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Class I and Class II: Two Ways to Be Safe, Two Different Test Sets
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Class I equipment relies on basic insulation plus a protective earth connection, so that a fault that breaches the basic insulation becomes a fault current rather than a shock. Class II equipment relies on double or reinforced insulation and has no protective earth connection at all. Both are legitimate; the evidence for each is different.
Class I evidence
Dielectric strength and clearance for the basic insulation, plus the bonding test on every accessible conductive part, plus touch current under normal and single fault conditions. The bonding test is the distinctive one: without it the class I claim has no meaning, because the earth path is the protection.
Class II evidence
Dielectric strength and clearances for reinforced insulation between live parts and every accessible surface, including surfaces that are not obviously conductive, such as a metallised label or a conductive coating. The distinctive test is the assessment of the whole accessible surface, because there is no earth to fall back on.
The mixed construction
Many instruments are Class I with Class II subassemblies, or Class II with a functional earth. The rules then apply separately to each part and to the interfaces between them. A report that assigns a single class to a mixed construction is under-specified, and the ambiguity tends to appear at the exact interface where the insulation changes.
