Test Procedures

The sequence, and why it is a sequence

Regulation 643.1 sets the order in which tests are carried out. It is not arbitrary: each test either depends on the result of the one before it, or would be unsafe without it.

Dead tests, before energising

1. Continuity of protective conductors (Reg 643.2)
2. Insulation resistance (Reg 643.3)
3. Protection by SELV, PELV or electrical separation, where used (Reg 643.4)
4. Polarity (Reg 643.6)

Live tests, after safe energising

5. Earth fault loop impedance (Reg 643.7)
6. Prospective fault current (Reg 643.7.3.201)
7. RCD operation (Reg 643.8)
8. Phase sequence, on polyphase circuits (Reg 643.9)
9. Functional testing (Reg 643.10)

Continuity comes first because everything downstream assumes the protective conductors are actually connected. Insulation resistance comes before polarity because a circuit with a breakdown in insulation should be found before anyone is thinking about energising it.

Continuity of protective conductors

The purpose is to prove there is an unbroken, low-resistance path from every exposed-conductive-part back to the main earthing terminal.

There are two accepted methods:

• Method 1 (R1+R2) — link the line conductor and CPC together at the board, then measure between line and CPC at each point on the circuit. The reading is that point’s R1+R2, which is exactly what you record on the Schedule of Test Results.
• Method 2 (wander lead) — measure from the main earthing terminal to each exposed-conductive-part with a long test lead, subtracting the lead’s own resistance.

Method 1 is generally preferred because it gives you the R1+R2 value directly rather than as a separate calculation.

Before either, null the test leads — a low-resistance ohmmeter reading of 0.05 Ω means nothing if 0.04 Ω of it is the leads.

Ring final circuit continuity

The three-step figure-of-eight test proves the ring is a ring: continuous, not broken, and not interconnected.

Step 1 — end-to-end readings. With both legs disconnected from the board, measure each conductor end to end: r1 (line), rn (neutral), r2 (CPC). On a ring wired in the same cable throughout, r1 and rn should be within a few percent of each other. A larger difference means a broken ring, an interconnection, or a spur mistaken for part of the ring. r2 is normally higher, because the CPC is usually a reduced size.

Step 2 — line to neutral. Cross-connect line of one leg to neutral of the other and vice versa, then measure line to neutral at each socket. Every reading should be substantially the same, at about (r1 + rn) / 4.

Step 3 — line to CPC. Repeat with line and CPC cross-connected. Again every reading should be substantially the same, at (r1 + r2) / 4 — and that figure is the circuit’s R1+R2.

A reading noticeably higher at one point indicates a spur. Readings that rise and fall as you move round the ring indicate the ring is not continuous.

Work it out with the ring final calculator →

Insulation resistance

BS 7671 Table 64 / Reg 643.3.2.

Before testing, disconnect or otherwise protect equipment that could be damaged by the test voltage, and anything that would give a misleading low reading — electronic dimmers, SPDs, RCDs with electronic components, and connected appliances.

A reading that just scrapes past 1.0 MΩ is not a comfortable pass. New installations typically read far higher; a value close to the minimum on an existing circuit indicates insulation that has deteriorated and is worth recording as an observation.

Polarity

Polarity confirms that:

• single-pole devices — switches, protective devices, fuses — are in the line conductor only
• centre-contact lampholders have their centre contact connected to line
• socket-outlets are correctly wired

It is tested dead as part of the continuity work, and confirmed again once the installation is live.

A single-pole protective device found in the neutral is a classic examination scenario: switching it off leaves the circuit’s line conductor still live while appearing to be isolated.

Earth fault loop impedance and prospective fault current

Zs is the total impedance of the earth fault loop for a circuit: the supply’s external impedance (Ze) plus the circuit’s own R1+R2. It determines whether enough current will flow during an earth fault to operate the protective device in the required time.

BS 7671 Reg 411.3.2.2/.3, Table 41.1, at U₀ 230 V.

Measured Zs is normally compared against 0.8 × the tabulated maximum from Table 41.3, which allows for the conductors being warmer in service than when you measured them.

Check a reading with the max Zs calculator →

Prospective fault current is measured at the origin and at any point where a device’s breaking capacity needs confirming. The value recorded is the higher of the prospective short-circuit current and the prospective earth fault current. Its purpose is to confirm every protective device can safely interrupt the largest fault current that could flow through it.

RCD operation

30 mA RCDs are used for additional protection — a backstop against faults, not a substitute for correct earthing and disconnection times.

BS 7671 Reg 415.1.1 / 643.8.

The integral test button on the device proves the mechanism operates. It does not prove the operating time, nor that the earthing is adequate — which is why an instrument test is required as well as the button.

Functional testing

The last step: assemblies, interlocks, controls, isolators and switching devices are operated to confirm they are properly mounted, adjusted and installed, and that they do what they are supposed to do. It is the point at which the installation is checked as a working whole rather than as a set of individual measurements.