BS 7671 Regulations 643.2 to 643.11 specify that tests must be carried out in a particular sequence. This is not optional – the order matters because later tests depend on the results of earlier ones. Testing out of sequence can produce misleading results or create safety risks.

What it confirms: That protective conductors (CPCs) provide a continuous path back to the source, and that ring final circuits are correctly wired without interconnections or breaks.

How it works: A low-resistance ohmmeter is used to measure the resistance of protective conductors. For R1+R2 testing, the line conductor and CPC are linked at the consumer unit and measured at each point on the circuit. This gives the total resistance of the line conductor plus the CPC for that circuit.

For ring final circuits: Three measurements are taken – end-to-end resistance of the line ring (r1), end-to-end resistance of the neutral ring (rn), and end-to-end resistance of the CPC ring (r2). The line and CPC are then cross-connected and measured at each socket. Each reading should be approximately (r1+r2)/4 at the midpoint, increasing equally toward each end.

What it confirms: That the insulation between live conductors and between live conductors and earth is adequate to prevent leakage current and reduce the risk of electric shock or fire.

How it works: An insulation resistance tester applies a DC voltage between the conductors being tested and measures the resistance of the insulation in megaohms (MOhm).

Tests required: Line to earth, neutral to earth, and line to neutral. Where circuits share a distribution board, they can be tested together (all lines connected, all neutrals connected) to save time. If the grouped reading is below the minimum, individual circuits must be tested separately to identify the fault.

What it confirms: That single-pole switching devices are connected in the line conductor only. That centre-contact bayonet and Edison screw lampholders have the line connected to the correct terminal. That socket-outlets and other accessories are correctly wired.

How it works: Polarity is normally confirmed during the R1+R2 continuity test – if the reading at a socket or accessory is correct, it proves polarity is correct because the test was taken between the line conductor and CPC linked at the board. Dedicated polarity checks using a voltage indicator or approved test lamp are also carried out during live testing.

What it confirms: That the earth fault loop impedance is low enough for the protective device to disconnect the supply within the required time in the event of a fault to earth.

Ze (external earth fault loop impedance): Measured at the origin of the installation with the main earthing conductor disconnected from the MET. This gives the impedance of the supply transformer winding, the line conductor from the supply, and the return path via earth. Typical Ze values for UK supplies: TN-C-S (PME) = 0.35 ohm max, TN-S = 0.8 ohm max, TT = varies widely (can be 20 ohm or more).

Zs (total earth fault loop impedance): Measured at each point of utilisation (socket, light fitting, etc). Zs = Ze + R1 + R2 for that circuit. The measured Zs must not exceed the maximum permissible value for the protective device type and rating. These values are found in BS 7671 Tables 41.2 to 41.6.

Note: These are the maximum values at the operating temperature of the conductor. When testing on a cold installation, apply the 0.8 correction factor to the tabulated value to get the maximum measured Zs. For example, a 32 A Type B MCB has a tabulated Zs of 1.44 ohm, so the maximum measured Zs on a cold circuit should not exceed 1.44 x 0.8 = 1.15 ohm.

What it confirms: That residual current devices operate within the required time at their rated residual operating current, and do not trip at 50% of rated current (to confirm discrimination and avoid nuisance tripping).

How it works: An RCD tester injects a known current between line and earth on the load side of the RCD and measures the time taken for the device to trip.

What it confirms: That the prospective fault current at every relevant point does not exceed the rated short circuit capacity of the protective devices installed.

How it works: PFC is measured at the origin of the installation using a loop impedance tester. Both line-to-neutral (Ipf) and line-to-earth prospective fault currents are measured. The higher of the two values is the one that matters. This must be recorded on the certificate.

What it confirms: That assemblies, switchgear, controls, interlocks and other devices operate correctly and are properly mounted and adjusted.

This includes operating RCD test buttons, checking isolator switches function correctly, verifying that circuit breakers can be manually tripped and reset, and confirming that any interlocking or control systems work as intended. While not a measured test with pass/fail values, functional testing is a BS 7671 requirement and must be recorded.

Every result above needs to be accurately recorded against the correct circuit on the correct certificate. On paper, that means columns of handwritten values that are easy to misread, easy to miss, and impossible to validate until someone reviews them manually.

Electrical testing software removes the transcription step entirely. An electrical testing app like iCertifi records R1+R2, insulation resistance, Zs, RCD trip times and all other results directly against each circuit on site. It checks Zs values against the maximum permissible for the protective device. It flags missing results. It generates the complete certificate as a PDF without any office admin.

iCertifi also includes Test Vision, which photographs your test meter display and populates the certificate fields automatically – regardless of whether you use a Megger, Fluke, Kewtech or Metrel instrument. That eliminates manual entry for continuity testing, insulation resistance testing, earth fault loop impedance testing and RCD testing entirely.

Record Every Test Result in One App

Continuity, insulation resistance, Zs, RCD, PFC – all on one certificate, checked automatically.

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