On-load tap-changers (OLTCs) are indispensable in regulating power transformers used in electrical energy networks and industrial applications. This thesis explains the static on-load transformer tap-changer. A mathematical model derived for two modes of operation. Zero-current crossing and zero-voltage crossing modes of operation will be presented. The general switching principles for OLTCs are discussed and OLTC applications are presented. Today’s OLTC design concepts, including the new generation of vacuum type OLTCs, are described. The vacuum switching technology used in OLTCs is the “state of the art” design now and in the foreseeable future. Examples of OLTC designs and the respective switching principles show the range of the usage of vacuum interrupters.