City & Guilds Electrical Installation (Level 3) Inspection, Testing and Commissioning Flashcards

24 sample cards from the Inspection, Testing and Commissioning deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Give typical maximum values of $Z_e$ for TN-S and TN-C-S supplies.

   TN-S (separate earth/cable sheath): maximum $Z_e \approx 0.8\,\Omega$. TN-C-S (PME): maximum $Z_e \approx 0.35\,\Omega$. These are the supplier's declared maximums used in design until measured values are obtained.

2. Write the relationship between measured $Z_s$, the device disconnection requirement, and how a temperature correction is applied.

   Compliance requires $Z_s \leq Z_{s(\text{max})}$ for the protective device and disconnection time. Because tabulated maximum values assume conductors at operating temperature, a measured cold value is compared using $Z_s \leq 0.8 \times Z_{s(\text{tabulated})}$ (the rule-of-thumb 0.8 factor), or by applying conductor resistance temperature correction factors.

3. How is prospective fault current (PFC) determined, and what does it comprise?

   PFC is the greater of the prospective short-circuit current (PSCC, line-to-neutral) and the prospective earth fault current (PEFC, line-to-earth). It is measured at the origin using a loop/PFC tester (or determined by enquiry of the DNO) and must not exceed the breaking capacity of protective devices.

4. What is the basic relationship between supply voltage, loop impedance and prospective fault current?

   $I_{pf} = \frac{U_0}{Z}$, where $U_0$ is the nominal line-to-earth (or line-to-neutral) voltage and $Z$ is the relevant loop impedance ($Z_e$/$Z_s$ for earth fault, or line-neutral loop for short circuit). For a single-phase supply, measuring L–N and L–E and taking the higher current gives the PFC at that point.

5. Why must the breaking capacity (Icn or Ics) of a protective device equal or exceed the prospective fault current?

   If the fault current exceeds the device's rated breaking capacity, the device may fail to safely interrupt the fault — it could weld closed, explode or fail to clear — allowing the fault to persist and causing fire or injury. Hence $I_{cn} \geq I_{pf}$ at the point of installation.

6. What are the standard RCD test trip-time requirements for a 30 mA general-purpose RCD?

   At rated residual current $I_{\Delta n}$ (30 mA): must trip within 300 ms (Type AC/A general use) — typically ≤200 ms expected. At $5 \times I_{\Delta n}$ (150 mA): must trip within 40 ms. A test at $\tfrac{1}{2} I_{\Delta n}$ (15 mA) must NOT trip the device.

7. For additional protection against electric shock, what maximum disconnection time and RCD rating apply, and what is the principle behind the $5\times$ test?

   Additional protection uses a 30 mA RCD that must disconnect within 40 ms at $5 \times I_{\Delta n}$. The $5\times$ test simulates a larger residual fault (e.g. direct contact) and confirms rapid disconnection within the time that limits the let-through energy to a non-fatal level.

8. Why is an RCD tested on both half-cycles (0° and 180°), and what value is recorded?

   The test is performed at both 0° and 180° points of the AC waveform because trip times can differ depending on the instant the fault is applied. The longest (worst-case) trip time of the two is recorded for comparison against the limit.

9. What is the purpose of the RCD test button on the unit itself, and why is it not a substitute for instrument testing?

   The integral test button verifies the mechanical tripping mechanism operates and should be operated quarterly by the user. It does not verify the trip time or that the residual current sensing meets BS 7671 limits, so an instrument test is still required at verification/periodic inspection.

10. What does functional testing verify, and give three examples.

    It verifies that assemblies and components operate correctly as intended in service. Examples: switchgear and control gear operate and isolate correctly; interlocks function; RCDs trip on test; motor controls/contactors operate; drives and isolators work; emergency stops and lighting function as designed.

11. How and why is phase sequence (phase rotation) verified on a three-phase installation?

    Using a phase-rotation indicator/meter connected to the three phases (live test). It confirms the correct rotational order (L1–L2–L3) so that three-phase motors and rotating machinery turn in the intended direction, preventing damage or hazard from reversed rotation.

12. Which certificate is issued for a brand-new installation, addition or alteration, and what are its three signatory sections?

    The Electrical Installation Certificate (EIC). It carries signatures for: Design, Construction (Installation), and Inspection & Testing — which may be one or more competent persons. It must be accompanied by a Schedule of Inspections and a Schedule of Test Results.

13. When is a Minor Electrical Installation Works Certificate (MEIWC) used instead of an EIC?

    For additions and alterations to an existing circuit that do NOT involve a new circuit — e.g. adding a socket to an existing ring, or replacing an accessory. It is not used for a new circuit or a new consumer unit.

14. What report is produced for periodic inspection of an existing installation, and what is its overall outcome statement?

    The Electrical Installation Condition Report (EICR). Its overall assessment is either 'Satisfactory' or 'Unsatisfactory'. An 'Unsatisfactory' outcome results from any C1, C2 or FI observation and means remedial work is required.

15. What three schedules/documents normally accompany an EIC, and why are they retained?

    (1) Schedule of Inspections, (2) Schedule of Test Results (Generic), and the certificate itself. They are retained as the baseline record of the installation's condition for future comparison at periodic inspection, and as legal evidence of compliance and competent work.

16. When recording test results, what does the column 'Zs (Ω)' represent and how is it cross-checked against the maximum permitted value?

    It is the measured earth fault loop impedance for that circuit at its furthest point. It is compared against the maximum permitted $Z_s$ for the circuit's protective device and required disconnection time (allowing the 0.8 temperature correction). If $Z_s$ measured $\leq Z_{s(max)}$, the circuit complies.

17. How would you interpret an insulation resistance reading of '>999 MΩ' versus a reading of '0.3 MΩ' on a 230 V circuit?

    >999 MΩ indicates excellent insulation with negligible leakage (a healthy circuit). 0.3 MΩ is below the 1.0 MΩ minimum for a 500 V test, indicating deteriorated insulation or a fault (e.g. moisture, damage, connected equipment) and is non-compliant — requiring investigation before energising.

18. What is the logical sequence for systematic fault diagnosis on an installation?

    (1) Gather information/symptoms; (2) analyse and identify the likely faulty area (divide and isolate); (3) safely isolate and prove dead; (4) test/measure to locate the exact fault; (5) interpret results against expected values; (6) rectify the fault; (7) re-test to confirm; (8) restore supply and document. Always work safely and methodically.

19. Classify common electrical fault types with a brief definition of each.

    Open circuit: a break in a conductor causing no current flow (e.g. loose/broken connection). Short circuit: a low-impedance fault between live conductors (L–N or L–L) causing high current. Earth fault: a fault between a live conductor and earth. High-resistance joint: a poor connection causing localised heating and volt drop. Insulation breakdown: deteriorated insulation causing leakage to earth.

20. What instrument test would distinguish an open-circuit cpc from a high-resistance cpc connection, and how?

    A low-resistance continuity (R2) test. An open circuit shows infinite/over-range resistance (no continuity), while a high-resistance joint shows a measurable but abnormally high resistance compared with the expected calculated value, indicating a poor but not broken connection.

21. What is commissioning, and how does it differ from inspection and testing?

    Commissioning is the process of bringing the completed installation into full working service — energising, setting, adjusting and functionally proving equipment and systems operate correctly and safely to the design intent. Inspection and testing verify safety and compliance; commissioning additionally confirms operational performance and readiness for use.

22. What information should be provided to the client at handover of a new installation?

    The Electrical Installation Certificate with schedules of inspection and test results, operation and maintenance (O&M) information, circuit charts/distribution board labelling, details of protective devices and settings, recommended date for next inspection, manufacturers' instructions for equipment, and guidance on user duties (e.g. quarterly RCD test button).

23. What label or notice must be fixed at the origin regarding periodic inspection, and what does it state?

    A notice stating the date of the last inspection and the recommended date for the next inspection and test, fixed in a prominent position at or near the origin. It alerts the duty holder/user when the next EICR is due, supporting ongoing compliance with the Electricity at Work Regulations.

24. Compare TN-S, TN-C-S and TT earthing systems in terms of the earth path and typical protective measures.

    TN-S: separate earth conductor (e.g. cable sheath) back to the source star point; low $Z_e$, fault protection by OCPD or RCD. TN-C-S (PME): combined neutral-earth (PEN) in the supply, lowest $Z_e$, requires main bonding; OCPD/RCD protection. TT: installation relies on its own earth electrode (high $R_A$), so RCD protection is essential because loop impedance is too high for OCPD-only disconnection.

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Inspection, Testing and Commissioning flashcards FAQ