Isolated electrical power systems have been in use in hospital operating rooms and critical care areas since the 1940’s. Their primary purpose was to prevent the possibility of electrical arcs and sparks which could provide a source of ignition for flammable anesthetics. The use of flammable anesthetics was discontinued in the U.S. in the 1970’s, however, the use of isolated power systems has continued because of the superior protection they also provide against electric shock in "wet locations".

The techniques and devices utilized for monitoring the "electrical isolation" of these power systems have evolved over time due to technological advances as well as experience factors with electrical safety for patients and personnel within the operating room and critical care areas.

Systems placed into service prior to 1971 typically used some variation of electromechanical devices which formed a balanced bridge. These units, called Ground Detectors, were simple in design, inexpensive to build and performed reliably within their limited sensing range. Common units were made by Crouse-Hinds (their GDA M2 and GDP devices) and Hevi-Duty (their Dynatector devices). The very notable disadvantage of these devices is their inability to detect balanced resistive and capacitive faults. In other words, if the ungrounded isolated power system became partially grounded through leakages that were equal in each conductor of the system, the Ground Detector would not recognize or sound an alarm in response to this fault condition. These inherent sensing limitations led to the design of subsequent generations of newer devices called Line Isolation Monitors or LIMs.

The aforementioned "experience factors" have progressively instigated many significant changes and new requirements in the NEC – NFPA 70 Article 517 and NFPA 99 – Standard for Health Care Facilities Section 3. Installation and periodic testing requirements have become significantly more stringent in the areas of line-to-ground fault detection and annunciation; grounding and ground system impedance; system leakage levels; and applied fault testing.

Each version of NFPA 70 (NEC) and NFPA 99 since the early 1970’s has clearly identified the requirements for fault detection in Isolated Power Systems (IPS). These requirements have included the use of Line Isolation Monitors (LIMs).

Most importantly, it must be understood that with the 1971 edition of NFPA 56A (now known as NFPA 99) Ground Detectors were banned for new construction and effectively rendered technologically obsolete. As the focus on electrical safety in the operating room changed from primarily explosion prevention to heightened concern over patient and personnel electrical safety, experience indicated that isolated power system monitoring needed to be improved. In order to provide this required enhancement in electrical safety to patients whose body resistance was vastly reduced due to invasive procedures and the presence of fluids, new monitoring technology was developed which resulted in the introduction of new devices known as Line Isolation Monitors or LIMs.

Even the early LIMs provided fault detection far beyond the capabilities of the obsolete Ground Detectors - particularly in the case of capacitive faults, hybrid (resistive and capacitive) faults, and balanced L1 & L2-to-ground faults. For these reasons, and in an effort to provide maximum electrical shock protection to patients and mitigate risk in the OR, many hospitals began retrofitting old Ground Detectors with new Line Isolation Monitors. This practice continues today.

NFPA 70 and NFPA 99 closely parallel each other with regard to Isolated Power Systems and should be used together as resource documents. Further, JCAHO Environment of Care guidelines for electrical systems maintenance and testing reference NFPA 99 as a fundamental base document.

There is some confusion over a statement contained in NFPA 99 Annex 2 wherein it states: