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Line Isolation Monitors (LIM)
What is a Line Isolation Monitor (LIM)?
The Line Isolation Monitor (LIM) system is the central safety device of the Isolated Power Panel. The LIM is continuously monitoring the Total Hazard Current (THC) of all equipment powered by the panel. In the event of a line-to-ground fault an audible and visual signal will notify the medical staff of a potentially hazardous condition, but the patient is still protected.
Isolated power systems maintain continuity of essential power to critical equipment in hospitals and surgery centers, and other “wet procedure locations” in the event of a line-to-ground fault.
In conjunction with the isolated power system, the LIM helps ensure uninterrupted power which is critical for equipment providing life support during surgery or intensive care. Additionally, the LIM is able to detect the first sign of electric fault and alert staff to help remove dangerously faulty equipment before it can create a safety risk.
How does a Line Isolation Monitor Work?
The Line Isolation Monitor (LIM) continuously measures the electrical impedance between each isolated conductor (L1 and L2) and ground. Instead of measuring load current or shutting off power, it calculated Total Hazard Current (THC) a predictive estimate of th amount of current that would flow if a second ground fault occured.
By tracking impedence, the LIM detecs when the isolation between conductors and ground is compromised and alerts staff before a hazardous condition develops all while maintaining uninterrupted power for critical equipment. If the current reaches a dangerous level, the monitor alarms, notifying caregivers of a surge in electric current. In the event of one line-to-ground fault, the Isolated Power Panel for hospitals provides uninterrupted power and also eliminates the danger of electrical shock. This allows the procedure to continue while keeping patients and caregivers safe
Why Isolated Power Is Used in Operating Rooms
Isolated Power is required in category 1 critical care hospital operating rooms and other “wet procedure locations” because if a fault were to occur, the electric current will flow through the path of least resistance. In a “wet procedure location,” patients are vulnerable, critical electrical equipment is often used, and fluids are present. This combination creates a dangerous environment should an electrical fault occur.
No Intentional Bond to Ground
Isolated power systems are different from grounded systems because neither L1 nor L2 is tied to earth ground. This floating design creates high impedance, which allows the system to remain energized even when one fault condition exists.
Hazard Current
In a grounded system, leakage fault current flows directly to ground. In an isolated system, the LIM calculates predicted hazard current based on system impedance, it does not measure actual current flowing through the patient or equipment.
As more equipment and wiring are connected to the IPS, cumulative impedance decreases and calculated THC increases.
Continuous Impedance Monitoring
The LIM continuously measures line-to-ground impedance on L1 and L2 and displays THC in milliamps:
- Higher impedance = Lower Hazard Current
- Lower impedance = Higher Hazard Current
This real-time calculation enables early warning of insulation degradation before a dangerous condition could occur.
First Fault Alarm at 5mA
When the Total Hazard Current exceeds 5 mA, the LIM activates an audible and visual alarm. This indicates a first fault condition. Importantly:
- The alarm does not mean a dangerous current is flowing.
- It does not mean equipment should be immediately shut down.
- It does not interrupt power to the room.
An alarm indicated that one layer of isolation had been compromised and that a second fault could now allow hazardous current to flow. This is the key advantage of isolation power systems: The first fault does not trip a breaker or shut down critical equipment.
Why the System Stays Energized
Unlike Ground-Fault Circuit Interrupters (GFICs) that trips on imbalance, isolated power systems limit potential fault current by maintaining high impedance, allowing essential systems to remain energized during a first fault.
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Learn MoreNFPA 99 Healthcare Facility Code Requirements:
All Isolated Power Systems and Line Isolation Monitors (LIM) are designed in compliance with NFPA 99 Healthcare Facility Code:
- 6.3.2.6.3.1 – "In addition to the usual control and protective devices, each Isolated Power System shall be provided with an approved, continually operating Line Isolation Monitors that indicates possible leakage or fault currents from either isolated conductor to ground."
Are Line Isolation Monitors required by code?
Special protection against electric shock is required by NFPA 99 Healthcare Facility Code and NEC (2014 Edition) to be installed in all “wet procedure locations”. Isolated Power Systems with apnels and line isolation monitors are the preferred method of achieving this protection where power interruption cannot be tolerated.
Do you need to update your current LIM system? Click here for Line Isolation Monitors retrofitting and installation services.
How often should a LIM be tested?
According to NFPA 99 standards, line isolation monitors must be tested and inspected after installation and on a routine, periodic basis.
It is recommended that clinical staff test the LIM before the first case of the day by pressing the "test" or "push-to-test" button to ensure the audible and visual alarms are functioning.
While annual testing to cover NFPA 99 requirements can be performed internally, PG LifeLink offers in-depth annual LIM testing to provide thorough information about the health of the system to prevent issues in the future. Visit our annual testing page to learn more.
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Line Isolation Monitor FAQs
An alarm indicates that a ground fault has occurred somewhere in the isolated power system. This could be due to faulty equipment, wiring issues, or excessive leakage currents caused by too many devices connected to the system. If your LIM enters an alarm, follow the troubleshooting steps below or call PG LifeLink Field Service at 877-445-4123 for assistance.
- An alarm does not mean there is imminent danger. Do not endanger patient by discontinuing procedure prematurely.
- If possible, immediately notify personnel responsible for maintaining equipment.
- If it is clear that the alarm is directly related to a particular device, disconnect that device from the system. Depending on the nature of the device, this may not be possible. For example, life-support equipment.
- Once the procedure is completed and the patient has been safely removed from the area, authorized personnel should attempt to identify the root cause of the fault. Likely causes may include damaged wire or power cord insulation, liquid intrusion into energized equipment or wiring, or faulty equipment.
- If the cause is not readily identifiable, turn off all circuit breakers on the isolation power panel. Then turn on individual circuit breakers one at a time and observe the circuit’s relative hazard current contribution as displayed on the LIM. Identify which circuits contribute the largest amount of hazard current. This will narrow down the number of devices that may be associated with the alarm.
- Contact PG LifeLink Field Service for further troubleshooting at 877-445-4123 or click here to email a customer service rep.
Yes. A first fault condition does not interrupt power and does not mean hazardous current is actively flowing. The purpose of the isolated power system is to allow procedures to continue while the issue is identified and corrected.