The present invention relates to a device making it possible to turn on a sub-assembly of an electrical item of equipment hosted in a bigger system, which, if turned on unchecked, may impair the operation of the other items of equipment of the system. An embodiment includes three make/break switches in series in the power supply line and a control of the opening or closing of these make/break switches. Two of the make/break switches are controlled by two independent discrete state indicators, to determine whether the power supply of the sub-assembly of the item of equipment may be energized without undue risk to other equipment of the system. The third make/break switch is controlled by a validation command based on the two discretes, actuating the third make/break switch only if the discretes are in the desired state and if a switching of these discretes has been noted. An embodiment of the invention applies to the protection of the onboard equipment of an aircraft in the flight phase against the turning on of ancillary equipment not used in the flight phase and transmitting radioelectric waves that may catastrophically impair the operation of the other items of equipment.