text{–}0.05\,\Omega.
• Tester lead resistance should be null-zeroed or subtracted from readings.

CPC Continuity Test Methods

Method 1: "R1 + R2" (Short-Link Procedure)

• Purpose: Simultaneously confirms polarity, CPC continuity, and provides R1 + R2 for later Z_s determination.
• Steps (adapted to a lighting circuit example with Edison screw lampholder):
  1. Safely isolate the circuit being tested.
  2. At the distribution board, install a temporary link between the phase (line) conductor and CPC of the circuit under test.
  3. Go to each accessory (switch, lampholder, socket, junction box):
  • With the circuit still isolated, place one tester probe on the earthing terminal, the other on the lampholder centre contact (or phase terminal of a socket).
  • Switch ON local switches to include them in the test path.
  1. Record displayed resistance = R1 + R2 for that point.
  2. Remove the temporary link before re-energising.
• Compliance references: BS 7671 Regulations 643.2 and 643.6.
• Acceptable values: Derived from cable length & c.s.a. using R = \rho \dfrac{L}{A}; must be low enough that when added to external earth loop impedance Ze total Zs satisfies disconnection time tables.

Method 2: "Wander / Long-Lead" Method

• Used when a parallel earth path doesn’t exist yet (e.g. testing a single bonding conductor) or where linking is impractical.
• Steps:
  1. Attach one end of a long test lead to the MET or CPC at the consumer unit.
  2. Route the lead safely to the remote point (tap, radiator, accessory, second building, etc.).
  3. Measure resistance between the remote conductive part and the long lead return.
  4. Subtract the resistance of the long lead itself to obtain actual conductor resistance.
• Additional uses: circuit identification, polarity confirmation, live conductor continuity (when paired with phase identification bar or tone generator).

Using R1 + R2 to Predict Earth-Fault Loop Impedance Z_s

• Relationship: Zs = R1 + R2 + Ze
  where Z_e is external earth impedance measured at the origin.
• If calculated Z_s (or measured directly later by loop tester) ≤ tabulated maximum for the protective device and earthing system, ADS will operate within required time.
• Typical disconnection times:
  • Final circuits ≤ 32\,\text{A} in TN systems: 0.4\,\text{s}.
  • Distribution circuits or TT system: 5\,\text{s} (unless RCD provides faster cut-off).
• If Z_s is too high: options include reducing circuit length, increasing c.s.a., fitting RCD/RCBO, additional bonding, or re-designing earthing.

Continuity of Live Conductors (Line & Neutral)

• Live conductors must also be electrically continuous; breaks could produce overheating, arcing, or single-pole isolation hazards.
• Same ohmmeter and procedures as CPC continuity, ensuring correct polarity and inter-connections.

Ring Final Circuit Continuity (BS 7671 643.2.1-(ii))

• Construction: Twin conductors leave the MCB/RCBO, loop through each accessory, then return to the same terminals, forming three complete rings (line, neutral, CPC).
• Purpose of test: Detect broken rings or unauthorized inter-links that could overload cable segments.
• Instrument: Low-resistance ohmmeter capable of discerning changes of 0.05\,\Omega.
• Three-Step Regimen (only Step 1 covered in transcript – included here in full context for completeness):
  1. Step 1 – End-to-End Resistance
  • Isolate, identify and disconnect all line, neutral and CPC conductors of the ring.
  • Measure each loop separately:
    • R1 (line), Rn (neutral), R_2 (CPC).
  • Expectations:
    • R1 \approx Rn (same c.s.a.).
    • R2 larger if CPC is smaller; for 2.5 mm²/1.5 mm² T&E, R2 \approx 1.67\,R_1.
  1. Step 2 – Cross-Connected (“Figure-of-Eight”) Test (implied)
  • Join one end of line to the opposite end of neutral; measure resistances at every socket between the remaining open ends; values should be consistent (theoretical mean = \tfrac{R1+Rn}{4}).
  1. Step 3 – Repeat with Line-CPC Cross (verifies polarity & CPC integrity).
• Interpretation: Abnormal readings pinpoint breaks, interconnections, high-resistance joints or wrongly wired accessories.

Practical, Ethical & Regulatory Implications

• Document every reading on the Electrical Installation Certificate (EIC) or Minor Works Certificate; include test instrument serial numbers and calibration dates.
• Ethical responsibility: Accurate records enable future electricians to assess safety; falsification can lead to injury, death, legal action.
• Real-world relevance: Proper continuity and loop impedance ensure MCBs/RCDs trip fast enough to prevent fire, especially with modern high-fault currents from PME supplies.

Connections to Earlier & Wider Learning

• Builds on principles of Ohm’s Law, resistivity, and protective devices discussed in previous modules (e.g. Module 3 Cable Sizing & Module 4 Overcurrent Protection).
• Provides groundwork for forthcoming topics: Insulation Resistance, Polarity, Earth-Fault Loop Impedance measurement, RCD testing, Certification & Part P compliance.

Key Numerical / Formula Summary

• Acceptable bonding resistance: \le 0.05\,\Omega.
• Relationship: Zs = R1 + R2 + Ze.
• Typical ring final ratios: R2 \approx 1.67\,R1 for 2.5/1.5 T&E.
• Disconnection times (TN final circuits ≤ 32\,\text{A}): 0.4\,\text{s}; (distribution circuits): 5\,\text{s} unless RCD provides faster protection.
• Resistive calculation: R = \rho \dfrac{L}{A} (copper resistivity \rho \approx 17.2\times10^{-9}\,\Omega\,\text{m}).

Checklist for Documentation

• Record:
  • Earthing arrangement type.
  • Protective device ratings & types.
  • Conductor sizes, materials, routes.
  • Test instrument identifiers & calibration.
  • All measured values: R1, R2, Rn, R1+R2, Ze, Z_s.
  • Labels installed / updated.
  • Deviations & remedial actions.
• Sign & date certificate; obtain client acknowledgment; retain copies for statutory period.