阴霾的天气,仿佛永远都停不下来的.pptVIP

ENTC 4350,Electrical Safety,Hospital electrical safety begins with the principles that we have discussed. An electrical shock is always unpleasant, but it can be lethal in the intensive care unit.,It is extremely important that all hospital personnel be constantly on the watch for manufacturing defects or wear and tear of critical parts. There are documented cases where equipment from reputable manufacturers was delivered with ground wires disconnected, cords broken, and improperly installed plugs. In the meantime, there is still the patient; it is your patient and your responsibility. You are the one who must be suspicious and check the equipment when it comes from the factory.,Even if equipment is in perfect condition when it arrives from the manufacturer, it is subject to the normal wear and tear of daily hospital use. This type of deterioration may be very severe if the equipment is dragged around, in a great rush, from one room to another in response to emergencies. Once again, the part of the system that is most likely to be damaged is the cord and plug assembly.,Quite often, the damage is not visible on a mere surface examination; you have to get out your VOM or continuity tester and test it. Connect the continuity tester or VOM between the ground plug on the end of the cord and the metal case of the instrument. If the test light goes out when you wiggle or pull on the wire, or If the resistance measured by the VOM is erratic when you move the cord, then the appliance is defective.,If your hospital has a red tag service that allows defective equipment to be marked for immediate pickup and repair, all is well. However, if there is any danger that the equipment might be used in patient service before the repair is done, the best thing to do is take your handy bandage scissors and cut the plug off. That may sound like a drastic measure (surgery is always drastic), but in this case, it is quite justified.,Another thing to watch for is someone elses home repair.” This is particularly apparent in hospitals where one sees cracked cords or broken plugs repaired with adhesive tape. That cord or plug cracked for a reason: either age or misuse is usually to blame.,If the insulation is cracked, most of the conducting wires may be broken, too. Just suppose that the last strand of ground wire broke when it was being used on your patient, and reflect upon the results of our computations with the current divider equations previously.,In this regard, you have to watch the other staff membersi.e., the orderlies, aides, and so onsince the natural human tendency is to put the broken item back on the shelf and take one that looks all right. Quite often, an aide will hesitate to report defective equipment for fear of being thought to be a troublemaker. Only endless repetition, and possibly a cash prize for reporting defects, will alleviate this situation.,It should he clear to everyone that if any defective equipment is noted, or if a tingle is sometimes felt when using a piece of equipment, this is a signal to stop using the equipment and report it.,The patient is truly at your mercy, and equipment that comes near to him or her must be in proper condition. At this point, you might be wondering just what proper condition is and how leakage occurs. The specifications on electrical leakage are complex and subject to change; however, two good points to keep in mind are the leakage to the chassis of hospital equipment and the leakage through any patient-connected leads.,With the ground wire disconnected, the chassis leakage is limited to 100 mA, and the patient-lead leakage must not exceed 50 mA. There are many causes for leakage: defective insulators, damaged wire, dirt, water, and the radiation leakage.,Figure 14.3 Let-go current versus frequency Percentile values indicate variability of let-go current among individuals. Let-go currents for women are about two-thirds the values for men. (Reproduced, with permission, from C. F. Dalziel, “Electric Shock,“ Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson IIII, 1973, 3, 223-248.),Figure 14.1 Physiological effects of electricity Threshold or estimated mean values are given for each effect in a 70 kg human for a 1 to 3 s exposure to 60 Hz current applied via copper wires grasped by the hands.,They all add up to a problem for the hospital.,The danger of having a single hospital appliance with a defective three-wire cord is illustrated below. Here we show a patient in an electrical bed with a three-wire cord that is good.,This means that when the patient puts his hand on the bed rail, he is grounded. too. There is nothing wrong with that until someone brings over a second appliance, like an ECG or an apnea monitor, which has a defective three-wire cord. The manufacturer designed the appliance with the idea that the three-wire cord would be operational and that stray leakage in his unit would be grounded off to the case and removed by the ground wire. Unfortunately. in this case, the third wire is broken, and the leakage current goes back to the powerhouse via the patient with disastrous results.,Every time you wheel a piece of electrical equipment up to a patient, you have to ask yourself, “Am I sure that the ground wire is OK?”,Figure 14.5 Effect of entry points on current distribution (a) Macroshock, externally applied current spreads throughout the body. (b) Microshock, all the current applied through an intracardiac catheter flows through the heart. (From F. J. Weibell, “Electrical Safety in the Hospital,“ Annals of Biomedical Engineering, 1974, 2, 126-148.),UNDERWRITERS LABORATORIES STANDARDS,There have been some gaps in the design of medical equipment, but these holes will be closed as more hospitals require that all new equipment meet the Underwriters Laboratories Standards for Medical and Dental Equipment (UL 544). The important thing about UL 544 is its marking code for guidance in equipment application.,Type A apparatus, the highest grade, is suitable for electrically susceptible patients. This means that the leakage current has been held to the lowest possible value, and the greatest measure of safety has been provided for patients in intensive care, cardiac care, or catheterization units.,Type A equipment is very costly, and for this reason, a somewhat lower standard is used for type B, which applies to equipment not suitable for electrically susceptible patients. This equipment is not defective or poorly built. The designation is simply a recognition that the precautions needed in the CCU, for example, are not appropriate for the general medical patient.,The last, or type C, equipment label is intended for laboratory apparatus where patient contact is unlikely. In some cases, no marking will be used on type C equipment, but the hospital may want to have stickers saying not for use outside the laboratory area or not for use on patients. The UL 544 code designation is one more item to be checked when new apparatus is brought in for patient use.,Regulation of Medical Devices,In 1976, the U.S. Congress passed what are known as the Medical Device Amendments (Public Law 94-295) to the Federal Food, Drug, and Cosmetics Act. Further amendments were made in 1990 in the Safe Medical Devices Act.,Medical devices are defined as: any item promoted for a medical purpose that does not rely on chemical action to achieve its intended effect.,Medical devices are classified in two ways: The division of such devices into Class I, Class II, and Class III. Based upon the principle that devices that pose greater hazards should be subject to more regulatory requirements.,Seven categories Preamendment, Postamendment, Substantially equivalent, Implant, Custom, Investigational, and Transitional.,Software used in medical devices has become an area of increasing concern. Several serious accidents have been traced to software bugs. Increased requirements for maintaining traceability of devices to the ultimate customer, postmarketing surveillance for life-sustaining and life-supporting implants, and hospital reporting requirements for adverse incidents were added to the law in 1990.,Class I: General Controls,Manufacturers are required to perform registration, premarketing notification, record keeping, labeling, reporting of adverse experiences, and good manufacturing practices. Apply to all three classes.,Class II: Performance Standards,These standards were to be defined by the federal government, but the complexity of procedures called for and the enormity of the task have resulted in little progress having been made toward defining 800 standards needed. The result has been overreliance on the postamendment “substantial equivalence” known as the 510(k)process.,Class III: Premarketing Approval,Such approval is required for devices used in supporting or sustaining human life and preventing impairment of human health. The FDA has extensively regulated these devices by requiring manufacturers to prove their safety and effectiveness prior to market release.,WALL RECEPTACLES,There are times when it is important for you to be able to find the neutral, hot, and ground jacks of the wall receptacles or outlets. Suppose. for example. that you move into a new (or new to you) facility with three-wire grounded receptacles. All is well until you begin worrying that the contractor might have forgotten to hook up the ground wire at some ot the receptacles. It has been our experience that many contractors do not install the receptacles correctly.,Figure 14.6 Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz.,The protection that you think you have is not there, and you might not want to find that out the hard way. When the hospital is new, the contractor will fix a little thing like this when the building is 10 years old, however, he may be somewhat hard to find.,We suggest that when you move into a facility, you whip out your VOM or outlet tester and check the receptacles. A typical outlet tester is shown below.,Figure 14.16 Three-LED receptacle tester Ordinary silicon diodes prevent damaging reverse-LED currents, and resistors limit current. The LEDs are ON for line voltages from about 20 V rms to greater than 240 V rms, so these devices should not be used to measure line voltage.,Its dimensions are those of a large spool of thread. These gadgets are not perfect: a poor groundi .e one with high resistancemight check out all right, because the neon bulbs inside the tester only require a small current to light up (a neon bulb operating at 65 volts and 0.25 watt will light up even when the ground resistance is as high as 13,000 ohms). However, that is no excuse to avoid using a gadget of this type, because something is always better than nothing.,A better method for checking the wall receptacles involves the use of your VOM.,The right-hand opening should be hot,” The left should be “neutral,” and The other opening should be “ground”,Figure 14.18 (a) Chassis leakage-current test. (b) Current meter circuit to be used for measuring leakage current. It has an input impedance of 1 k and a frequency characteristic that is flat to 1 kHz, drops at the rate of 20 dB/decade to 100 kHz, and then remains flat to 1 MHz or higher. (Reprinted with permission from NFPA 99-1996, “Health Care Facilities,“ Copyright 1996, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject, which is represented only by the standard in its entirety.),Grounding-contact switch (use in OPEN position),Polarity- reversing switch (use both positions),Appliance power switch (use both OFF and ON positions),This connection,is at service,entrance or on,supply side of,separately derived,system,Building ground,(a),(b),H (black),N (white),Appliance,G,N,H,H = hot,N = neutral (grounded),G = grounding conductor,I, 500 A for facility,owned housekeeping and maintenance appliances,I, 300 A for appliances intended for use in the patient vicinity,120 V,G (green),mV,Figure 14.17 Ground-pin-to-chassis resistance test,Any hospital that hopes to avoid lawsuits should have a regular program for checking the outlets. One of the things that we have found to be important is the periodic testing of the actual resistance of the hospitals ground circuit. This requires a special electrical gadget that the Maintenance Department will have to buy and use regularly.,Another good rule requires the replacement of all cracked receptacles, even when they test out “OK.” A cracked receptacle is like high blood pressure; it is a signal for you to do something about it or expect to pay the penalty.,ELECTRICAL BEDS,THE ELECTRICAL BED,Electrical beds are great gadgets; they have saved thousands of nurses millions of hours of cranking up and down. Like most gadgets, however, they have their bad aspects. A patient in a standard hospital bed on the usual rubber tiled floor is effectively “floating” in the electrical sense. The term floating simply means that he or she is not electricaIy connected to the ground.,The conventional electrical bed has its frame and motor case connected to electrical ground via a three-wire cord. This could be hazardous if the patient came in contact with an appliance that had an electrical leak” while he was touching a grounded, metal bed rail. To avoid this hazard, manufacturers have introduced a variety of systems ranging from insulated bed rails to doubly insulated motors.,Three points to keep in mind are:,Bed-related injuries to patients, such as falls and shock, are the leading cause of patient and visitor trauma. Such injuries outnumber, by a factor of five, any other type of in-hospital injury.,The conventional electrical bed has its frame and motor case connected to electrical ground via a three-wire cord. One hand on the grounded bed rail and the other on a defective bed lamp can be a formula for disaster. Other problems can occur if any conductive liquids, such as water, blood, or urine, spill on the bed and leak through to the motor. This might allow current to flow from the motor to the patient via the grounded bed rail.,If the bed rails are ungrounded or are not connected to the grounded frame, the above hazards might be alleviated but you had better check this with your handy VOM to make sure no electrical path exists. Ungrounded bed components, however, can act as antennas and pick up signals that interfere with in-bed ECG or EEG studies.,A doubly insulated system has the usual layers of functional insulation in the motor. In addition, the motor itself is isolated from the bed frame by a layer of insulating material. Mechanical power is transmitted from the motor to the bed via a nonconductive coupling.,This does provide an added measure of safety, provided we recognize that even the best insulation will lose its insulating qualities if it gets dirty, wet, or both. Dirt holds moisture, and the combination of dust and water, blood, or urine could be a pretty good conductor.,The Double Insulation System,The doubly insulated system can be purchased with several different arrangements: Double insulated with the motor electrically isolated from the bed frame. If there is a two-wire cord, the bed and the motor are “floating” in the electrical sense. Double insulated with a three-wire cord that grounds the motor but not the bed, because the motor is electrically isolated from the bed. Here again, the bed is “floating” rather than grounded. Double insulated with a three-wire cord that grounds the bed and the motor. In this case, the bed is grounded.,The Use of Isolated Power,To appreciate isolation, we have to introduce a new electrical gadget: the transformer. A transformer has two coils of wire; One is called the primary, and The other the secondary coil.,When an AC voltage is applied to the primary coil, a voltage appears at the secondary coil. The important point here is that the transfer of power from the primary to the secondary occurs without any physical connection, such as a wire, between the two coils. All transfer occurs by means of electromagnetic waves.,The circuit diagram for a transformer is shown below. Note the primary and secondary coils and the input and output voltages.,A transformer for an isolated electrical bed has the dimensions of a cube about one foot on each side and weighs about 50 pounds. The important thing to note in the figure is that there is no voltage between either output “A” or output B” and ground.,This can provide an important measure of patient protection; For example, if the bed is grounded and the patient puts one hand on the bed rail, he or she will be grounded, too, If something should go wrong with the bed and if either wire A or wire B should touch the patient, there would be no chance of injury, because there is no voltage between either A or B and ground.,Figure 14.7 Power-isolation-transformer system with a line-isolation monitor to detect ground faults.,The next question might be, “Which one is best?” Our choice might be 1 or 2 above, because the “floating” bed provides an added measure of patient protection. However, it may well be that NFPA will stick to their ruling that all beds must be grounded, in which case you may as well purchase the third type and save money.,If the patient, or anyone else, touches hot wire A and wire B at the same time, however, they have just grabbed 115 volts. To maintain isolation of power, it is most important that neither wire A nor B touch the bed or patient, because once contact is made with one wire, the next accidental contact with the other wire could be fatal.,It thus is a system that gives you one error for free, in that contact of either wire A or wire B with the patient does not produce any shock, but, if one wire rema