transportable power supply substation is a power supplying equipment in which power supplying devices such as, for example, a transformer, an interrupter, a disconnecting switch unit and so on are mounted on the trailer truck. Accordingly, the transportable power supply substation is very conveniently used for various purposes and in various places, because it is very easy to set in place by the simple construction thereof. There are, however, serious and troublesome problems in case where a power supply substation is transported. Particularly, a current interrupter unit used for the high voltage is convenient to install and to transport, because it is large in size and heavy in weight. Specifically, the electrical power supplying machines such as the transformer, the current interrupter and a disconnecting switch unit are,in general, arranged on the trailer truck along with the length of the trailer truck. The electrical power machines must also be installed on the trailer truck spaced apart at a given distance from each of adjacent machines in order to maintain the insulation distance. The three-phase current interrupter must be mounted on the trailer truck so that its arranging direction is perpendicular to the length direction of the trailer truck to obtain the insulation distance from the other neighbouring machines. In this case, it is very inconvenient to transport, since the length of the three-phase current interrupter extends over the limit of transportation.