Three-phase testing is single-phase testing done three times, plus a phase sequence check. The line-to-earth voltage Uo is still 230 V, so the maximum Zs values in Table 41.3 are the same as on a domestic board, and a 32 A Type B device is still limited to 1.37 Ω. What changes is the number of tests, the 400 V between lines that your leads and habits need to respect, the phase rotation requirement in Regulation 643.9, and a schedule of test results that has to make sense across three lines. This guide takes an electrician who is comfortable with domestic EICRs through what to do differently at a three-phase board.
- Uo is 230 V line to earth on a 400/230 V system, so the Table 41.3 maximum Zs values are unchanged from single-phase.
- Regulation 643.9 requires phase sequence to be verified on multiphase circuits; check it at the origin and at every three-phase outlet.
- Insulation resistance on a three-phase circuit with neutral is up to ten pairs at 500 V DC; minimum 1 MΩ, and linked lives to earth is acceptable.
- Measure Zs on each line and record the highest; measure PFC line-to-line and line-to-neutral and record the highest.
- Neutral current is not zero on an unbalanced or harmonic-rich load, so treat the neutral as live and check its terminations.
- Pre-2006 installations use red, yellow, blue and black; mixed colours need a caution notice under Regulation 514.14.1.
What is different about a three-phase supply?
A three-phase supply provides three lines, L1, L2 and L3, each at 230 V to neutral and 400 V to one another, with the waveforms displaced by 120°. Single-phase loads are connected between one line and neutral, three-phase loads such as motors, large heaters and lifts are connected across all three lines. The main advantages are three times the power through a similar cable and the rotating magnetic field that makes induction motors simple. For the inspector the practical differences are the higher voltage between lines, the need for correct rotation, and a neutral that may carry significant current when the phases are not balanced.
The supply arrangement at the origin is typically a three-phase cut-out with three fuses, a three-phase meter, then a main switch or switch-fuse feeding a distribution board with the three bus bars. Domestic three-phase supplies are becoming common with heat pumps and EV chargers, and the DNO will typically provide 3 × 60 A, 3 × 80 A or 3 × 100 A.
| Quantity | Single-phase domestic | Three-phase 400/230 V |
|---|---|---|
| Line to neutral voltage (Uo) | 230 V | 230 V |
| Line to line voltage | n/a | 400 V |
| Max Zs, 32 A Type B (Table 41.3) | 1.37 Ω | 1.37 Ω |
| Max Zs, 63 A Type C | 0.35 Ω | 0.35 Ω |
| Max Ze, TN-C-S (typical DNO declared) | 0.35 Ω | 0.35 Ω |
| Insulation resistance test voltage | 500 V DC | 500 V DC |
| Phase sequence test | n/a | Required, Regulation 643.9 |
How do I test phase sequence?
Regulation 643.9 requires verification that the phase sequence is maintained for multiphase circuits. In practice that means checking at the origin, at each distribution board, and at every three-phase socket-outlet or motor isolator that L1-L2-L3 gives the same rotation throughout. Connect a phase rotation tester to the three lines; the indicator shows clockwise for L1-L2-L3 in order, which is the expected direction. Many MFTs include a rotation function, and some three-phase socket testers show it directly.
The consequence of getting it wrong is a motor that runs backwards. On a fan that is a nuisance; on a pump, a compressor or a lift it can be a damaging or dangerous fault. If a three-phase outlet shows reverse rotation it usually means two lines have been swapped at some point in the circuit, which is also worth finding because it tells you the identification is wrong somewhere. Record the result on the schedule of test results in the phase sequence column; the current model schedule has a column for it since the 18th Edition brought the requirement in.
Before any of this, confirm the supply voltages: each line to neutral should read about 230 V and each pair of lines about 400 V. A line reading 230 V to the other two lines and zero to neutral usually means a lost line at the cut-out or a blown DNO fuse, which is a call to the DNO before any further work.
How do I do insulation resistance on three-phase circuits?
The principle is unchanged from single-phase: isolate, confirm dead, disconnect or switch off anything vulnerable such as drives, controllers, surge protective devices and electronic ballasts, and test at 500 V DC for circuits up to 500 V nominal. The number of tests increases. For a circuit with three lines and a neutral, the full set is L1-L2, L1-L3, L2-L3, L1-N, L2-N, L3-N, then L1-E, L2-E, L3-E and N-E. Guidance Note 3 accepts linking all live conductors together and testing to earth as a single test, which is what most of us do on a large board, so the routine becomes six between-live tests and one linked-lives-to-earth test per circuit. See Insulation Resistance Testing Guide: Voltages, Minimums, Method for the general method.
The minimum acceptable insulation resistance from Table 64 is 1 MΩ for circuits up to 500 V, but Guidance Note 3 recommends further investigation below 2 MΩ. On a three-phase motor circuit, the motor windings are normally disconnected at the isolator or the contactor before testing the wiring, then the motor is tested separately if required, because windings will read well below the wiring and the reading tells you about the machine rather than the cable.
A practical warning about variable speed drives: a 500 V insulation test into a VSD or a soft starter can destroy it, and a three-phase board in a workshop or pump room often has several. Isolate and disconnect at the drive input, or omit that circuit's IR test and record the limitation, rather than press the button and hope.
How do I measure Zs and PFC on each phase?
Fault protection on a three-phase circuit is a line-to-earth fault at Uo of 230 V, so the required disconnection time and maximum Zs are exactly those for a single-phase circuit with the same protective device. The difference is that a three-phase circuit has three lines and the impedance of each must be checked. Measure Zs at the furthest point between each line and earth, and record the highest of the three as the circuit value. A large spread between the lines points to a poor termination on one line, so investigate rather than average. Maximum values are in Maximum Zs Values BS 7671: Tables, 80% Rule and Ze; for the loop test itself see Earth Fault Loop Impedance Testing: Ze, Zs and Ipf Explained.
At the origin, measure Ze on each line to earth with the earthing conductor disconnected and record the highest. Measure PFC between each pair of lines and between each line and neutral. The line-to-line prospective short-circuit current is about √3 times the line-to-neutral value, because the driving voltage is 400 V rather than 230 V, and it is the highest value that must be below the rated breaking capacity of every device on the board under Regulation 434.5.1. A 10 kA rated MCCB at the origin feeding a 6 kA board on a supply with 8 kA line-to-line PFC is a common finding in older commercial boards and is normally a C2 on an EICR unless the upstream device provides back-up protection confirmed by the manufacturer. See What is prospective fault current (PFC)? Explained.
For RCD testing, most three-phase RCDs and RCBOs are tested on each line individually, because the tester derives its test current from one line to neutral. Test each line at rated current and record the longest time. The 40 ms limit at 5 × IΔn for a 30 mA device and the 300 ms limit at IΔn apply as usual. A three-phase RCD with no neutral connection cannot be tested with a standard MFT and needs a tester that injects between lines.
What about the neutral and conductor identification?
On a balanced three-phase load the neutral current is zero, which is why single-phase electricians sometimes forget about it. Real loads are not balanced: a board with a run of lighting on L1, a kitchen on L2 and offices on L3 will have neutral current equal to the imbalance, and harmonic currents from LED drivers, computer power supplies and drives add together in the neutral rather than cancelling. Regulation 523.6.3 and Appendix 4 section 5.5 deal with derating cables for triple-N harmonics, and a neutral running warm at a busy commercial board is an observation worth making. Check neutral terminations with the same care as the lines, and never disconnect a shared neutral under load.
Conductor identification is the other trap. Installations from before the colour harmonisation, completed between 2004 and 2006, use red, yellow and blue lines with a black neutral. Current colours are brown (L1), black (L2), grey (L3) and blue neutral. A black conductor is therefore a neutral in an old installation and L2 in a new one, and a blue conductor is L3 in an old installation and neutral in a new one. Where an installation contains both, Regulation 514.14.1 requires a caution notice at the distribution board warning that two colour schemes are present. On an EICR, mixed colours without the notice is normally a C3; a black conductor connected as a line with no identification sleeve where it enters a modern accessory is worth a C2 if someone could reasonably treat it as neutral. See Polarity Testing Explained: Checks, Methods and EICR Codes for the related polarity checks.
How do I record it on the schedule?
The model schedule of test results is the same form for single-phase and three-phase circuits, with columns for the number of points, cable csa, reference method, device type and rating, R1+R2 or R2, insulation resistance between lives and to earth, polarity, Zs, RCD times and phase sequence. Guidance Note 3 and the scheme assessors expect one line per circuit with the worst-case value where a circuit has three lines, or three lines on the schedule where the values need to be shown separately, with the line identified. What matters is that a later inspector can see which line a value belongs to. Record the highest Zs, the lowest insulation resistance, the longest RCD time, and note the phase sequence as satisfactory or otherwise. For the general form see How to fill in a schedule of test results, column by column.
Supply characteristics need the number of phases, the nominal voltages 400/230 V, and PFC and Ze per line or the highest. Under earthing arrangements, a three-phase TN-C-S is common on commercial supplies but TN-S is still widespread in older premises, and a TT commercial supply is a different testing regime again.
On safety: 400 V between lines is enough to cause a serious arc flash from a dropped probe across a bus bar. Use GS38-compliant fused leads with finger guards and shrouded probes, test leads and meter rated CAT III 600 V or better, and prove your voltage indicator on a proving unit before and after each dead test. Keep the board cover on while live testing wherever the board design permits, and work from the line you are testing towards the meter, not across the bus bars.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £5 a month.