Smart Meters, Isolators and Meter Tails on an EICR

Most domestic EICRs now start at a smart meter, and a big share of those intakes have no isolator between the meter and the consumer unit. Neither is a defect in the installation. Regulations 132.15.201 and 462.1.201 of BS 7671:2018+A4:2026 require a main linked switch or circuit-breaker at the origin, and the consumer unit main switch does that job. What the missing isolator changes is how you isolate, what you can measure, and what goes in the limitations. This guide covers that, plus meter tails, cut-out defects, fuse withdrawal and how to code what you find.

Who owns what at the intake?

The intake is three sets of equipment with three owners. The service cable, cut-out, cut-out fuse and distributor's earth terminal are the DNO's. ESQCR 2002 Regulation 24 puts the duty on the distributor to make sure that equipment on the consumer's premises is suitable, protected and safe. The meter belongs to the energy supplier and is installed by its meter operator. Where the supplier has fitted an isolator, that and the short tails from meter to isolator are usually theirs too. Everything after that point is the consumer's installation, and that is what your EICR reports on.

BS 7671 says the same thing from the other side: Regulation 110.2 excludes distributors' equipment from its scope. That is why a cracked cut-out is a phone call to the DNO and a note on the report, not a line on your quotation.

Record on the report that you told the person ordering it to contact the DNO or supplier. The IET's July 2025 Wiring Matters article on removing the cut-out fuse makes that point: defects in the distributor's equipment found during periodic inspection should be reported promptly, in writing, to the person ordering the work, who then notifies the distributor or supplier.

Is a missing isolator a defect?

No. The demand for one is practical, not regulatory. An isolator lets you change a consumer unit or disconnect the main earthing conductor without touching the DNO's fuse, so suppliers fit them at meter exchanges and DNOs encourage them. Nothing in BS 7671 makes a separate isolator mandatory where the consumer unit has a main switch.

On the A4 model form, a supply with no isolator belongs in Section D (extent and limitations), not Section K (observations). Typical wording:

No isolator between meter and consumer unit. Installation not isolated at the origin. Ze measured with the main earthing conductor connected; parallel earth paths present.

It is not a C3, because nothing departs from BS 7671. If you're quoting a board change, the sensible advice is to get the supplier to fit an isolator first. That goes in the recommendations, not as a coded item.

When can you withdraw the cut-out fuse?

Only with permission. The cut-out is the distributor's equipment, and the Electricity Act 1989 limits who can restore a supply. The IET's July 2025 article sets out the current position:

• Historically only the DNO, the supplier's meter operator, or someone the DNO had expressly authorised could withdraw the fuse. BS 7671:2008+A3:2015 said so directly in Regulation 537.1.3: "A cut-out fuse may be withdrawn only by a person authorized to do so by the distributor."
• Some DNOs, SSEN for example, now let electricians who belong to a recognised trade association break the seal, withdraw the fuse and reinstate it. Not every DNO does this, so get your own network operator's procedure in writing.
• Safe Isolation Providers authorised under the Retail Energy Code can withdraw and reinstate the fuse. That permission does not extend to the supplier's meter.

Whatever the permission, the Electricity at Work Regulations 1989 still apply: Regulation 12 (means of cutting off the supply and isolation), 13 (work on equipment made dead), 14 (work on or near live conductors) and 16 (competence). Do not withdraw the fuse if:

• the cut-out is phenolic (roughly 1967 to 1992 vintage), which can track, go conductive or overheat;
• it has a fused neutral, which should not be retained under ESQCR (DNOs must remove these within 28 days of being told);
• the carrier may contain asbestos;
• there are signs of overheating, distress or tampering;
• you are not certain which cut-out feeds the property, for example in flats with shared intakes.

In each case report it to the DNO and plan the EICR without isolation at the origin.

What does a smart meter change for the inspector?

A SMETS2 smart meter has an internal supply contactor that the supplier can open remotely, for example on a prepayment cut-off. It is a load switch under someone else's control. You can't lock it off, prove it, or stop it reclosing, so it is never your means of isolation. It does not meet Section 537 or EAWR Regulation 12. Treat everything upstream of the consumer unit main switch as live.

Smart meters cause a few things that look like faults but aren't:

• Prepayment supplies can drop out mid-test if the credit runs low. Check the meter display before you go chasing a "supply failure".
• Reconnection after the contactor closes is not always instant; some meters need the customer to press a button.
• Auxiliary load control relays on dual-rate and storage-heater supplies switch on a schedule. The click is the meter, not a loose connection.

None of these is an installation defect. If the meter has reported faults or a tampered seal, record it and tell the person ordering the report to contact the supplier.

Are the meter tails the right size?

This is where most of the real intake observations come from. For a 100 A cut-out fuse, the norm is 25 mm² double-insulated single-core tails. Table 4D1A gives 70 °C thermoplastic single-core cables clipped direct (Reference Method C, two cables, single-phase) the following ratings:

Tails grouped in a meter box, run through insulation or bunched in trunking will rate lower than this, so apply the correction factors.

Whether 16 mm² tails are a C2 or C3 depends on what's in front of you: the actual DNO fuse rating (older cut-outs often carry 60 A or 80 A fuses, which you can confirm by asking the DNO, not by pulling the fuse), the maximum demand, and any sign of overheating. ESF Best Practice Guide 4 (Issue 7.4, August 2026) leaves this to the inspector's judgement. A reasonable approach:

• C3: tails cool, undamaged, fuse 80 A or less or a modest demand.
• C2: discolouration or heat damage, or a heavy load (electric shower plus EV charger, heat pump) on a confirmed 100 A fuse.

How long can meter tails be?

The 3 m figure is often called a DNO rule, but it starts in BS 7671. Regulation 433.2.2 lets the overload device sit along the run rather than at the point where the cross-section reduces, and Regulation 434.2.1 allows the same for fault protection up to 3 m, provided the conductor is installed to keep the risk of fault and fire to a minimum and isn't near combustible materials. Voltimum's guidance on maximum meter tail length says the same thing and adds the practical point: the DNO may impose its own maximum for tails protected by its cut-out fuse.

On an EICR:

• Tails over 3 m with no switch-fuse near the meter: usually C3, or C2 if the run is unprotected, damaged or in a position where a fault is likely.
• Tails run without mechanical protection across a garage or cellar wall: C3, or C2 if the sheath is damaged.
• A switch-fuse after the meter feeding long tails is good practice. Record its rating on the form.

How do you measure Ze and PFC without isolation?

Ze is measured at the origin with the main earthing conductor disconnected, so parallel paths through bonded pipework don't flatter the reading. You can't safely disconnect it without isolating the installation. GN3 accepts a reading taken with the earthing conductor connected, as long as it is recorded that way and the limitation is stated.

That reading is the same as or lower than the true Ze, which is the unsafe direction for the disconnection-time check. If it is close to the DNO's typical maximum, flag it:

PFC is measured live at the origin, line to neutral (PSCC) and line to earth (PEFC), and the higher value is recorded as Ipf. Isolation doesn't affect it. A typical domestic PME reading is 1 to 3 kA. The DNO's declared maximum for a single-phase supply is normally 16 kA. That is a design figure, not something you should expect to measure. Domestic consumer units to BS EN 61439-3 with 6 kA devices rely on a 16 kA conditional rating when fed through the DNO's 100 A fuse, so 6 kA breakers on a 2 kA supply aren't a problem. The A4 EICR lets you record Ipf by enquiry or measurement, and asks for Zdb and Ipf at each board.

See Ze explained and Prospective fault current (PFC) explained.

How do you code intake defects on an EICR?

This table follows ESF Best Practice Guide 4 Issue 7.4 and the IET Wiring Matters guidance. As always, the code is the inspector's decision for the installation in front of them.

That last row matters more than it looks. A Ze of 0.2 to 0.4 Ω doesn't tell you whether you have a PME or TN-S earth. Look for the PME label, check the cable and the cut-out, and if you're still not sure, ask the DNO. The A4 form now has a separate PNB tick box, so get the arrangement right. See earthing arrangements TN-S, TN-C-S and TT explained.

For the wider coding framework see EICR observation codes: C1, C2, C3 and FI, and for the overall process see how to do an EICR step by step.

What should go in the limitations section?

Be specific. "Supply not isolated" on its own tells the next inspector nothing. A good intake limitation covers:

1. Why the installation wasn't isolated at the origin (no isolator, DNO permission not given, cut-out unsuitable).
2. What that stopped you doing (Ze with earthing conductor disconnected, tails insulation resistance).
3. What you did instead (Ze measured with parallel paths, circuits isolated at the consumer unit main switch).
4. Who you reported intake defects to, and when.

Agree the limitations with the person ordering the report before you start, as the A4 EICR Section D expects. See EICR sampling and limitations explained.

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 £4.99 a month, charged when you sign up.

Written by the Certio team. Last checked 31 October 2026.