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SELV, PELV and electrical separationSELV or PELV. Which has an earth on the secondary?
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Guidance Note 3 · p.75 · Tables 2.13, 2.14 and 2.15. First, what the letters mean:
| Term | Stands for | Earth |
|---|---|---|
| SELV | Separated extra-low voltage | On the primary side only — no earth on the secondary |
| PELV | Protected extra-low voltage | A cpc carries through to the secondary side |
That is the whole difference. SELV is separated from earth on the output side; PELV is not. A SELV transformer has more windings on the primary, and the output is no more than 50 V.
Testing a shaver socket — electrical separation, not SELV
Fit a temporary link across the conductors on the supply side. Fit a temporary link across the conductors on the separated output side. One test lead on the supply side link, the other on the output side link. Test at 500 V d.c. — minimum 1.0 MΩ. You are proving the supply and the separated circuit are completely separated from each other — not testing either to earth.
A shaver socket is not SELV. Its output is a separated circuit at mains-type voltage, not extra-low voltage. Guidance Note 3 · 2.6.9 · Table 2.15 · p.75 gives the electrical separation test as 500 V d.c., minimum 1.0 MΩ, for a source voltage up to 500 V.
You will mostly meet SELV. PELV turns up rarely in domestic work.
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