Heading 8535 or 8536 covers individual apparatus such as relays or fuses, or multiples of one such apparatus, that are principally used for switching or protecting or for making connections to or in electrical circuits. On the other hand, heading 8537 covers assemblies of the apparatus described in heading 8536, for example, combinations of relays and fuses on a board, panel or other base whose principal use is to control or distribute electricity. Assemblies (other than simple switch assemblies) of the apparatus of headings 8535 or 8536 are classified under heading 85.37.