An interesting history for isolated power systems

Patients undergoing acute care in healthcare establishments (such as hospitals) require enhanced reliability and safety of the electrical installation, as well as safe and reliable operation of the medical electrical (ME) equipment used. These criteria need to be met both to ensure security of supplies, and to minimise the risk of electric shock. In medical locations, the risk to patients is increased, due to:

The presence of liquids, such as blood and saline solutions, will also add to the risks, due to an increased conductive area, and subsequent lower body resistance. Any prolonged loss of the mains supply to life-supporting ME equipment may put the patient’s life at risk. This equipment must have secure supplies to provide adequate safety.

The introduction of medical IT, or, as they are also known, isolated power systems, has eliminated the operational hazards listed above. Earth fault currents in a medical IT system depend only on the capacitive coupling between the line conductors and earth. As these capacitances are quite low in value, the resultant capacitive coupling current is quite low, i.e. not dangerous.  Similarly, a single earth fault will not cause a trip; it simply creates a reference to earth (as in a TN system), so continuity of supply is present.