25.20 Do supply cord entering the metal enclosure need an additional layer of insulation at the vias?
25.20: The conductors of the supply cord for type Y attachment and type Z attachment shall be insulated from accessible metal parts by basic insulation for class 0 appliances, class 0I appliances and class I appliances, and by supplementary insulation for class II appliances. This insulation may be provided by the sheath of the supply cord or by other means.
This clause should be considered with clause 16.3, generally condition, all most countries employed supply cord with double insulation except those countries based on main power voltage less than 150V, such as Japan, America, Mexico etc.
I guess most people would think that the white insulation enclosure on the outside of supply cord should be the sheath or other means. But it is not. Here, the supply cord do need an additional sheath or other means, please pay attention, a sheath is only required when the supply cord passes directly through the accessible metal enclosure of the appliance, because the first sentence of clause 25.20 mentions the insulating sheath between accessible metal parts and the supply cord.
Why I could get such a conclusion? We could refer to clause 16.3, the following content:
A test voltage is applied between accessible metal parts and the supply cord which is wrapped with metal foil at the point where the supply cord is located in an inlet bushing and where, for appliances having a type X attachment, the supply cord is located in a cord guard or a cord anchorage with their clamping screws, if any, being tightened to two-thirds of the torque specified in Table 14. The test voltage is 1 250 V for class I appliances and 1 750 V for class II appliances.
Clause 16.3 requires that the supply cord be wrapped with a metal foil at the inlet before the electrical strength test. If there is no insulating sheath outside the power cord, the test will fail.
Combining the requirements of two different clause of the standard, we can infer the standard’s intent.
What type of sheath should be used? The standard does not require a sheath, other types are acceptable. The following photo show an acceptable structure.
The method shown in the image serves both as insulation and as a cable clamp.
In the image below, because the supply cord via is made of non-metallic plastic, clause 25.20 is not applicable.
