How does a Line Isolation Monitor (LIM) detect ground faults?

The Line isolation Monitor or the LIM is a censorious component of an outlying power system. The line isolation monitor constantly monitors the impedance of the isolated power system from all of the conductors to ground. Impedance includes both capacitive and resistive readings. The Total Hazard Current or the THC is then calculated by the value of these readings, which dictates if a patient were in contact to a ground with conductor what current could flow. The THC readings in advance warn about the issues that are present in the system.

A person’s body need be in contact with two distinct conductive materials, to receive a shock, at different voltage likely to complete a circuit. The most usual scenario of occurrence of shock is where a grounded body that lies in neutral voltage touches a hot wire that exists at a voltage of non-zero potentiality.

The grounded wiring system in modern housing, the neutral that is connected to the neutral wire of the Power Company and grounded wires that are connected to equipment, both are grounded. This is an absolute case like a hairdryer where a fault in equipment can prove to be disastrous. If the connection of hairdryer was done to an underground system and if a fault occurred, the person who is in touch with the hairdryer might offer an alternative pathway of low resistance receiving a shock, whereas if it was grounded then maximum electricity would pass all through the ground wire.

How Ground Faults are detected by LIM:

• In Isolated System: – The Line Isolation Monitor in an operating room (OR) is mounted somewhere, where it gives reading of how much connection is there in between earth ground and the isolated wall power wires. The connection is probably below 2 miliAmperes (mA). Even though it is an effective leakage of current, you may think it as a current leakage that may or may not exist.

In your operating room the LIM may be placed into the circuit breaker box that will have a portion of remote alarm located somewhere in the OR if the circuit breakers are out the OR. It may also be on the wall of your OR.

In hot wire and isolated system case, the LIM immediately detects the connection of either of the isolated wires or between them, and also to the case of appliances since all metallic cases are connected to the ground. The LIM repeatedly scrutinise for possible current connections and then displays the existing or potential leakage. If the leakage exceeds above a safe threshold then the LIM sends an alarm immediately.

I such situation one should unplug all devices to find out where the problem lies.

• In Broken Ground & Hot Wire & Isolated System: – In this case the circuit breaker will not protect the victim. Even if it was the case where the equipment is grounded through an intact with the wire, still there will be no current flow in the system since in an isolated system; ground is not a part of the power circuit. Whatsoever, if the ground wire and equipment are properly connected, and the Line Isolation Monitor is also active, then it would have given a strong alarm with a very high current indication. With even a single fault somewhere in the present devices inside the room, it would have warned you. No alarm goes off, with the broken ground and in that case of the device is then connected to the power system voltage.

The Line isolation systems protect victims from electrocution. It does so by turning a general grounded system which needs only a single fault to cause any electrocution within a protected system outside the operating room, where to deliver a shock two faults are needed. The LIM detects the isolation degree between the ground and the two power wires, and predicts what amount of current could flow if there develops a second short-circuit. If there is possibility of an intolerable amount of current to the ground the alarm goes off. This means the isolate system is no more in an isolated state, rather it is now grounded. This could result a shock with just any additional fault.

When the monitor alarms means there is a single fault in the system, but it requires one more to deliver a shock. If the alarm is going off means the last piece of equipment that is plugged is in suspect and should be unplugged.