Protection class I, II and III — IEC 61140

During NEN 3140 inspection of portable and connected equipment (tools, extension cords, appliances), the protection class of a device determines what the inspector may expect in terms of earthing, insulation and plug design. IEC 61140 ("Protection against electric shock — Common aspects for installation and equipment") defines three classes.

Class I — earthed enclosure

For class-I equipment, basic protection consists of basic insulation of the live parts, supplemented by a connection of all accessible conductive parts (the enclosure) to protective earth (PE) via the plug. If a live conductor touches the enclosure due to an insulation fault, the earth connection (together with the installation's overcurrent/residual-current protection) causes disconnection. Class-I equipment therefore always has an earthed plug (with earth contacts or an earth pin).

Class II — double or reinforced insulation

Class-II equipment requires no earth connection: protection relies entirely on double insulation (basic insulation + supplementary insulation) or reinforced insulation (a single insulation system providing a level of protection equivalent to double insulation), such that a single insulation fault cannot lead to a dangerous touch voltage on the enclosure. Class-II equipment is recognizable by the double-square symbol (⧈) and has no earth pin on the plug — a class-II device with an earthed plug is an indication of an incorrectly fitted or replaced plug.

Class III — SELV/PELV, extra-low voltage

Class-III equipment is supplied with a safety extra-low voltage (SELV or PELV, generally not exceeding 50V AC or 120V DC), low enough that a dangerous touch voltage cannot occur under a single fault. Examples include low-voltage hand lamps fed from an isolating transformer, or battery-powered hand tools.

Why this is relevant for NEN 3140 inspection

The protection class determines which measurement/check is relevant:

For class I, PE continuity (continuity measurement of the earth connection) is one of the standard measurements.
For class II, a PE continuity measurement does not apply (there is no earth connection) — here the emphasis is on visual inspection of the enclosure/insulation and the class-II symbol.
For class III, the supply voltage itself is already so low that many regular NEN 3140 measurements (insulation resistance to the mains, PE continuity) do not apply in the same form.

Note: the exact measurement procedure and limits per protection class follow from the applicable NEN 3140 measurement instructions for the specific device type — this article only gives the classification principle, not the full measurement procedure.

Common mistakes

Fitting an earthed plug to a class-II device (for example when replacing a damaged cord/plug) — this does not necessarily break the class-II principle, but is confusing and can lead to an incorrect (unnecessary) PE continuity measurement during inspection.
Measuring PE continuity on a class-II device and concluding that it "failed" because there is no earth connection — the measurement simply does not apply here.
Confusing class III (SELV/PELV) with "always safe, no inspection needed" — even at extra-low voltage, general safety aspects (mechanical damage to cords, correct power source) remain relevant.