Initial Verification: The BS 7671 Testing Sequence
Initial verification follows a fixed order for a reason: dead tests first to prove the installation is safe to energise, then live tests to prove it works — get the sequence wrong and you can either miss a fault or create a danger.
Before a new installation, addition or alteration is energised and certified, BS 7671 requires it to be inspected and tested. The tests are not done in a random order — there is a defined sequence, and it exists to keep you and the installation safe as much as to produce the numbers on the certificate.
This guide walks through the initial verification sequence as set out in BS 7671 and the IET On-Site Guide: the inspection, the dead tests in order, and the live tests in order, with the reasoning for why each step comes where it does.
Key takeaways
- Inspection comes first, then dead (de-energised) tests, then live (energised) tests.
- Dead tests: continuity of protective conductors and bonding, ring final continuity, insulation resistance, polarity.
- Do not energise until the dead tests confirm it is safe to do so.
- Live tests: polarity, earth fault loop impedance (Ze then Zs), prospective fault current, RCD operation.
- Recording earth-fault disconnection: 0.4s for most final circuits, 5s for distribution circuits.
Inspection before any testing
Initial verification starts with a visual inspection, not a meter. You confirm the installation has been constructed and installed correctly — correct devices, correct conductors, secure connections, appropriate protection against electric shock, thermal effects and the correct external influences — before you test anything.
The inspection catches the things a test never will: a device of the wrong rating, an accessory installed in the wrong location, a missing barrier. Only once the installation looks right do you begin the dead tests.
Dead tests, in order
With the installation isolated and proven dead, the dead tests are carried out in this order: (1) continuity of protective conductors, including main and supplementary bonding; (2) continuity of ring final circuit conductors — the three-step r1, rn, r2 test; (3) insulation resistance; and (4) polarity by continuity.
The order matters. You prove the protective conductors are continuous before anything else, because they are what keeps people safe once the power is on. Insulation resistance comes before energising because a low reading tells you the installation is not fit to be made live. Polarity is confirmed dead first so that no wrong-polarity circuit is ever energised.
Prove dead properly
Isolate, lock off, and prove your voltage indicator on a known live source before and after testing. Dead testing on a circuit that isn't actually dead is how people get hurt.
Insulation resistance — what good looks like
Insulation resistance is tested between live conductors and between live conductors and earth, typically at 500V DC for a 230/400V installation. The minimum acceptable value for a low-voltage circuit is 1MΩ, though a healthy new installation should read far higher — into the hundreds of megohms or off the top of the scale.
Test with electronic equipment, dimmers and similar disconnected or with live and neutral linked to avoid damage and false readings. A reading that meets the 1MΩ minimum but is markedly lower than the rest of the installation is worth investigating even though it technically passes.
Live tests, in order
Once the dead tests confirm it is safe, energise and carry out the live tests in order: (1) confirm polarity live; (2) measure the external earth fault loop impedance Ze at the origin; (3) measure prospective fault current (PFC) at the origin; (4) measure earth fault loop impedance Zs at each circuit; and (5) test RCD operation.
Ze and PFC come first because they characterise the supply and everything downstream depends on them. Zs at each circuit confirms disconnection times will be met. RCD testing comes last of the live tests, confirming each device trips within its required time at its rated current — 300ms at rated current, and within 40ms at 5× rated current for a 30mA device.
Loop once, read it twice
The Zs and PFC measurements come from the same loop test. Record the loop impedance and read it both ways — Zs against disconnection limits, PFC against device breaking capacity.
Disconnection times and the certificate
The whole sequence exists to prove two things: that the installation is safe under fault, and that faults disconnect fast enough. For final circuits up to 63A supplying socket-outlets or up to 32A supplying fixed equipment on a TN system, the required earth-fault disconnection time is 0.4 seconds; for distribution circuits and larger final circuits it is 5 seconds. On TT systems the times are shorter and RCDs carry the fault protection.
Every measured value — R1+R2, insulation resistance, Ze, Zs, PFC and RCD times — is recorded on the schedule of test results that accompanies the Electrical Installation Certificate. The certificate is only valid once the full sequence has been completed and the results recorded.
Frequently asked questions
Why are dead tests done before live tests?
Dead tests prove the installation is safe to energise — that protective conductors are continuous, insulation is sound and polarity is correct. Energising before those are confirmed risks creating a danger. Only once the dead tests pass do you make the installation live.
What is the minimum insulation resistance?
For a low-voltage circuit tested at 500V DC, the minimum acceptable insulation resistance is 1MΩ. A healthy new installation should read far higher; a value close to the minimum warrants investigation even though it passes.
What disconnection times apply?
On a TN system, 0.4 seconds for most final circuits up to 63A, and 5 seconds for distribution circuits. TT systems require faster times met by RCDs. Zs is measured at each circuit to confirm the relevant time is achieved.
From guidance to action
Related guides
How to Test R1+R2 on a Ring Main
The three-step ring final continuity test, cross-connections and the readings that prove it.
Max Zs Values Table (BS 7671 18th Edition)
Full maximum Zs tables for Type B/C/D MCBs and RCBOs, the Cmin formula and the 80% rule.
What Is Prospective Fault Current (PFC)?
PSCC vs PEFC, how to measure PFC, and why breaking capacity has to exceed it.
