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In Europe, a double insulated appliance must be labelled Class II or double insulated or bear the double insulation symbol: ⧈ (a square inside another square). As such, the appliance should not be connected to an earth conductor because the high-impedance casing will cause only low-fault currents that are unable to trigger the fusible cut-out.
Class II insulation means that the device is double insulated. This is used on some appliances such as electric shavers, hair dryers and portable power tools. Double insulation requires that the devices have both basic and supplementary insulation, each of which is sufficient to prevent electric shock. All internal electrically energized ...
Double insulation is a design concept where failure of one insulation system will not expose the user to a shock hazard due to the presence of a second independent layer of insulation. Reinforced insulation is a supplemental insulation system that is strong enough to effectively perform as if a double insulation system was present.
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For example, power semiconductors connected to the line voltage may be switched by opto-isolators driven from low-voltage circuits, which then need not be insulated from the higher line voltage. Transformers allow the output of a device to float relative to ground to avoid potential ground loops. Power isolation transformers increase the safety ...
Moulded plugs for unearthed, double-insulated appliances may instead have a non-conductive plastic pin (an Insulated Shutter Opening Device or ISOD) the same size and shape as an earth pin, to open the shutters. When looking at the plug pins with the earth uppermost the lower left pin is live, and the lower right is neutral.
Wire crossover symbols for circuit diagrams. The CAD symbol for insulated crossing wires is the same as the older, non-CAD symbol for non-insulated crossing wires. To avoid confusion, the wire "jump" (semi-circle) symbol for insulated wires in non-CAD schematics is recommended (as opposed to using the CAD-style symbol for no connection), so as to avoid confusion with the original, older style ...
Standards permit ungrounded plugs where the appliance does not require grounding due to low risk of leakage current, such as on double-insulated devices. In older plug designs both blades were the same width, so the plug could be inserted into the receptacle either way around.