Standard electrical circuit symbols. Diagram of standard symbols used to represent electrical equipment in electrical circuit diagrams. The symbols are arranged in six rows of sets of three. From top, these rows show symbols for: conductors, power supplies, resistors, capacitors, diodes and transistors. These symbols have varied over time and from country to country, and so various national and international standards have been devised to ensure consistent use of standard symbols that can be recognised worldwide. These symbols conform to British Standard 3939 (BS 3939).