The modular back support (1) is adapted to be placed side by side with one or more identical back supports(l), to form rows both in horizontal and vertical directions, and includes: a box-like structure (2) on whose front face (2A) is fixed a circuit breaker (100); three conductive rails (41, 42, 43) made of conductive material, housed in said box-like structure (2) and insulated from each other, intended to be connected to an external three-phase power supply line; three holes (21, 22, 23), made in the box-like structure (2), in which are inserted plug connectors (101, 102, 103) of said circuit breaker (100) to connect the latter to the three conductive rails ( 41, 42, 43); three openings (24, 25, 26), made starting from each of the four side faces (2B, 2C, 2D, 2E) of the said box-like structure (2) which are perpendicular to said front face (2A), adapted to make accessible from the outside the said conductive rails (41, 42, 43); connecting means (5), provided between each of said modular back supports (1) a
