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Electrical Equipment Grounding
Electrically powered equipments, be they medical instrumentation or devices employed in some other environment, operate by using electrical voltages and currents to represent information. Information is received and passed on by the instrumentation in the form of electrical voltages and currents passed through a point of debarkation called an interface.
By its very definition "voltage" is the "work" that the equipment performs in moving an electrical charge to some point in space from a reference point which is called "ground." Any electrical equipment has a ground reference point. In many cases this reference point is electrically tied to actual earth ground, by a wire connection to a cold water pipe for example. In other cases some continuous metal surface, usually, the equipment's enclosure, is designated as the reference point and referred to as frame ground, and connected to the AC power ground.
In any environment, be it that associated with health care or some other, problems occur when different electrical equipments-instruments- must be interconnected in order to pass information from one to the other.
The first area of concern is proper representation and interpretation of instrument measurements. If the different instruments are employing different ground references there will immediately be problems of accuracy. Basically, measurements are being made by the two instruments, but using a different reference point. It is a little like measuring the height above the surface of the earth from two different points on the slope of a hill- which is why people usually measure height from a sea level. In passing the information from an electrical instrument employing one ground to an instrument employing another misinterpretations about the true level of measurements occur.
The second area of concern occurs because of the generation of ground currents. When two electrically powered instruments are tied together, with different grounds, an electrical current will be generated going from one instrument to the other through the ground. This current is generated in an attempt to reach electrical equilibrium and bring both grounds to the same electrical potential. The problem with this ground current is that it is not confined to any wire, cable or waveguide. It can "spill" over the entire environment especially through places where the electrical conductivity is high. In particular, it can reach the patient and his cardiovascular system and cause damage especially if it has an alternating component.
Impact of the Electrical Problem on the Health Care Environment
The use of the multiplicity of electrical and electronic devices required in the care of patients exposes them to ground currents directly (and secondarily to ElectroMagnetic and Radio Frequency Interference -EMI and RFI). This exposure results in two types of hazards in the health care environment. The first is the risk of electrical shock, burns, internal organ damage and potential cardiac arrhythmias directly due to leakage current resulting from the improper grounding. This can be further exasperated by the electrical conductivity of body fluids and the presence of spilled electrolyte solutions in the patient care area-posing a risk not only to patients but also to medical care givers. The second type of hazard results from electrical interference among devices employed in the facility. This effect causes a degradation of the performance of the diagnostic and monitoring devices resulting in inaccurate readings which may in turn cause ineffective and/or inappropriate care.
Standard alternating current of 60 Hz resulting from ground currents is particularly dangerous to the heart as it can induce ventricular fibrillation at lower energy levels than either direct current or alternating current at either higher or lower frequencies. This risk is especially important when alternating current devices are coupled to and power catheters such as those used in electrophysiologic diagnostic or therapeutic procedures. The conductivity of blood and the proximity of those devices to the cardiac conduction system can amplify the effects of even small leakage currents.
Improperly grounded externally applied diagnostic and therapeutic devices can also expose the patient to risk of electrical burns or cardiac arrhythmias. These devices include electrocautery, cardiopulmonary monitors and defibrillators. All use a powered device coupled to an electrical pathway to the skin. Often the high resistance of the skin (>50,000 Ohms) is reduced by mechanical or chemical means to allow proper operation of these devices.
Patients in the operating room, critical care units and cardiac procedure rooms (catheterization and electrophysiology laboratories) are particularly at risk to errant ground currents because of the number and complexity of devices with which they routinely come into contact. Medical personnel can be exposed to biohazards in these situations. It is in these areas that the control of extraneous ground currents is vital for both patient safety and fidelity of diagnostic monitoring. Thus, use of proper grounding and electrical equipment isolation is an absolute requirement.
Examples of clinical problems caused by improperly grounded electrical equipment abound. Improperly grounded defibrillators have sustained ventricular fibrillation when used to abolish the rhythm. Ventricular fibrillation has also occurred, in one experience, when an electrically leaking electrophysiologic stimulator was used in a cardiac arrhythmic study. This malfunctioning device almost convinced the physician that the patient had an inducible ventricular tachyarrhythmia when in fact, none was present.
Available Solutions to the Electrical Problem
What then is the answer to such hazards and risks caused by vulnerability to ground currents? Three (3) solutions are discussed below.
The first solution seems the most obvious- simply tie all instruments to the same physical point as the ground. Unfortunately, except in cases of the simplest and smallest equipment, collocated in the most confined environment, this can not be done. Instrumentation is just not close enough to employ the same ground.
The second solution is not to tie the instruments together electrically by metallic wire or cable but instead to tie them together optically and employ fiber optic cable. Here the voltage representing information coming out of an interface is converted to light at a certain intensity. This propagates to the other instruments by fiber optic cable. At the other instrument the light intensity is converted back into a voltage and inputs the interface. Replacing the electrical interconnection of the instruments with the optical connection essentially separates the two instruments into two (2) totally different "electromagnetic universes" with no possible electrical connection between them- and therefore no ground current. From an electromagnetic perspective the conversion at an interface of an output information bearing voltage to light appears to be the same as driving an infinite impedance- with no current then being generated. The conversion of incoming light to an information bearing voltage at an interface appears to be the same as a current source. Besides being a supplier of the fiber optic cable Telebyte has several products which allow this second solution to be realized. These are the Model 271, Model 272 and Model 276 and are shown in Figure 1. These products are respectively employed when the instruments interface is either EIA-232 or RS-422 or RS-485.
Figure 1: Model 271 - Auto Powered Fiber Optic Line Driver, Model 272 - RS-422 to Fiber Optic Line Driver and Model 276 - RS-485 to Fiber Optic Line Driver
The optical solution is very attractive. Besides protecting against ground currents it also protects the information being dealt with from a variety of other deleterious effects such as ElectroMagnetic Interference (EMI) and Radio Frequency Interference (RFI). However, the optical solution while not prohibitively expensive does come at some cost. Fiber optic cable is much more expensive than metallic cable, either twisted pair or coaxial cable, although year by year its price gets lower. The list prices of the Model 271, Model 272 and Model 276 are $145, $152 and $152. These are not excessively high, but used in pairs all over a health care facility the additional burden can be significant.
The third solution provides the protection of the second solution, although, not as all inclusive, but at a lower cost. This solution uses a device called an "Opto Isolation Module" between instrument interfaces. The Opto Isolation Module is like a mini optical (but not fiber optic) communication system contained within a box. The Opto Isolation Module would plug into the instrument's interface. Within a Module's case the information bearing voltage is converted to light using a Light Emitting Diode (LED). Then, without a metallic wire connection the information bearing light is reconverted back to a voltage using a photodiode. These functions are performed by a class of integrated circuits called "optocouplers." This voltage can then be transferred to another instrument's interface. This mini optical communication system allow separate grounds to be used on different equipments without the worry of generating any ground currents which can harm the patient. Telebyte manufactures several different Opto Isolation Modules for equipment with different interfaces. Examples are the Model 281 for RS-422 and Model 268 for RS-232 which are shown in Figure 2 . The list price of these units is $158 and $106 respectively.
Figure 2: Model 281 - RS-422 Opto Isolation Module and Model 268 - RS-232 Opto Isolation Module
This third solution has the additional advantage of being able to provide interface conversion along with the isolation in the same unit. In order for two instruments with different interfaces "to communicate" the output of one interface must be made compatible with the other in addition to having the equipments' ground differences dealt with. Another reason for carrying out the interface conversion may be to deal with the distance separating the instruments.
In some cases the instruments communicating may both have an EIA-232 interface, but may be separated by several hundred feet. The EIA-232 interface is specified for reliable communication of data only up to fifty(50) feet. One way of handling this distance separation would be to use modems to interconnect the equipments. However, modems maybe overkill having the ability to transport data several thousand feet in this environment. A solution more tailored to this problem could be to convert the EIA-232 interface at either end to an interface which supports interchange of information over much longer distances. RS-422 is one such interface. Furthermore, it has the added advantage of using differential signalling and providing additional protection against electrical noise, EMI and RFI, although not as much as a fiber optic connection. Thus, it is often necessary to carry out interface conversion along with protection against ground currents.
Figure 3: Typical Usage of Model 245 in a hospital environment
The Model 245 is an example of an Opto Isolated interface converter that is selectable to convert interfaces RS-422 or RS-485 to EIA-232. It has a list price of $226. Another example of an optically isolated interface converter is the Model 287 EIA-232 to RS-422 Opto Isolated Interface Converter. This product is unique in the fact that it does not require and separate AC or DC power supply. It derives its power from the interface. It has a list price of $155. We show how the Model 245 may be used in a typical hospital environment in Figure 3. The Model 245 and the Model 287 are Figure 4.
Figure 4: Model 245 - Opto-Isolated Converter and Model 287 - Auto Powered Optically Isolated Interface Converter