Abstract: A transformer is formed by a housing having a main transformer compartment, a first compartment for the input terminals for the main power supply, a second compartment for the output terminals to supply the load and a third compartment for a manually operable switching unit, where each of the compartments is maintained separately from each of the others and each has a manually openable interlocked cover which can be opened to provide access to an interior of the respective compartment. The switch assembly has switch contacts mounted within a switch container located within the third compartment and a manually operable member mounted one on side wall located within the third compartment outside the switch container for moving the switch contacts between open and closed positions. The switch container has a transparent window in a wall thereof through which the contacts can be observed and an insulating liquid bath maintained separate from an insulating liquid bath for the transformer assembly.