The NSI Special Bulletin No. 03/05 provides guidance on interpreting specific clauses of PD 6662:2004 and prEN 50131-1:2004, aimed at NSI-approved installers of security systems. It clarifies areas that may lead to misinterpretation and emphasizes that while the guidance is not mandatory, compliance with PD 6662:2004 and related standards is required. The bulletin also outlines the context of these standards within the framework of European regulations for intruder and hold-up alarm systems, effective from October 1, 2005. Manual review may be needed for complete details, as the document contains extensive technical specifications and interpretations.