Line isolation alarm

Basic, Basic Sciences

Line isolation systems (isolation transformer + line isolation monitor) protect from electrocution by turning a normal “grounded system” (exists outside the operating room) which only needs a single fault to cause electrocution into a “protected” system in which two faults are needed to deliver a shock. The line isolation monitor determines the degree of isolation between the two power wires and the ground and predicts how much current could flow if a second short-circuit were to develop. 

LIM alarm goes off if an unacceptable amount of current to the ground is possible i.e the “isolated” system is no longer isolated, but rather is grounded, thus only one additional fault could result in a shock. In other words, when the monitor is alarming, there is a single fault in the system, but there still needs to be another one in order to deliver a shock. If the alarm is going off, the last piece of equipment plugged in is usually suspect and should be unplugged unless it is critical piece of life-support. If it is, just remember that the protection of the LIM is no longer operative and if possible, no other electrical equipment should be connected at least until the faulty piece of equipment can be safely removed.

The LIM alarms when a single fault occurs, such as when leakage exceeds 2mA (5 mA for newer systems). The LIM does not detect current below 2 mA, therefore even with the LIM in place and working, it is possible to deliver a microshock. If a faulty piece of equipment is connected to the isolated power system, this will change the system back to a conventional grounded system. Also, the faulty piece of equipment will continue to function normally. 

LIM alarm also goes off when many normal pieces of equipment are connected to the system. Although each piece of equipment has a small amount of leakage current, if the total leakage current exceeds 2 mA then the LIM will trigger an alarm.


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Year asked

Hamy Kassahun, MD