7.3 Verifying SELV, PELV & Electrical Separation

Three Protective Measures That Share One Test

Ordinary low-voltage circuits are protected by automatic disconnection of supply: a fault produces enough current to operate a device quickly. SELV, PELV and electrical separation work on a completely different principle — they prevent a dangerous shock current from arising in the first place, by keeping the circuit apart from everything else.

Because all three depend on separation, all three are verified the same way: by measuring the insulation resistance across the separation. That is what Regulation 643.4 requires — the separation of the live parts from those of other circuits and from Earth is verified by a measurement of insulation resistance, with the values taken from Table 64.

The source matters as much as the wiring. SELV and PELV must come from a safety isolating transformer to BS EN 61558-2-6 (or BS EN 61558-2-8), or an equivalent source such as a motor generator, a battery, or an electronic supply meeting the same requirements. Electrical separation uses an isolating transformer to BS EN 61558-2-4. A basic double-wound transformer with no such marking is neither.

What SELV and PELV Actually Require

SELV is the strictest arrangement. There is no connection to earth at any point — not on the live conductors, and not on any exposed-conductive-part of the SELV circuit. Because neither pole is referenced to earth, a person touching one live conductor while standing on the ground completes no circuit.

PELV is identical except that the circuit is earthed, usually because the equipment needs an earth reference (control circuits and some instrumentation). PELV therefore does not offer the same protection against a single touch to a live conductor, and BS 7671 places additional requirements on it in higher-risk locations.

For both, the wiring must be kept apart from other circuits — separate enclosures, or cables insulated for the highest voltage present, and no shared multicore unless the multicore is insulated for the highest voltage. Plugs and socket-outlets for SELV and PELV must not be interchangeable with those of any other system, so extra-low voltage equipment cannot be plugged into a 230 V outlet.

Voltage limits

The nominal voltage must not exceed 50 V AC or 120 V ripple-free DC. In locations containing a bath or shower, and in other special locations, lower limits apply.

The Test

The instrument is the same insulation resistance tester used in section 7.1, but the connections and the selected voltage differ.

The principle behind the two rows in the middle is worth stating plainly: the barrier is tested at the voltage of the higher circuit it has to withstand, not at the voltage of the extra-low voltage circuit it protects. Testing the transformer’s primary-to-secondary separation at 250 V would not prove it can hold off 230 V with a margin.

Procedure

1. Isolate and prove dead, then disconnect the extra-low voltage equipment where the manufacturer’s data requires it — SELV and PELV circuits commonly feed electronics that 500 V DC would destroy.
2. Test within the extra-low voltage circuit at 250 V DC — live conductors to each other, and live conductors to the exposed-conductive-parts and Earth for SELV.
3. Test across the separation at the higher circuit’s voltage — primary to secondary of the source, and the separated circuit to other circuits and to Earth.
4. Record each value on the schedule of test results, identifying which separation each figure refers to.

Electrical Separation

Electrical separation supplies one item of equipment from an isolating transformer to BS EN 61558-2-4, with the separated circuit deliberately left unearthed. Because there is no earth reference, a single fault from a live conductor to earth produces no fault current and no shock current — the classic example is a shaver socket in a bathroom.

The key requirements the inspector verifies:

• The source is an isolating transformer to BS EN 61558-2-4 or equivalent, and the secondary voltage does not exceed 500 V.
• No live conductor of the separated circuit is connected to earth, and no exposed-conductive-part of the separated circuit is connected to the protective conductor of the supplying circuit or to any other circuit.
• Flexible cables in the separated circuit are visible throughout any length liable to mechanical damage.
• Where the arrangement supplies more than one item of equipment, additional requirements apply, including an unearthed local equipotential bonding system linking exposed-conductive-parts — and the protection is lost if a second fault occurs on a different conductor.

The exam trap: candidates connect the separated circuit’s exposed metalwork to the installation earth "to be safe". Doing so destroys the protective measure, because it restores the path that separation exists to break.

Regulation 643.5 — Insulation of Floors and Walls

This is the least common test in the sequence, and it applies only where non-conducting location (Regulation 418.1) is used as the protective measure — a specialised arrangement found in some test areas and laboratories, not in general installations. If the installation does not use it, the test does not apply and the schedule entry is N/A.

Where it does apply, the resistance or impedance of insulating floors and walls is measured at a number of points in the location. The measured value must be not less than:

• 50 kΩ where the nominal voltage of the installation does not exceed 500 V; or
• 100 kΩ where the nominal voltage exceeds 500 V.

Knowing that it exists, what it is for, and that it is normally N/A is enough for the multiple-choice paper.