Ask a 2391-52 candidate to list the tests and most can. Ask them to list them in order, and the failure rate climbs sharply. Ask them why that order, and it climbs again.
The sequence is set by Regulation 643.1, and it is not arbitrary. Every test either depends on the result of the one before it, or would be unsafe without it.
Dead tests, before energising
- Continuity of protective conductors (Reg 643.2)
- Insulation resistance (Reg 643.3)
- Protection by SELV, PELV or electrical separation, where used (Reg 643.4)
- Polarity (Reg 643.6)
Live tests, after safe energising
- Earth fault loop impedance (Reg 643.7)
- Prospective fault current (Reg 643.7.3.201)
- RCD operation (Reg 643.8)
- Phase sequence, on polyphase circuits (Reg 643.9)
- Functional testing (Reg 643.10)
Why continuity comes first
Every test after it assumes the protective conductors are actually connected. Loop impedance readings, RCD operating times, the whole disconnection-time argument — all of it rests on there being a continuous, low-resistance path from every exposed-conductive-part back to the main earthing terminal.
Prove that first, and the rest of your results mean something. Skip it, and you may spend an afternoon recording impressive numbers about a circuit with no earth.
There is a practical point here too: null your test leads. A low-resistance ohmmeter reading of 0.05 Ω is worthless if 0.04 Ω of it is the leads.
Why insulation resistance comes before polarity
A circuit with a breakdown in insulation should be found before anyone is thinking about energising. Insulation resistance is the test most likely to reveal a fault that would be dangerous to energise into — and it is done dead, at a test voltage that will reveal deterioration a visual inspection would not.
| Circuit nominal voltage | Test voltage (DC) | Minimum insulation resistance |
|---|---|---|
| SELV and PELV | 250 V | 0.5 MΩ |
| Up to and including 500 V | 500 V | 1.0 MΩ |
| Above 500 V | 1000 V | 1.0 MΩ |
BS 7671 Table 64 / Reg 643.3.2.
Learn those as rows, not as six separate numbers. The classic exam trap pairs the SELV test voltage with the low-voltage minimum resistance, or the reverse. A 230 V single-phase circuit falls in the “up to and including 500 V” band: 500 V DC test, 1.0 MΩ minimum.
Before testing, disconnect or protect anything that could be damaged by the test voltage or that would give a misleadingly low reading — electronic dimmers, SPDs, RCDs with electronic components, and connected appliances.
Why polarity is tested dead, then again live
Polarity confirms that single-pole devices are in the line conductor only, that centre-contact lampholders have their centre contact connected to line, and that socket-outlets are correctly wired.
Consider the classic failure: a single-pole protective device installed in the neutral. Switch it off and the circuit looks isolated. It is not — the line conductor is still live, and anyone working on that circuit is working live while believing they are not.
Finding that dead costs you five minutes. Finding it live costs considerably more.
Polarity is then confirmed again once the installation is energised, because the live confirmation catches things the dead test cannot — a supply-side issue, for example.
Why the live tests are in that order
Loop impedance before RCD testing is the important one. Zs tells you whether enough current will flow during an earth fault to operate the protective device in the required time.
| System | Final circuits up to 63 A | Distribution circuits and final circuits over 63 A |
|---|---|---|
| TN | 0.4 s | 5 s |
| TT | 0.2 s | 1 s |
BS 7671 Reg 411.3.2.2/.3, Table 41.1, at U₀ 230 V.
RCDs provide additional protection. They are a backstop, not the primary means of disconnection. Establishing that the primary protection works, and only then confirming the backstop, is the right order — and it reflects how the regulations are structured.
Functional testing comes last for the obvious reason: it is the point at which the installation is checked as a working whole, rather than as a collection of individual measurements. Assemblies, interlocks, controls and switching devices are operated to confirm they are properly mounted, adjusted and installed.
The reordering mistakes that cost marks
- Polarity before insulation resistance. The most common wrong answer.
- RCD test before loop impedance. Backstop before primary protection.
- Prospective fault current before loop impedance. PFC is measured to confirm devices can safely interrupt the fault current available; it belongs with the loop impedance work, after it.
- Treating the integral RCD test button as the RCD test. The button proves the mechanism operates. It proves nothing about the operating time, and nothing about whether the earthing is adequate. An instrument test is required as well.
Practise it
The test procedures study guide covers each test in detail, including the three-step ring final circuit method. The dead and live test sequence reference cards give you the ordered lists with the regulation numbers, and there are free test procedures practice questions when you want to check it has stuck.
