A device for fastening support brackets to vertical frame components at the forward side of switch boxes whereby connecting brackets provided with threaded connectors are connectable to the frame components, so that the front plates of the support brackets can be screwed to the connecting bracket. In order for the vertical interval of the support bracket on the switch box to be continuously variable and the support brackets to be removable even with telescoping extensions, the invention provides that, on both frame pieces forming the front side of the switch box, connecting brackets are mounted, which are aligned facing one another with a vertical connecting bar directed parallel to the front side, and that on this connecting bar, sliding elements may be mounted to which support brackets may be attached in the desired position with a threaded connector.