About 1973, in the USA, was proposed the adoption of an isolated electrical energy supply system to the operating room. Such system was known as the IT system (IT system) and it was standardized by IEC 60364-7-710 Ed. 1.0 b -"Electrical Installations of Buildings - Requirements for Special Installations or Locations - Medical Locations, Part 710.413.1.5", 2002. This system, shown in a simplified way in Figure 1, has the main function of preventing a first fault interrupt to the energy supply during the surgery. The IT system was also designed so that the first fault does not produce an electric spark, enabling inflamed gases to be produced by the anesthetic evaporation, widely used in 1973. In the TNS System, which uses neutral and phase conductors, this first fault is the accidental contact of the phase conductor with a grounded point in the surgery room. In an IT System, in which feed conductors do not have electrical tension referred to ground, an accidental contact of F1 with the ground would not provoke any spark. This F1 accidental contact with the grounding, in addition to a second fault putting F2 in contact with another grounded point, would provoke spark and feeding, accomplished by circuit breakers. In order to increase the security of the IT system, an Insulation Monitoring Device (IMD) was added to the isolator transformer, which is able to signal, through a sound alarm, when the electrical resistance between F1 or F2 and the grounding falls below a selected value, normally 50kOhms.