Line isolation monitor

Basic, Basic Sciences, Physics, Monitoring, & Devices

Intro: Electrocution

To receive a shock, persons need body contact with two separate conductive materials at different voltage potentials to complete a circuit. The most common scenario in which a shock occurs is where a grounded (“neutral voltage”) patient touches a “hot” wire which exists at a non-zero voltage potential – the electrical current that subsequently passes through the patient can be described as I = V/R, where V is the difference in voltage potential between the “ground” (0) and the “hot” line (~ 120V for a standard wall outlet).

Grounded Wiring

Modern housing systems have grounded wiring, in which both the neutral (connected to power company neutral wire) and grounded wires (connected to equipment) are grounded. This is ideal for something such as a hairdrier, in which an equipment fault can be disastrous. If a hairdryer were connected to an ungrounded system, and developed a fault, the person touching the hairdryer might offer a low resistance alternative pathway and thus receive a shock, whereas if the hairdryer were grounded, most of the electricity would pass through the ground wire (although a small amount might pass through the individual)

Primary vs. Secondary Wiring

Primary wiring in an operating room (from the main alternating current power supply) is normally grounded. Equipment used in the OR is NOT normally grounded, but the equipment casing is – this configuration protects this patient, as the secondary wiring system, which is not grounded, requires two faults in order to shock the patient (fault no. 1 = convert to grounded, fault no. 2 = patient touching the secondary wiring system). An electrical wire which is not grounded is thus considered an “isolated” line – if an ungrounded patient were to touch an “isolated” ie ungrounded line, nothing would happen. In order for a piece of equipment to take advantage of this safety mechanism, it must be connected to the primary wiring system (which is grounded) via an isolation transformer (for a schematic diagram, see Morgan and Mikhail Figure 2.7, page 25) [See image]

What the Line Isolation System Does

Line isolation systems (isolation transformer + line isolation monitor) protect persons from electrocution by turning a normal “grounded system” (that 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. An alarm goes off if an unacceptably 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).

What the Alarm Means

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.

Regulations Post-1984

Isolated power systems are no longer required in operating rooms as of 1984, thus some modern equipment may offer the potential for electrocution, although this risk is reduced by the use of ungrounded batteries for power, double insulating equipment, and isolating patients from equipment (Morgan and Mikhail 23-23).

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