Isolated Power Specialist
Isolated Power Specialist

Line Isolation Monitors and the NFPA

Early Days - No Line Isolation Monitors

In the early days, the operating room was a very dangerous place. There were no isolated power systems or line isolation monitors. The operating rooms used a flammable agent for putting their patients to sleep. There was little thought given to an electrical spark that would cause an explosion. 

In 1951, the NFPA required isolated power systems in the operating room. The requirement was to keep the path from each electrical conductor to ground as high as possible. This would remove any spark from occurring and causing a fire. Therefore, the system would need a static ground detector to monitor the resistance to ground.

The idea was spot on, but the technology had a long way to go. The monitoring device was either in “Alarm” or “Safe” mode. It may be hard to believe, but some of these devices are still around. The NFPA required monthly testing of these devices. In today’s world where law suits are so common place, these devices pose a physical safety risk to the patient and staff. The financial and political risks are huge to the hospital. 

In 1971 the second-generation monitor was a Dynamic Ground Detector, but it had flaws:

  1. Its monitor hazard leakage was anywhere from 500 to 1000 ma
  2. It used a switching relay to test one line to ground then switch over, and test the second line to ground
  3. At times, this caused interference with other equipment, like heart monitors

Code Changes

The 1996 edition of NFPA changed the code to allow for self-testing, self-calibrating line isolation monitors. The NFPA required annual testing and logging of these LIMs, instead of monthly. This was a cost saving. Additionally, this resulted in better monitoring of the operating room’s electrical system.

NFPA 99 3.3.95 says the LIM is a test instrument. Therefore, it requires annual certification to make sure it is following NFPA 99

In 2015, MSNBC published that in the United States there are some 600 surgical fires a year. In the fire triangle; fuel, oxygen, and heat source, the electrical spark serves as that heat source very well. It would behoove all facility directors to be sure your Log Book is up to date, and all line isolation monitors tested to their respective intervals (monthly or annually).

Isolated Power Regulations

The Joint Commission standards are the basis of an objective evaluation process that can help healthcare organizations assess and improve performance. The standards focus on important patient, individual, or residential care. They give attention to the facility functions that are essential to providing safe, high quality care. We can ensure your isolated power systems are compliant before your next Joint Commission review.

NFPA 99 2018 Edition states: “Operating Rooms shall be considered to be a wet procedure location, unless a risk assessment conducted by the health care governing body determines otherwise.”

NFPA 99 -3.3.95 referring to a line isolation monitor “A test instrument designed to continually check the balance and unbalanced impedance from each line of an isolated circuit to ground and equipped with a built-in test circuit to exercise the alarm without adding to the leakage current hazard.”  LIM


Line isolation monitors are the watchdogs of an isolated power system. It is very important that they function correctly for the facility to remain NFPA compliant. The NFPA code requires that hospitals and healthcare facilities test all line isolation monitors. The model of the LIM determines the testing frequency necessary. For example, LIMs that have an analog meter the testing is monthly. If the LIM does not have self-test features, then testing is monthly. For LIMs with digital meters and self-testing features testing is annually.

Isolation Power Specialist works with your facility to provide testing and certification. If needed, we will make repairs, or install retrofits for any LIM manufacturers. Also, the NFPA requires that the staff has continual education on the “wet procedure” location, or operating room. Helping the staff to understand their work environment is important to the hospital and the patients. We provide staff training to personnel that maintains the isolated power system in a classroom environment or hands-on training in your operating room. 

Let us know if you have any problems, questions, or concerns.


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© 47 Years of Experience with Line Isolation Monitors