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To reference this educational web simulation according to the APA style for Web references, use: Gibby GL, Lizdas DE, Lampotang S. (2013): Electrical Power and Safety in the Operating Room. Retrieved <insert date of retrieval here>, from University of Florida Department of Anesthesiology Virtual Anesthesia Machine Web site:
Simulation: How much do you know about the power system of an operating room? Do you know when they can and cannot protect the patient? This simulation shows selected circuit pathways in an isolated system, a grounded system, and a grounded system with a GFCI (Ground Fault Circuit Interrupt.) Scroll down for a in-depth narrative |
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ISOLATED SYSTEM
Note that in an operating room (OR) with isolated power, there is a Line Isolation Monitor (LIM) mounted somewhere, which gives you a reading of how much "connection" there is between the supposedly isolated wall power wires, and earth ground. The "connection" should be below 2 milliAmperes (mA). You can think of it as a "leakage" current, even though it is a virtual leakage current; it may or may not exist.
This Line Isolation Monitor may be built into the circuit breaker box for your operating room (in which case it will have a remote alarm portion located somewhere in your operating room if the circuit breakers are outside your OR), or it may be on the wall in your OR.
In an isolated system, neither of the 120 volts AC power wires available at the power outlets are connected to earth ground. They are both "floating". Thus touching ground has little consequence to you, if you are in such a room.
In a room powered by an Isolated Power System, you can stand with bare feet on a wet floor and touch an outlet wire with little chance of being harmed. However, you will have just made the granddaddy of all leakage scenarios, and the Line Isolation Monitor should alarm strongly.
GROUNDED SYSTEM
A grounded power system for the wall outlets in an operating room is just like what you probably have in your home's living room or TV room. For various safety reasons (there are advantages to grounded systems), one of the 2 wires supplying your outlet with power is connected to earth ground, via a grounded connection back at your service entrance box , somewhere in your house. The wire that is connected to this ground connection is supposed to be colored "white" and is call "cold" because if all is done correctly, you can stand on wet ground and touch this wire and not get shocked -- because it also is connected to the earth. However, the other wire, supposed to be "black" in color and known as the "Hot" wire -- different story. If you accidentally touch this wire (never do this!) while it is powered, you have a significant likelihood of dying.
In your home, additional protection is therefore provided in portions of your home where occupants are likely to be near water -- the kitchen, bathrooms, basements and the like. In these locations, a "Ground Fault Current Interrupter" (GFCI) is a part of the power circuit, and it serves in an attempt to very quickly shut off power if a potentially deadly leakage to ground (e.g., though a human) is detected. These are great devices and are highly recommended.
However, in many grounded system operating rooms, GFCI protection is not utilized, primarily because in the event of a leakage current --whether a person is being harmed or not -- the power will be shut OFF and that carries a risk as well.
GFCI
The Ground Fault Current Interrupter is an ingenious device that usually takes advantage of some magnetic properties, in order to compare, continuously, the current actually flowing in the black and white ("hot" and "cold" ) wires of a grounded power system. These two currents should be exactly identical, whatever they are. All devices should obtain their electrical current through their connection to the black and white wires -- they should not return ANY current via their grounding connection (usually a green wire).
Humans, however, generally get harmed by being in physical contact with the ground (e.g., by standing on it, or on a floor or other physical structure that has good conductivity to the earth) and then by accidentally touching just one wire -- the black ("hot") wire. Thus the current flow through the human involves connection to the black wire only -- and back to the power system via the house grounding connection back at the service entrance. If a human is being harmed, there is a high likelihood that the current in the black wire ("hot") temporarily exceeds the current in the white wire ("cold"). The GFCI detects this quickly, and shuts off power to the system. The hope is that this will occur prior to the onset of ventricular fibrillation in the victim or other damage.
COMBINATIONS
GROUNDED SYSTEM & BROKEN GROUND
The ground wire to appliances is very important as it is one of the primary defense systems to prevent electrical shock. Devices with broken or damaged ground wires should be taken out of service and repaired. However, when just the ground wire is broken and NO OTHER FAULTS are present, no harm occurs. In order for harm to occur, electrical current must pass through the patient (or other human) in a complete circuit from one of the two conductors from the power company, to the other.
GROUNDED SYSTEM & BROKEN GROUND & GFCI
The ground wire to appliances is very important as it is one of the primary defense systems to prevent electrical shock. Devices with broken or damaged ground wires should be taken out of service and repaired. However, when just the ground wire is broken and NO OTHER FAULTS are present, no harm occurs. In order for harm to occur, electrical current must pass through the patient (or other human) in a complete circuit from one of the two conductors from the power company, to the other.
Because the mere presence of a broken ground wire does not cause current to flow through a human, the GFCI will not alarm.
ISOLATED SYSTEM & BROKEN GROUND
The ground wire to appliances is very important as it is one of the primary defense systems to prevent electrical shock. Devices with broken or damaged ground wires should be taken out of service and repaired. However, when just the ground wire is broken and NO OTHER FAULTS are present, no harm occurs. In order for harm to occur, electrical current must pass through the patient (or other human) in a complete circuit from one of the two conductors from the power company, to the other.
Because the broken ground wire does not necessarily provide a leakage path from one of the 2 power wires to ground, the Line Isolation Monitor will not detect this problem.
GROUNDED SYSTEM & GFCI BROKEN GROUND & HOT WIRE TO CASE
In this scenario, the circuit breaker will not protect you, because of the broken ground wire. Normally the metallic case of appliances is connected to earth ground via the ground wire. Should the hot wire contact the case (which could result in the case having a dangerous electrical voltage) the current will flow right to ground via the ground wire connected to the case -- we call this a "short circuit" because the current never made it to the intended device but diverted "short of the goal" and made it back to the power system through the ground wiring. Thus the circuit breakers and the grounded cases of appliances provide one means of protecting users. However, if that ground wire is broken, the case will become "hot" and the circuit breaker will not blow. The GFCI, if present, will also not make any disconnection because, up to this point, current is not making it out of the hot wire and not returning through the cold wire.
ISOLATED SYSTEM & BROKEN GROUND & HOT WIRE TO CASE
In this scenario, the circuit breaker will not protect you. Even had the case of the equipment been grounded through an intact wire, still no current will flow in the system, because "ground" is not a part of the power circuit in an isolated system. However, had the ground wire been properly connected to the equipment case, and the Line Isolation Monitor working, it would have given a very strong alarm with a very high current indication. This would have warned you of the single fault somewhere in the devices present in the room. With the broken ground, no alarm goes off, and now the case of the device is connected to the power system voltage.
ISOLATED SYSTEM & HOT WIRE TO CASE
In this situation, the Line Isolation Monitor will immediately detect the connection between EITHER of the isolated wires, and the case of the appliance, because all metallic cases are connected to ground. The Line Isolation Monitor constantly probes for possible current connections and displays the potential or existing leakage. If that leakage rises above a safe threshold, the Line Isolation Monitor will alarm.
In this situation, one should unplug devices to find out which one may have the problem. It may be the "last one plugged in", making it easier to find.
Because the patient is not actively being harmed, one does not need to interrupt surgery, but the risks in the room are now much greater and prudent efforts to correct the problem are warranted. Disabling the alarm -- unless the surgery is emergent -- is not a prudent response.
GROUNDED SYSTEM & BROKEN GROUND & HOT WIRE TO CASE & PATIENT GROUNDED
This scenario can easily occur in a non-isolated power system operating room with faulty equipment and metal surgical equipment (or the bed) touching the patient. Rigid metal endoscopes, urological equipment and other metal used in surgery can make the connection to ground. To avoid these risks, generally all wires from anesthesia monitoring equipment are themselves "isolated" and do not have the ability to "ground" the patient. The dispersal pad from modern (but not all ancient) electrosurgery / electrocautery devices is NOT grounded, precisely to avoid this and other risks.
In this deadly scenario, the current can be thought of as proceeding from the hot wire, to the case of the faulty device, to the patient, through the patient, out of the patient at the point where the patient is grounded, then through the grounded wiring (or even earth ground) back to the power source's white wire, which is also connected to ground.
A fairly modest current can cause ventricular fibrillation through the patient's skin, and a very small current indeed can do the same if the heart or tissues immediately adjacent to the heart, are connected in this manner.