A question about whether the second part of clause 21.2 needs to be tested.

We know that clause 29.3.2 and clause 21.2 are two separate clauses and they are not very much related, which means that the product being assessed needs to fulfill both clauses if applicable.
A reinforced insulation system consisting of three separate layers of insulating material or what could be considered a double insulation system, Each layer of material can withstand the electric strength test of 16.3, so how do we decide if we need to do clause 21.2 testing?
I think the question should be viewed in this way.
clause 21.2 does not mention how many layers of insulation are required? As long as the thickness requirements are met.
In the first case, suppose all three layers are 0.7mm thickness, and all three layers together are 2.1mm thickness. These three layers are combined together to meet the thickness requirements of the reinforced insulation, while the outside two layers are combined together for a thickness of 1.4mm, also able to meet the thickness requirements of the supplementary insulation.
The second situation, assuming that the basic insulation (and live parts in direct contact with the insulation) is 0.5mm, outside the two layers of insulation or 0.7mm, the three layers of insulation together is 1.9mm, does not meet the requirements of the thickness of the reinforced insulation of 2mm, but if the outside of the two layers together as an supplementary insulation, the thickness of 1.4mm to meet the requirements of the thickness of the supplementary insulation of 1mm.
The third situation, assuming that the basic insulation thickness is 1.4mm, the thickness of the two layers of insulation outside are 0.4mm, the thickness of the three layers of insulation together is 2.2mm, to meet the requirements of the thickness of the reinforced insulation, however, the two layers of insulation outside of the 0.4mm insulation as supplementary insulation, the thickness is not enough to meet the requirements of the 1mm.
The fourth situation, assuming that the thickness of the three layers of insulation are 0.4mm, combined together with the thickness of 1.2mm, the overall thickness does not meet the thickness requirements of 2mm reinforced insulation, the outside two layers do not meet the supplementary insulation 1mm thickness requirements.
My personal opinion is that none of the previous three cases require the test in clause 21.2. The basic principle is that whether the additional insulation to meet the thickness requirements, or reinforced insulation to meet the thickness requirements, do not need to consider the clause 21.2 of the test.
There are two considerations here, the first is because clause 21.2 assesses the mechanical strength, the standard considers the mechanical strength to be sufficient to meet the thickness, mainly the thickness of the outer insulation to meet the thickness of the outside, because that is the parts that can be touched, easy to receive damage. The second is because of clause 22.33, this provision is actually considered multiple insulation insulation will be better than single insulation, although clause 22.33 consider more is the effect of water on the insulation effect of the insulation material, as well as the effect of aging of the insulation material itself on the insulation effect.