An insulating support (20) for insulating an electrical conductor contained in aligned casing portions (34, 35), is held in a housing (39) defined by the flanges (36, 37) for joining the casing portions (34, 35) together, the flanges presenting contact faces (38) around the periphery (24) of the support (20). A single sealing gasket (41) is sufficient to prevent the atmosphere in the casing leaking to the outside, and that simplifies assembly of the electrical equipment, while at the same time protecting the support (20) from the outside. For application to medium- and high-voltage electrical equipment.