What are the principles of Double Safeguard constructions as they relate to accessibility requirements to ES3 energy sources?
More specifically you asked: “0.6.2 states that "A SUPPLEMENTARY SAFEGUARD may be additional electrical insulation (SUPPLEMENTARY INSULATION) or a protectively earthed conductive barrier or other construction that performs the same function." Initially leading us to believe that the two following solutions are acceptable between Ordinary and ES3: 1- Two concentric barriers (for example hard plastic). One closer to the energy source (Basic) and one closer to the person (Supplementary) 2- One single barrier made of conductive material and connected to protective earth.
The physical barrier is the Basic and the Protective earthing is the Supplementary. Assuming this is correct, 126.96.36.199 and 188.8.131.52 make it difficult to see how the option 2 is viable given it says "For ORDINARY [or INSTRUCTED] PERSONS, the following shall not be ACCESSIBLE:– an ES3 BASIC SAFEGUARD. This seems to hint that a single conductive barrier protectively earthed cannot be touched because it's a basic safeguard.
Unless the real intent is that when it's protectively earthed, the user only touches the first layer which is the supplementary safeguard. As it stands 184.108.40.206 and 220.127.116.11 lead us to think that no matter the protective earthing, 2 concentric barriers are always required, one to act as Basic and the other one to prevent users from accessing the ES3 basic safeguard.”
In response, the general safeguard requirement between a Class 3 energy source and Ordinary Persons is defined in sub-clause 18.104.22.168, which consists of a Basic and a Supplementary Safeguard as shown in Figure 12. This is a Double Safeguard as defined in 22.214.171.124, which in the context of electrical insulation, also is reflected in 126.96.36.199 (Double Insulation, consisting of two independent safeguards – one as Basic and the second as Supplementary). The equivalency between Double and Reinforced Insulation is covered under clause 188.8.131.52.
The most common construction of Class I Equipment’s is having Basic Insulation (clearance and creepage distance) to a metal Enclosure connected to Protective Earthing, in which case the Protective Earthing / protective earthed parts are considered the second independent safeguard, defined as the Supplementary Safeguard. (Also, see 184.108.40.206 for the definition of Class I equipment which further clarifies this principle).
The key point that needs to be made related to your questions is that parts connected to protective earthing (e.g., metal enclosure) are associated with a Supplementary Safeguard / Insulation, not a Basic Safeguard / Insulation. Basic and Supplementary Safeguards / Insulation generally are not interchangeable as Supplementary Safeguards / Insulation typically have more onerous requirements than Basic Safeguards / Insulation. For example, in sub clause 220.127.116.11 for minimum distance through insulation requirement, a min. thickness of 0.4 mm is required for Supplementary Insulation, whereas Basic Insulation has no minimum thickness requirement.
Because of the above, there are restrictions on access to a Basic safeguard from ES3.
A complete answer to this topic depends on the application of the Standard to a specific construction. As there appears to be a specific, detailed construction that needs review / analysis, you are encouraged to contact UL for an in-depth consultation, either via your local account executive or via https://62368-ul-solutions.com/contact-ul.html.