The main function of an opto-isolator is to block such high voltages and voltage transients, so that a surge in one part of the system will not disrupt or destroy the other parts. Historically, this function was delegated to isolation transformers, which use inductive coupling between galvanically isolated input and output sides. Transformers and opto-isolators are the only two classes of electronic devices that offer "reinforced protection" — they protect both the equipment "and" the human user operating this equipment. They contain a single physical isolation barrier, but provide protection equivalent to double isolation. Safety, testing and approval of opto-couplers are regulated by national and international standards: IEC 60747-5-2, EN (CENELEC) 60747-5-2, UL 1577, CSA Component Acceptance Notice #5, etc. Opto-isolator specifications published by manufacturers always follow at least one of these regulatory frameworks.