CHAPTER 2: ELECTRICAL SAFETY

Transcription

1 Contents: CHAPTER 2: ELECTRICAL SAFETY 2.1 Physiological Effects of Electricity 2.2 Macroshock and Microshock 2.3 Approaches Towards Protection Against Shock 2.4 Medical Safety Test 2.5 Medical Equipment: Class and Types 2.1 Physiological Effect of Electricity The increased complexity of medical devices and their utilization in most procedures results in high injury each year. Most of the injuries are attributable to improper use of a device as a result of inadequate training and lack of experience. Thus, the need of designing and developing a safer devices. For a physiological effect to occur, the body must become part of an electric circuit. Current must enter and leave the body. Magnitude of current = applied voltage/ (impedance of body tissues) Three phenomena occur when electric current flows through body; Electric stimulation of excitable tissue Resistive heating of tissue Electrochemical burns and tissue damage 1

2 Threshold of Perception When local current density is large enough to excite nerve endings in the skin, the subject feels a tingling sensation. Current at the threshold of perception is the minimal current that individual can detect. Threshold varies among individuals. For dc current is 2 to 10mA. Let-Go Current For higher level of current, nerves and muscles are stimulated resulting to pain and fatigue. Involuntary muscle contraction may cause secondary physical injury. Let-go current is the maximum current which the subject can withdraw voluntarily. Min threshold of let-go current is 6mA. Respiratory Paralysis, Pain and Fatigue Higher current cause involuntary contraction of respiratory muscles severe enough to bring out asphyxiation if current is not interrupted. Respiratory arrest has been observed at 18 to 22mA. Strong involuntary contractions of muscle and stimulation of nerve can be painful and cause fatigue if long exposed. Ventricular Fibrillation Parts of current passing through the chest flows through the heart. Only part of muscle is excited, normal propagation of electrical activity is disrupted. If sufficiently disrupted, HR can rise to 300bpm! Ventricular fibrillation is the rapid and disorganized cardiac rhythm, and does not stop even when current was removed. VF is the major cause of death. Threshold varies about 75 to 400mA. 2

3 Sustained Myocardial Contraction When current is high enough, entire heart muscle contracts. Although heart stop beating while current is applied, normal rhythm happens when current is interrupted. Min current for complete myocardial contraction range 1 to 6A. Burns and Physical Injury Resistive heating causes burns, usually on skin at entry point as skin resistance is high. Voltage greater than 240V can puncture the skin. Brain and other nervous tissue lose all functional excitability when high current pass through. 2.2 Macroshock and Microshock There are two situations which account for hazards from electrical shock They are macroshock and microshock Macroshock In macroshock, the current flows through the body of the subject, e.g. as from arm to arm A physiological response to a current applied to the surface of the body that produced unwanted stimulation, muscle contraction or tissue injury Macroshock is less dangerous than microshock with limits of 10mA 11 Macroshock Factors that reduce the danger of VF; High resistance of dry skin Spatial distribution of current throughout body Design of electric equipment that minimize contact to high voltage Skin and body resistance; Resistance of skin limit the current flows, which can vary with presence of amount of water and natural oil. Most of resistance is in outer layer of epidermis. Any medical procedure that reduce the resistance of skin increase the possibilities of current flow and makes patient more vulnerable to macroshock. Eg: Electrode gel, electronic thermometer, intravenous cathethers. 3

4 Macroshock Electric faults in equipment; Many device have metal chassis and cabinet that medical personnel and patient may touch. If not grounded, insulation failure or shorted component between black hot power lead and chassis results in 115V potential between chassis and grounded object. if patient touch both, macroshock occurs. Chassis and cabinet can be grounded using third green wire in power cord. The ground conductor is not needed for protection against macroshock until hazardous fault develops. Fault inside electric devices may result from failure of insulation, shorted components or mechanical failures that cause shorts. Often, macroshock accidents result from carelessness and failure to correct known deficiencies in the power distribution system and in electric devices Microshock In microshock, the current passes directly through the heart wall This is the case when cardiac catheters may be present in the heart chambers Here, even very small amounts of currents can produce fatal results A physiological response to a current applied to the surface of the heart that produced unwanted stimulation, muscle contraction or tissue injury 15 Microshock Microshock accidents in patients who have direct electric connections to the heart are usually caused by circumstances unrelated to macroshock hazards. Microshock results from leakage current in line-operated equipments or from difference in voltage between grounded conductive surface due to large current in grounding system. Leakage current; Is the small current that flow between any adjacent insulated conductors that are at different potentials. Most important source of leakage current is current that flow from all conductors in electric device to lead wire connected either to chassis or to patient. Leakage current flowing to chassis flow safely to ground if low-resistance ground wire is available. If ground wire broken, chassis potential rise above ground, patient who touch the chassis and has grounded electric connection to heart may receive microshock. If there is connection from chassis to patients heart and connection to ground anywhere on body, microshock could also happens. 4

5 Microshock Conductive surface; Small potentials between any two conductive surface near patient can cause microshock if either surface make contact with the heart and the other surface contact any other part of body. Conductive paths to the heart; Following devices make patient susceptible to microshock; 1. Epicardial or endocardial electrodes of externalized temporary pacemakers 2. Electrodes for intracardiac electrogram (EGM) measuring and stimulating devices 3. Liquid-filled cathethers placed in heart to measure BP, withdraw blood samples, inject substances like dye or drug 5

6 2.3 Approaches Towards Protection Against Shock Two fundamental methods of protecting patients against shock; Patient can be completely isolated and insulated from all grounded objects and all sources of electric current. All conductive surfaces within reach of patient can be maintained at the same potential, which is not necessary at ground potential. In practical environment, those two cannot be fully achieved, so combination is needed. Protection from macroshock to all patients, visitors and staff. Extra protection to patient with reduced skin resistance (wearing electrodes), invasive connection (intravenous cathethers), exposure to wet condition (dialysis). Necessary to consider cost-benefit ratios with respect to both purchase cost of safety equipment and periodic maintenance costs of those equipments. 2.4 Medical Safety Test Why we perform safety test? To ensure the medical equipment is safe to be used for all even the faulty happen to the machine When electrical safety test is performed? During equipment commissioning After perform repair After transportation During routine PPM Before apply to the patient Safety Test Parameters Safety Test Parameters Measure the voltage from the mains Protective Earth Resistance Power cord resistance Parameters Mains Voltage Current Consumption Measure the voltage from pin to pin Measure the current consumption of the equipment Parameters Leakage Current Earth leakage Enclosure leakage current Patient leakage current Insulation Resistance Measure the dielectric strength from L to N or applied 23 part to earth Patient auxiliary 24 leakage current 6

7 Electrical safety analyzers Useful for testing both medical-facility power system and medical appliances. Ranges in complexity from simple conversion boxes used with any volt-ohm meter to computerized automatic measurement systems with barcode readers that generates written reports of test results. Features to consider; Accuracy, Ease of use, Testing time and Cost. Reduce errors caused by incorrect test setup and reduce risk of shock to person performing tests. Testing the electric system 1.Test of receptacles; Receptacles should be tested for proper wiring, adequate line voltage, low ground resistance, and mechanical tension. Common three-light receptacle tester; can only check wiring, Indicate only 8 of 64 possible states for an outlet, have only two states, where each of three outlet contacts has four states hot, neutral, ground and open. Give an OK reading when ground and neutral wires are transposed, and when green and white wires are hot and black wire is grounded. Ground resistance can be measured by passing up to 1 A through ground wire and measuring voltage between ground and neutral. Testing the electric system 2.Test of the grounding system in patient-care areas; NFPA 99 requires both voltage and impedance measurements with different limit for new and existing construction. Voltage between a reference grounding point and exposed conductive surface should not exceed 20mV for new construction. For existing construction, limit is 500mV for general-care areas and 40mV for critical-care areas. Impedance between reference grounding point and receptacle grounding contacts must be less than 0.1Ω for new construction and less than 0.2Ω for existing one. 7

8 Testing the electric system 3.Test of isolated power systems; Isolated power systems should have equipotential grounding that is similar to that of unisolated systems. Line isolation monitor should trigger a visible and audible alarm when total hazard current reaches threshold of 5mA under normal line-voltage condition. Tests of Electric Appliances 1.Ground-pin-to-chassis resistance; Resistance between the ground pin of the plug and equipment chassis, and exposed metal objects should not exceed 0.15Ω during the life of appliance. During the measurement, power cord must be flexed at its connection to the attachment plug and its strain relief where it enters the appliance. Tests of Electric Appliances 2.Chassis leakage current; Leakage current from chassis should not exceed 500µA for appliances with single fault not intended to contact patient and should not exceed 300µA for appliances that are intended for use in patient care vicinity. Limits apply; Whether the polarity of the power line is correct or reversed Whether the power switch of the appliance is in the on or off position Whether or not all control switches happen to be in the most disadvantageous position at the time of testing. When several appliances are mounted together in one rack or cart, and all appliances are supplied by one power cord, the complete rack or cart must be tested as one appliance. 8

9 Tests of Electric Appliances 3.Leakage current in patient leads; Leakage current in patient lead is important because these leads are the most common low-impedance patient contacts. Limits on leakage current in patient leads should be 50µA. Isolated patient leads must have leakage current less than 10µA. Leakage current between individual or interconnected patient leads and ground should be measured with the patient leads active.-fig 2.9 Leakage current between any pair of leads or between any single lead and all other patient leads should be measured.-fig 2.10 Leakage current that would flow through patient leads to ground if line voltage were to appear on the patient should be tested called isolation current or risk current Application of power line voltage and frequency to the isolated patient leads should produce an isolation current to ground that is less than 50µA.-Fig

10 2.5 Medical Equipment: Class and Types The class of equipment defines the method of protection against electric shock All electrical equipment is categorised into classes according to the method of protection against electric shock that is used For mains powered electrical equipment there are usually two levels of protection used, called basic and supplementary protection The supplementary protection is intended to come into play in the event of failure of the basic protection EQUIPMENT CLASSIFICATION Classes Equipment Classification Classes Types Class I Class II Class III Class I Class II Class III Type B Type BF Type CF Classes

11 Class I Equipment Class I equipment has a protective earth The basic means of protection is the insulation between live parts and exposed conductive parts such as the metal enclosure In the event of a fault that would otherwise cause an exposed conductive part to become live, the supplementary protection (the protective earth) comes into effect A large fault current flows from the mains part to earth via the protective earth conductor, which causes a protective device (usually a fuse) in the mains circuit to disconnect the equipment from the supply It is important to realise that not all equipment having an earth connection is necessarily class I Class I Grounded System Class I medical electrical equipment should have fuses at the equipment end of the mains supply lead in both the live and neutral conductors, so that the supplementary protection is operative when the equipment is connected to an incorrectly wired socket outlet. There is no agreed symbol in use to indicate that equipment is class I and it is not mandatory to state on the equipment itself that it is class I Where any doubt exists, reference should be made to equipment manuals Live (L1) Neutral (L2) Ground Class II Equipment The method of protection against electric shock in the case of class II equipment is either double insulation or reinforced insulation In double insulated equipment the basic protection is afforded by the first layer of insulation If the basic protection fails then supplementary protection is provided by a second layer of insulation preventing contact with live parts In practice, the basic insulation may be afforded by physical separation of live conductors from the equipment enclosure, so that the basic insulation material is air The enclosure material then forms the supplementary insulation Class II medical electrical equipment should be fused at the equipment end of the supply lead in either mains conductor or in both conductors if the equipment has a functional earth The symbol for class II equipment is two concentric squares illustrating double insulation Reinforced insulation is defined in standards as being a single layer of insulation offering the same degree of protection against electric shock as double insulation Class II Double Insulated System Hot (L1) Neutral (L2)

12 Class III Equipment Class III equipment is defined in some equipment standards as that in which protection against electric shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present SELV is defined in turn in the relevant standard as a voltage not exceeding 25V ac or 60V dc In practice such equipment is either battery operated or supplied by a SELV transformer If battery operated equipment is capable of being operated when connected to the mains (for battery charging) then it must be safety tested as either class I or class II equipment It is interesting to note that the current IEC standards relating to safety of medical electrical equipment do not recognise Class III equipment since limitation of voltage is not deemed sufficient to ensure safety of the patient All medical electrical equipment that is capable of mains connection must be classified as class I or class II Medical electrical equipment having no mains connection is simply referred to as internally powered Types The degree of protection for medical electrical equipment is defined by the type designation The reason for the existence of type designations is that different pieces of medical electrical equipment have different areas of application and therefore different electrical safety requirements For example, it would not be necessary to make a particular piece medical electrical equipment safe enough for direct cardiac connection if there is no possibility of this situation arising Type B Type BF Types Type CF

13 Type B The applied part is connected to earth External application to the patient Provides protection against electric shock by taking into account the fault current Not suitable for direct application to the heart Type BF The applied part is insulated from the equipment (no connection to earth) External application to the patient, higher-quality protection against electric shock than in insulation class B Not suitable for direct application to the heart Type CF The applied part is insulated to a high level from the equipment (no connection to earth) External and intracardiac application to the patient, higher-quality protection against electric shock than in insulation class BF Suitable for direct application to the heart 49 The End 13