5.1 The Correct Sequence of Tests

Quick Answer: Initial verification follows a strict dead-then-live sequence laid down by GN3 and BS 7671. The dead tests (continuity of conductors, insulation resistance, verification of SELV/PELV or electrical separation, polarity and earth electrode resistance) are completed while the installation is de-energised. Only then is the installation energised for the live tests: earth fault loop impedance (Ze then Zs), prospective fault current, additional protection by RCD, phase sequence, functional testing and voltage drop. The order is exam-critical — reversing it can injure the tester or damage equipment.

Why a Fixed Sequence Exists

The sequence is not a checklist convention. Each test depends on the previous one for its validity and for the safety of the tester. Continuity of the protective conductor path must be confirmed before any test that relies on that path to clear a fault. Insulation resistance must be proved before energising so that a latent fault does not flash over when voltage is applied. Polarity is checked dead so that equipment is not energised with reversed line and neutral. The dead-then-live split is therefore a safety boundary as well as a technical one.

Dead Tests (Before Energising)

The dead tests are performed in this order, with the installation isolated and proved dead. The order follows BS 7671 Regulation 643, and the regulation numbers are worth flagging in your copy because open-book questions often quote them.

1. Continuity of conductors (Reg 643.2) — protective conductors including main protective bonding and supplementary bonding, measured end-to-end using the R1+R2 method (line conductor and circuit protective conductor linked at the board) or the long-lead method where one end is inaccessible. This is first because it confirms the fault path that every subsequent test relies on.
2. Continuity of ring final circuit conductors (Reg 643.2) — the r1, rn, r2 test. Each conductor of the ring is measured end-to-end, then the crossover steps confirm the ring is continuous and correctly wired. The standard procedure: measure r1, rn, r2 end-to-end; cross-connect line and neutral at the board and measure between L and N at each socket (should be (r1+rn)/4); cross-connect line and cpc and measure between L and E at each socket (should be (r1+r2)/4). Detects spurs, interconnections, and broken rings.
3. Insulation resistance (Reg 643.3, Table 64) — measured between live conductors and between the live conductors and earth, at 250 V DC for SELV and PELV circuits, 500 V DC for circuits up to and including 500 V, and 1000 V DC for circuits above 500 V. The minimum values in Table 64 are 0.5 MΩ for the 250 V test and 1.0 MΩ for both the 500 V and 1000 V tests. Electronic equipment may need to be disconnected or its terminals linked to avoid damage from the test voltage.
4. Protection by SELV, PELV or electrical separation (Reg 643.4) — where these protective measures are used, the separation between circuits is verified by an insulation-resistance measurement between the separated circuit and other circuits and earth. Covered in section 7.3.
5. Insulation resistance or impedance of floors and walls (Reg 643.5) — only where non-conducting location is used as a protective measure, which is rare outside specialised installations.
6. Polarity (Reg 643.6) — confirmed dead at every point to prove that single-pole protective devices and switches are in the line conductor and that socket-outlets and lampholders are correctly connected. Often established during the continuity test, because the R1+R2 measurement already proves the line-to-cpc path.
7. Earth electrode resistance (Reg 643.7.1) — for TT systems, measured by the dead fall-of-potential method using a dedicated earth electrode tester with current and potential spikes, with the electrode disconnected from the main earthing terminal. The 62% method places the potential spike at 62% of the distance to the current spike to minimise measurement error. It may alternatively be determined live from the loop impedance.

Once the dead tests are satisfactory, the installation is energised and these tests are performed in order:

1. Protection by automatic disconnection of supply (Reg 643.7) — Ze first (the external loop impedance at the origin, with the main earthing conductor disconnected so the installation cannot influence the reading), then Zs for each circuit (Ze + R1+R2, measured or calculated). Confirms the fault path will operate the protective device within the required disconnection time. Where an RCD provides fault protection, its operation is verified here (Reg 643.7.1).
2. Prospective fault current (PFC) — the maximum current that could flow in a fault. Measured at the origin and at each distribution board. The recorded figure is the greater of the prospective short-circuit current (PSCC, line-to-neutral or line-to-line) and the prospective earth fault current (PEFC, line-to-earth), so the breaking capacity of the protective devices can be checked.
3. Additional protection (Reg 643.8) — where a 30 mA RCD provides additional protection, its effectiveness is verified. Under the current standard the required verification is an alternating-current test at the rated residual operating current IΔn: a general (non-delay) type must operate within 300 ms, and a Type S within 130–500 ms. Detail in section 9.1.
4. Check of phase sequence (Reg 643.9) — for polyphase circuits, that the phase sequence is maintained at all relevant points. It is listed here because that is where it sits in the Regulation 643 running order and where an indicator reading is normally taken, but the IET points out that the verification itself is a dead check on how the line conductors are terminated; phase rotation is the live check. Covered in section 11.1.
5. Functional testing (Reg 643.10) — assemblies, switchgear, controlgear, interlocks and RCD test facilities are operated to confirm they are properly mounted, adjusted and installed. Covered in section 11.2.
6. Verification of voltage drop (Reg 643.11) — where required, verified by measuring the circuit conductor resistance and calculating the drop, rather than by direct measurement under load. Covered in section 11.3.

The examiner will not usually ask you to recite all thirteen items, but the relative order matters: continuity before insulation resistance, everything dead before anything live, Ze before Zs, and the functional and voltage-drop checks last.

Why the Order Matters — Exam Reasoning

The examiner tests the reasoning, not just the list:

• Continuity first — confirms the protective conductor path is intact before any test that depends on it for safety.
• Insulation before energising — proves no fault exists that could cause a short or flashover on energisation.
• Polarity dead-checked — so equipment is not energised with reversed polarity.
• Dead before live — protects the tester and the installation from a fault that dead tests would have revealed.
• Ze before Zs — because Zs = Ze + (R1+R2); the external loop is established before per-circuit values are verified.
• RCD after loop impedance — the loop measurement first confirms the fault path the RCD operates into.
• Phase sequence, functional testing and voltage drop last — they check that the installation works as a system, which is only meaningful once every protective measure has been verified.

A common exam trap is to place insulation resistance before continuity. This is wrong — if the protective conductor is not continuous, the insulation test could place test voltage on an exposed conductive part, creating a hazard. Continuity must come first.

Periodic Inspection — A Permitted Variation

Periodic inspection has no prescribed sequence — that is the point of assessment criterion 4.3b, which asks you to explain why one exists for initial verification but not for periodic inspection. Because the installation already exists and its condition is unknown, it is common practice to perform live tests first (Ze, Zs, PFC) to confirm the installation is safe to work on, then isolate for dead tests under safe isolation procedures. This reversal is a safety measure, not a shortcut — the inspector must still complete the full set of tests. GN3 recognises this distinction and the 2391-52 examiner accepts it as a legitimate periodic-inspection practice. Some tests may also be unnecessary or impracticable at periodic inspection — see section 5.3.

The key difference: initial verification starts with a new, proven-de-energised installation, so dead-then-live is always correct. Periodic inspection starts with an energised installation of unknown condition, so live-first may be necessary to establish whether it is safe to isolate.