Extra-low voltage under BS 7671:2018+A4:2026 means not more than 50 V AC or 120 V ripple-free DC, but the letters in front of "ELV" decide how much protection the circuit needs. A SELV circuit fed from a safety isolating transformer to BS EN 61558-2-6 needs no fault protection at all, and below 25 V AC in dry conditions needs no basic protection either. A FELV circuit fed from an ordinary transformer gets no such relief and must be earthed and protected like the 230 V circuit supplying it. This guide sets out the three types, the limits in bathroom and pool zones, how to test ELV circuits and what to record on the certificate.
- ELV is up to 50 V AC rms or 120 V ripple-free DC. SELV and PELV are protective measures in Section 414; FELV is covered by Regulation 411.7 and is not a protective measure in its own right.
- A SELV source must be a safety isolating transformer to BS EN 61558-2-6, a battery, a motor-generator or an electronic device that cannot deliver more than ELV even under an internal fault. Most branded LED drivers qualify; autotransformers and unmarked supplies never do.
- SELV is separated from earth and from every other circuit. PELV may be earthed. FELV has no separation, so exposed metalwork must be connected to the main earthing terminal.
- Basic protection can be omitted in dry conditions for SELV up to 25 V AC or 60 V DC, and for PELV at the same voltages where exposed parts are earthed. Above that you need insulation or IPXXB enclosures.
- Insulation resistance on SELV and PELV is tested at 250 V DC with a minimum of 0.5 MΩ. Separation from LV circuits is tested at 500 V DC with a minimum of 1 MΩ.
- Bathroom and pool zone 0 allow SELV only, at 12 V AC or 30 V DC maximum, with the source outside the zones.
What do SELV, PELV and FELV actually mean?
The definitions in Part 2 of BS 7671 are short but the consequences are large. Separated extra-low voltage (SELV) is an ELV circuit whose live parts are electrically separated from earth and from all other circuits by at least double or reinforced insulation, and whose exposed-conductive-parts are not deliberately earthed. Protective extra-low voltage (PELV) is identical except that the circuit, or its exposed-conductive-parts, may be connected to earth. Functional extra-low voltage (FELV) is any ELV circuit that fails to meet the separation or source requirements of SELV or PELV, typically because it is fed from an autotransformer, a non-safety transformer or an electronic supply without a safety rating.
| Type | Regulation | Source | Separated from earth? | Fault protection needed? | Typical example |
|---|---|---|---|---|---|
| SELV | 414 | Safety isolating transformer, battery or equivalent | Yes, and from all other circuits | No | 12 V pond lighting, LED strip on a marked driver, bell transformer to BS EN 61558-2-8 |
| PELV | 414 | As SELV | May be earthed at one point | No | Control circuits in switchgear, some door-entry and BMS supplies |
| FELV | 411.7 | Any transformer or supply without safety separation | No | Yes, as for the primary circuit | 12 V halogen on an autotransformer, unmarked "power supply" bricks |
The distinction matters because Section 414 treats SELV and PELV as complete protective measures: both basic protection and fault protection are provided by the extra-low voltage and the separation. FELV provides neither, so Regulation 411.7 sends you back to Chapter 41 to earth the exposed-conductive-parts of the FELV equipment to the protective conductor of the primary circuit and to rely on the primary circuit's automatic disconnection.
What counts as a SELV source?
Regulation 414.3 lists the permitted sources. The one you will meet most is a safety isolating transformer complying with BS EN 61558-2-6, which is built with double or reinforced insulation between the windings and marked with the double-square symbol and the transformer-with-line "safety isolating" symbol. The others are a source of current providing an equivalent degree of safety, an electrochemical source such as a battery, a motor-generator with equivalent separation, and an electronic device where the output cannot exceed ELV even under an internal fault.
That last category is where LED drivers and switch-mode supplies sit. A driver marked SELV and made to BS EN 61347-2-13, or a supply to BS EN 61558-2-16, satisfies 414.3. A driver that carries no SELV marking, or a 12 V halogen transformer that turns out to be an autotransformer, does not. On an EICR the quickest check is the marking on the unit; if you cannot find a safety isolating or SELV mark, treat the secondary circuit as FELV and inspect it on that basis.
Bell transformers to BS EN 61558-2-8 are safety isolating types, so the bell circuit is SELV; older units and the modules bundled with some chimes may not be, and if the housing gives no standard, note it as a limitation rather than assuming SELV.
How far does the separation have to go?
Regulation 414.4 is where installers slip. It is not enough for the transformer to be a safety type; the wiring on the secondary side must keep the separation. Regulation 414.4.2 requires SELV and PELV conductors to be either in a separate wiring system from other circuits, or insulated for the highest voltage present when they share a cable or containment, or separated by an earthed metallic screen or sheath. A 12 V circuit run in the same trunking as 230 V lighting using ordinary 0.5 mm² flexible cable is not compliant unless that flex is rated for 230 V, which most "speaker cable" and "LED wire" is not.
Regulation 414.4.1 also requires that plugs and socket-outlets for SELV and PELV systems are not interchangeable with those of other systems in the same premises, and that SELV plugs have no protective conductor contact. Fitting a 13 A socket on a 12 V supply for a pond pump fails this outright, and the reverse, a 12 V connector that could accept a 230 V plug, is just as bad.
For SELV, and this is the point most often forgotten, live parts must not be connected to earth or to any protective conductor, and exposed-conductive-parts must not be intentionally connected to earth, to a protective conductor of another system, or to the exposed-conductive-parts of another system. If a metal luminaire on a SELV circuit is also bolted to earthed metalwork, the circuit is no longer SELV; it may still be PELV if the source and separation are otherwise fine, and PELV is acceptable in most places outside bathroom and pool zone 0.
When can basic protection be left off?
This is the practical payoff of SELV. Regulation 414.4.5 states that basic protection is generally not required in normal dry conditions for a SELV circuit where the nominal voltage does not exceed 25 V AC rms or 60 V ripple-free DC, and for a PELV circuit at the same limits where exposed-conductive-parts or live parts are connected by a protective conductor to the main earthing terminal. That is why bare-terminal 12 V garden lighting connectors and open LED strip are permissible indoors.
Above those voltages, or in wet or conductive locations, basic protection by insulation capable of withstanding a 500 V AC test for 60 seconds, or by barriers and enclosures to at least IPXXB or IP2X, is required. Guidance Note 3 from the IET is clear that the dry-condition exemption should not be stretched to cover outdoor equipment; a 24 V DC garden supply in an IP44 box still needs its terminals covered.
| Location | SELV limit | PELV allowed? | Basic protection needed? |
|---|---|---|---|
| Dry indoor, general | 50 V AC / 120 V DC | Yes | Not below 25 V AC / 60 V DC |
| Bathroom zone 0 (701) | 12 V AC / 30 V DC | No | Yes, IPX7 equipment |
| Bathroom zone 1 (701) | 25 V AC / 60 V DC | Yes | Yes |
| Pool zone 0 (702) | 12 V AC / 30 V DC | No | Yes, IPX8 equipment |
| Pool zone 1 (702) | 25 V AC / 60 V DC | No | Yes |
| Pool zone 2 (702) | 50 V AC / 120 V DC | Yes | Yes |
The source for a SELV circuit serving zone 0 or 1 of a bathroom or pool must itself be outside zones 0, 1 and 2, and Section 702 prohibits FELV in zones 0 and 1 altogether. See Bathroom Zones Explained: Section 701 of BS 7671 and Hot Tub and Swimming Pool Wiring: Section 702 Explained for the full zone rules.
How do you test an ELV circuit?
Insulation resistance is the main test and the test voltage is lower than for a 230 V circuit. Table 64.1 of BS 7671 sets 250 V DC as the test voltage for SELV and PELV circuits with a minimum acceptable reading of 0.5 MΩ; FELV circuits fall in the "up to and including 500 V" band and are tested at 500 V DC with a minimum of 1.0 MΩ. Disconnect LED drivers, electronic transformers and any control gear before testing, both to protect them and because their internal circuitry will drag the reading down to a value that means nothing.
Regulation 643.4.1 then requires the separation of a SELV or PELV circuit from other circuits and from earth to be verified. GN3 recommends testing between the SELV secondary conductors, joined together, and the primary conductors and earth, at the test voltage for the higher-voltage circuit present, which for a 230 V primary is 500 V DC with a minimum of 1.0 MΩ. If that reading is low the circuit is not separated and cannot be recorded as SELV. Our general method is in Insulation Resistance Testing Guide: Voltages, Minimums, Method.
Earth fault loop impedance, R1+R2 and RCD tests do not apply to a SELV circuit because there is no protective conductor and no fault path; record them as N/A. For FELV you test the primary circuit as normal, since that provides the fault protection, and check continuity of the protective conductor to the FELV equipment's exposed metalwork.
Overcurrent protection is not waived by Section 414. The secondary conductors must be protected against overload and short circuit under Chapter 43 unless the transformer is marked as inherently short-circuit proof or non-inherently short-circuit proof with an internal fuse. A 300 W 12 V transformer can push 25 A into a 0.75 mm² flex, and the resulting melted cable is a common find in loft spaces feeding old halogen downlights.
How do you record ELV on the certificate?
On the schedule of test results, list the ELV circuit in its own row, giving the circuit designation, the type of wiring, the conductor size, and the source, for example "Circuit 6: 12 V AC SELV garden lighting via BS EN 61558-2-6 transformer in garage". Enter the insulation resistance with the test voltage used, and put N/A against Zs, R1+R2 and RCD columns for SELV and PELV. For FELV the row shows the primary circuit's protective device and its Zs, because that device is what disconnects the fault. See How to fill in a schedule of test results, column by column for the layout.
On an EICR the typical observations, using the code guidance from Electrical Safety First's Best Practice Guide 4, are these. A 230 V LED driver or electronic transformer mounted inside bathroom zone 1 or 2 without the required IP rating is C2. FELV lighting with unearthed metal fittings and no protective conductor is C2, because fault protection is absent. A 13 A socket-outlet used on a 12 V supply is C2 if a 230 V appliance could be plugged in and damaged or a 12 V load plugged into 230 V, and C3 where interchangeability is theoretical. An unmarked transformer where SELV cannot be confirmed is best recorded as a limitation with an FI where the equipment is in a special location. Our observation-writing guide at How to Write EICR Observations That Stand Up has the wording patterns.
One last point: a shaver supply unit to BS EN 61558-2-5 is a safety isolating transformer but its output is 230 V or 115 V, so it is not SELV and should not be recorded as an ELV circuit.
How Certio helps
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