Biomedical Instrumentation (BME420 ) Chapter 8: Electrical Safety John G. Webster 4 th Edition Dr. Qasem Qananwah

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1 Biomedical Instrumentation (BME420 ) Chapter 8: Electrical Safety John G. Webster 4 th Edition Dr. Qasem Qananwah 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 1 INTRODUCTION The patient in hospital is the center of care, but he is also helpless in the center of potential dangers, which are in the industry, long time ago, as such identified (i.e. chemicals, electricity, radiation). Safety in hospital means firstly patient safety, but it means also safety of operators and others. Electrical safety is a very important element in hospital safety. The electrical safety of the medical equipment in hospital is the most important of it. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 2 1

2 Med. Eng. & El. Safety Assurance the highest possible level of med. Equipment safety in hospital is one of the most important tasks of the med. / clinical engineer. The med. / clinical engineer, therefore, must be aware of and very familiar with the issues of the electrical safety of the medical equipment in hospital. Electrical Safety means electrical shock protection. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 3 The Mechanism of the El. Shock El. Shock occurs when a victim is a part of an electrical circuit (an element closing it), in which an electrical current can flow and has the ability to harm the victim or even cause death (electrocution). That means consequently that there must be a simultaneous two-points contact of the victim with the electrical shock circuit. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 4 2

3 El. Shock = Closing the El. Shock Circuit Accordingly, el. Shock must be a very rare and unusual event, which requires unusual circumstances. But it is not. Why? Usually el. Shock occurs when the victim contacts one voltage carrying line only. How is the shock circuit completed? The 2 nd necessary contact point is usually with things connected to earth, which are everywhere. What is the secret of this apparent paradox with the statement that earthing makes safety higher? 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 5 El. Power Distribution System For technical reasons, neutral point (and consequently the neutral line) is deliberately connected to earth. It is this connection that makes the electrical service a grounded system. Understanding this is the key for understanding the mechanism of electric shock and electrocution. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 6 3

4 Two Kinds of Grounding / Earthing Grounding of Electrical Systems: Connecting N-line of the service side to earth due to technical reason and for protection of systems and plants (removing the floating high voltage in the secondary (service) side of the distribution transformer). Protective Grounding: Connecting conducting parts, which are not intended for carrying current in normal circumstances (enclosures; switch-, fuse-, outlet- metal boxes; etc.) via 3rd conductor (which, in normal situations, does not carry current) to earth. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 7 Leakage Currents: Caused by stray capacitances, which are always present between conducting surfaces. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 8 4

5 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 9 From John G. Webster (ed.), Medical instrumentation application and design, 4th ed., John Wiley & Sons, This material is reproduced with permission of John Wiley & Sons, Inc. Figure 14.1 Physiological effects of electricity Threshold or estimated mean 6/2/2014 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. BME 420 Department of Biomedical Systems and Informatics Engineering 10 5

6 Touch Current Effects effect Current (ma) Un sensed A1 Muscle contraction. Let-go possible A2 Pain. Increasing probability of let-go impossibility 6 15 B1 range Passing let-go threshold. Minor effects on breathing & blood circulation. Let-go impossible. Increased heart beat & blood pressure (BP). Arrhythmia. Breathing irregularities B B3 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 11 Touch Current Effects effect Current (ma) range Increased probability of ventricular fibrillation (VF) if shock interval more than one second (or the interval between 2 heart beat). Arrhythmia. Cardiac arrest. Severe breathing irregularities. Increased BP. Often VF if shock interval more than one second (or the interval between 2 heart beat). Nevertheless as in C C C2 Increasing VF probability, even if shock interval less than one second (or the interval between 2 heart beat). Nevertheless as in C1 & C2. If shock lasts for more than on second (or the interval between 2 heart beat), it has lethal consequences D 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 12 6

7 Touch Current Effects effect Current (ma) range Often VF even if shock interval is less than (0.1) second. Thermal effects appear if shock lasts for more than (10) second E Like E concerning heart. Increasing probability of burns of muscles and limbs if shock lasts for more than (5) second. More than 2000 ma Continuous cardiac contraction. Temporary breathing paralyzed. Burns More than 5000 ma F 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 13 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 14 7

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10 Figure 14.6 Effect of entry points on current distribution (a) Macroshock, 6/2/2014 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, ) BME 420 Department of Biomedical Systems and Informatics Engineering 19 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 20 10

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12 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 23 Figure Macroshock due to a ground fault from hot line to equipment cases for (a) ungrounded cases and (b) grounded chassis. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 24 12

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14 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 27 Figure Microshock leakage-current pathways. Assume 100 μa of leakage current from the power line to the instrument chassis, (a) Intact ground, and 99.8 μa flows through the ground, (b) Broken ground, and 100 μa flows through the heart, (c) Broken ground, and 100 μa flows through the heart in the opposite direction. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 28 14

15 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 29 Protection: Power Distribution Grounding System Isolated power distribution syytem Ground fault circuit Interrupters (GFCI) 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 30 15

16 Grounding System Low resistance grounds that can carry currents up to the circuitbreaker ratings are clearly essentials for protecting patients against both macroshocks and microshoks, even when isolated power sytem is used. A grounding system protects patients by keeping all conducive surfaces and receptacle grounds in the patient s enviroment at the same potential. It also prtects the patient from graound faults at other locations. The grounding system ha a patient-equipment grounding point, a reference grounding point and connections. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 31 Figure Grounding system All the receptacle grounds and conductive surfaces in the vicinity of the patient are connected to the patient-equipment grounding point. Each patient-equipment grounding point is connected to the reference grounding point that makes a single connection to the building ground. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 32 16

17 Isolated power-distribution system Unfortunatly, even a good equipotential grounding system cannot eliminate voltages produced between grounds by large ground faults that cause large ground currents. The isolation transformer prevents this unlikely hazard. The IPS also reduces leakage current somewhat but not below the 10 microa. IPS protects againts microshock. IPS provide considerable protection against macroshock. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 33 Figure 14.9 Power-isolation-transformer system with a line-isolation monitor to detect ground faults. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 34 17

18 Isolation of both conductors from ground is commonly acheived with an isolation transformer. In an isolation system such as this, if a single ground fault from either conductor to ground accours, the system simply reverts to a normal grounded sytem. A second fault from the other conductor to ground is then required to get large currents to ground. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 35 Ground Fault circuit Interrupters (GFCI) Ground-fault circuit Interrupters disconnect the source of electric power when a ground fault greater that about 6mA occurs. The GFCI senses the difference between thease two currents and interrpts power when this difference, which must be flowing to ground somewhere, exceeds the fixed rating. The device makes no distinction among paths the currents takes to ground: that path may be via the ground wire or through the person itself. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 36 18

19 Figure Ground-fault circuit interrupters (a) Schematic diagram of a solid-state GFCI (three wire, two pole, 6 ma). (b) Ground-fault current versus trip time for a GFCI. [Part (a) is from C. F. Dalziel, "Electric Shock," in Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson III, 1973, 3: ] 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 37 This Week Safety in the clinical environment: Electrical safety Physiological effects of electricity Susceptibility parameters Distribution of electrical power Isolated power systems Macroshock hazards Microshock hazards Electrical safety codes and standards Protection Power distribution Ground fault circuit interrupters (GFCI) Equipment design 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 38 19

20 Safety in Clinical Environment Electrical hazards Electrical shocks (micro and macro) due to equipment failure, failure of power delivery systems, ground failures, burns, fire, etc. Mechanical hazards mobility aids, transfer devices, prosthetic devices, mechanical assist devices, patient support devices Environmental hazards Solid wastes, noise, utilities (natural gas), building structures, etc. Biological hazards Infection control, viral outbreak, isolation, decontamination, sterilization, waste disposal issues 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 39 Radiation hazards Electrical Safety Many sources of energy, potentially hazardous substances, instruments and procedures Use of fire, compressed air, water, chemicals, drugs, microorganisms, waste, sound, electricity, radiation, natural and unnatural disaster, negligence, sources of radiation, etc. Medical procedures expose patients to increased risks of hazards due to skin and membranes being penetrated / altered 10,000 device related injuries in the US every year! Typically due to Improper use Inadequate training Lack of experience 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 40 20

21 Physiological Effects of Electricity For electricity to have an effect on the human body: An electrical potential difference must be present The individual must be part of the electrical circuit, that is, a current must enter the body at one point and leave it at another. However, what causes the physiological effect is NOT voltage, but rather CURRENT. A high voltage (KV,(10 3 V)) applied over a large impedance (rough skin) may not cause much (any) damage A low voltage applied over very small impedances (heart tissue) may cause grave consequences (ventricular fibrillation) The magnitude of the current is simply the applied voltage divided by the total effective impedance the current faces; skin : largest. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 41 Physiological Effects of Electricity The real physiological effect depends on the actual path of the current Dry skin impedance:93 kω / cm 2 Electrode gel on skin: 10.8 kω / cm 2 Penetrated skin: 200 Ω / cm 2 Physiological effects of electricity. Threshold or estimated mean values are given for each effect in a 6/2/ kg human BME 420 for a 1 to 3 s exposure Department to 60 of Hz Biomedical current applied Systems via copper and wires Informatics grasped by Engineering the hands

22 Physiological Effects of Electricity Threshold of perception: The minimal current that an individual can detect. For AC (with wet hands) can be as small as 0.5 ma at 60 Hz. For DC, 2 ~10 ma Let-go current: The maximal current at which the subject can voluntarily withdraw. 6 ~ 100 ma, at which involuntary muscle contractions, reflex withdrawals, secondary physical effects (falling, hitting head) may also occur Respiratory Paralysis / Pain / Fatigue At as low as 20 ma, involuntary contractions of respiratory muscles can cause asphyxiation / respiratory arrest, if the current is not interrupted. Strong involuntary contraction of other muscles can cause pain and fatigue Ventricular fibrillation 75 ~ 400 ma can cause heart muscles to contract uncontrollably, altering the normal propagation of the electrical activity of the heart. HR can raise up to 300 bpm, rapid, disorganized and too high to pump any meaningful amount of blood ventricular fibrillation. Normal rhythm can only return using a defibrillator Sustained myocardial contraction / Burns and physical injury At 1 ~6 A, the entire heart muscle contracts and heart stops beating. This will not cause irreversible tissue damage, however, as normal rhythm will return once the current is removed. At or after 10A, however, burns can occur, particularly at points of entry and exit. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 43 Important Susceptibility Parameters Threshold and let-go current variability Distributions of perception thresholds and let-go currents These data depend on surface area of 6/2/2014 contact, BME moistened 420 hand grasping Department AWG No. of 8 Biomedical copper wire, Systems 70 kg human, and 60Hz, Informatics 1~3 s. of Engineering exposure 44 22

23 Important Susceptibility Parameters Frequency Note that the minimal letgo current happens at the precise frequency of commercial power-line, 50-60Hz. Let-go current rises below 10 Hz and above several hundred Hz. Let-go current versus frequency Percentile values indicate variability of let-go current among individuals. Let-go currents for women are about twothirds the values for men. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 45 Important Susceptibility Parameters Duration The longer the duration, the smaller the current at which ventricular fibrillation occurs Shock must occur long enough to coincide with the most vulnerable period occurring during the T wave. Weight Fibrillation threshold increases with body weight (from 50mA for 6kg dogs to Fibrillation current versus shock duration. Thresholds for ventricular fibrillation in animals for Hz AC current. ma for Duration 24 kg of dogs. current (0.2 to 5 s) and weight of animal body were varied. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 46 23

24 Important Susceptibility Parameters Points of entry The magnitude of the current required to fibrillate the heart is far greater if the current is applied directly to heart; externally applied current loses much of its amplitude due to current distributions. Large, externally applied currents cause macroshock. If catheters are used, the natural protection provided by the skin 15 kω ~ 2 MΩ) is bypassed, greatly reducing the amount of current req d to cause fibrillation. Even smallest currents (80 ~ 600 μa), causing microshock, may result in fibrillation. Safety limit for microshocks is 10 μa. The precise point of entry, even externally is very important: If both points of entry and exit are on the same extremity, the risk of fibrillation is greatly reduced even at high currents (e.g. the 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 47 Important Susceptibility Parameters Points of entry 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 6/2/2014 heart. BME 420 Department of Biomedical Systems and Informatics Engineering 48 24

25 Distribution of Electrical Power (230 V) Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 49 Distribution of Power If electrical devices were perfect, only two wires would be adequate (hot and return), with all power confined to these two wires. However, there are two major departures from this ideal case: A fault may occur, through miswiring, component failure, etc., causing an electrical potential between an exposed surface (metal casing of the device) and a grounded surface (wet floor, metal case of another device etc.) Any person who bridges these two surfaces is subject to macroshock. Even if a fault does not occur, imperfect insulation or electromagnetic coupling (capacitive or inductive) may produce an electrical potential relative to the ground. A susceptible patient providing a path for this leakage curren to flow to the ground is subject to microshock. The additional ground line provides a good line of defense! (how / why?) 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 50 25

26 GROUND! The additional line that is connected directly to the earth-ground provides the following: In case of a fault (short circuit between hot conductor and metal casing), a large current will use the path through the ground wire (instead of the patient) and not only protect the patient, but also cause the circuit breaker to open. The ability of the grounding system to conduct high currents to ground is crucial for this to work! If there is no fault, the ground wire serves to conduct the leakage current safely back to the electrical power source again, as long as the grounding system provides a lowresistance pathway to the ground Leakage current recommended by ECRI are established to prevent injury in case the grounding system fails and a patient touches an electrically active surface (10 ~ 100 μa). 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 51 Isolated Power Distribution Not grounded! Normally, In fact, in such when an there isolated a ground-fault system, if a single from hot ground-fault wire to ground, occurs, a the large system current simply is drawn reverts causing back to a potential the normal hazard, ground-referenced as the device will system. stop functioning when the circuit breakers open! A This line can isolation be prevented monitor by using is used the with isolated such system, that which continuously separates ground monitors from for neutral, the first making ground 6/2/2014 neutral fault, during and hot which electrically case it simply identical. informs A single the ground-fault operators to will fix the not problem. cause large The currents, single ground as long fault as both does NOT hot BME conductors constitute 420 are a hazard! initially Department isolated from of Biomedical ground! Systems and Informatics Engineering 52 26

27 Macroshock Most electrical devices have a metal cabinet, which constitutes a hazard, in case of an insulation failure or shortened component between the hot power lead and the chassis. There is then 115 ~ 230 V between the chassis and any other grounded object. The first line of defense available to patients is their skin. The outer layer provides 15 kω to 1 MΩ depending on the part of the body, moisture and sweat present, 1% of that of dry skin if skin is broken, Internal resistance of the body is 200Ω for each limb, and 100Ω for the trunk, thus internal body resistance between any two limbs is about 500Ω (somewhat higher for obese people due to high resistivity of the adipose tissue)! Any procedure that reduces or eliminates the skin resistance increases the risk of electrical shock, including biopotential 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 53 Macroshock Hazards Direct faults between the hot conductor and the ground is not common, and technically speaking, ground connection is not necessary during normal operation. In fact, a ground fault will not be detected during normal operation of the device, only when someone touches it, the hazard becomes known. 6/2/2014 Therefore, ground wire in devices and receptacles must be periodically BME 420 Department of Biomedical Systems and Informatics Engineering tested

28 Macroshock Hazards 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 55 Microshock Hazards Small currents inevitably flow between adjacent insulated conductors at different potentials leakage currents which flow through stray capacitances, insulation, dust and moisture Leakage current flowing to the chassis flows safely to the ground, if a low-resistance ground wire is available. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 56 28

29 Microshock Hazards If ground wire is broken, the chassis potential rises above the ground; a patient who has a grounded connection to the heart (e.g. through a catheter) receives a microshock if s/he touches the chassis. If there is a connection from the chassis to the patient s heart, and a connection to the ground anywhere in the body, this also causes microshock. Note that the hazard for microshock only exists if there is a direct connection to the heart. Otherwise, even the internal resistance of the body is high enough top prevent the microshocks. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 57 Microshock via Ground Potentials Microshocks can also occur if different devices are not at the exact same ground potential. In fact, the microshock can occur even when a device that does not connected to the patient has a ground fault! A fairly common ground wire resistance of 0.1Ω can easily cause a a 500mV potential difference if initiated due to a, say 5A of ground fault. If the patient resistance is less then 50kΩ, this would cause an above safe current of 10μA 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 58 29

30 Microshock Via Ground Potentials 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 59 Safety Codes & Standards Limits on leakage current are instituted and regulated by the safety codes instituted in part by the National Fire Protection Association (NFPA), American National Standards Institute (ANSI), Association for the Advancement of Medical Instrumentation (AAMI), and Emergency Care Research Institute (ECRI). 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 60 30

31 Basic Approaches to Shock Protection There are two major ways to protect patients from shocks: Completely isolate and insulate patient from all sources of electric current Keep all conductive surfaces within reach of the patient at the same voltage Neither can be fully achieved some combination of these two Grounding system Isolated power-distribution system Ground-fault circuit interrupters (GFCI) 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 61 Grounding Systems Low resistance (0.15 Ω) ground that can carry currents up to the circuit-breaker ratings protects patients by keeping all conductive surfaces and receptacle grounds at the same potential. Protects patients from Macroshocks Microshocks Ground faults elsewhere (!) The difference between the receptacle grounds and other surface should be no more then 40 mv) All the receptacle grounds and conductive surfaces in the vicinity of the patient are connected to the patient-equipment grounding point. Each patientequipment grounding point is connected to the reference grounding point that makes a single connection to the building ground. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 62 31

32 Isolated Power Systems A good equipotential grounding system cannot eliminate large current that may result from major ground-faults (which are rather rare). Isolated power systems can protect against such major (single) ground faults Provide considerable protection against macroshocks, particularly around wet conditions However, they are expensive! Used only at locations where flammable anesthetics are used. Additional minor protection against microshocks does not justify the high cost of these systems to be used everywhere in the clinical environment 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 63 Ground Fault Circuit Interrupters (GFCI) Disconnects source of electric current when a ground fault greater than about 6 ma occurs! When there is no fault, I hot =I neutral. The GFCI detects the difference between these two currents. If the difference is above a threshold, that means the rest of the current must be flowing through elsewhere, either the chassis or the patient!!!. The detection is done through the monitoring the voltage induced by the two coils (hot and 6/2/2014 neutral) in the differential transformer! BME 420 Department of Biomedical Systems and Informatics Engineering 64 32

33 GFCI The National Electric Code (NEC ) requires that all circuits serving bathrooms, garages, outdoor receptacles, swimming pools and construction sites be fitted with GFCI. Note that GFCI protect against major ground faults only, not against microshocks. Patient care areas are typically not fitted with GFCI, since the loss of power to life support equipment can also be equally deadly! 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 65 Protection through Equipment Design Strain-relief devices for cords, where cord enters the equipment and between the cord and plug Reduction of leakage current through proper layout and insulation to minimize the capacitance between all hot conductors and the chassis Double insulation to prevent the contact of the patient with the chassis or any other conducting surface (outer case being insulating material, plastic knobs, etc.) Operation at low voltages; solid state devices operating at <10V are far less likely to cause macroshocks Electrical isolation in circuit design 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 66 33

34 Electrical Isolation Isolation barrier CM CMRR SIG CM ~ ~ Error ~ ISO IMRR * ~ Error R F ISO Isolation Capacitance and resistance (a) Input common *IMRR in v/v ISO Output common o = 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 67 SIG ± Main features of an isolation amplifier: High ohmic isolation between input and output (>10MΩ) High isolation mode voltage (>1000V) High common mode rejection ration (>100 db) ~ CM CMRR ± ISO IMRR Gain Transformer Isolation Amplifiers SIG FB In - In + In com - + ± 5 V F.S. Mod Signal Demod AD202 ± 5 V F.S. Hi Lo o (b) + ISO Out - ISO Out V V Rect and filter 25 khz Power 25 khz Oscillator + 15 V DC Power return 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 68 34

35 Optical Isolation Amplifier Isolation barrier CR 3 CR 1 CR 2 R K = 1M W i i i 2 R G o +V + - ~ i i 1 - AI + - i 2 AII i o -V - (c) Input control o = i R K R G Output control 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 69 Electrical Safety Analyzers Wiring / Receptacle Testing Three LED receptacle tester: Simple device used to test common wiring problems (can detect only 8 of possible 64 states) Will not detect ground/neutral reversal, or when ground/neutral are hot and hot is grounded (GFCI would detect the latter) 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 70 35

36 Internal Circuitry 6/2/2014 Electrical Safety Analyzers Testing Electrical Appliances Ground-pin-to-chassis resistance: Should be <0.15Ω during the life of the appliance Ground-pin-to-chassis resistance test 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 71 Electrical Safety Analyzers Testing Electrical Appliances Chassis leakage current: The leakage current should not exceed 500μA with single fault for devices not intended for patient contact, and not exceed 300 μa for those that are intended for patient contact. Open switch for appliances not intended to contact a patient 120 V Grounding-contact switch (use in OPEN position) H (black) N (white) Appliance power switch (use both OFF and ON positions) Polarity- reversing switch (use both positions) H N G Appliance To exposed conductive surface or if none, then 10 by 20 cm metal foil in contact with the exposed surface Building ground G (green) This connection is at service entrance or on supply side of separately derived Current meter I Test circuit Insulating surface H = hot N = neutral (grounded) G = grounding conductor I < 500 μa for facility Ðowned housekeeping and maintenance appliances I > 300 μa for appliances intended for use in the patient vicinity 6/2/2014 BME 420 system Department of Biomedical Systems and Informatics Engineering 72 36

37 Electrical Safety Analyzers Testing Electrical Appliances Leakage current in patient leads: Potentially most damaging leakage is the one with patient leads, since they typically have low impedance patient contacts Current should be restricted to 50μA for non-isolated leads and to 10 μa for isolated leads (used with catheters / electrodes that make connection to the heart) Leakage current between any pair of leads, or between a single lead and other patient connections should also be controlled Leakage in case of line voltage appearing on the patient should also be restricted. 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 73 Leakage current Testers Test for leakage current from patient leads to ground 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 74 37

38 Leakage Current testers Test for leakage current between patient leads 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 75 Leakage Current Testers Test for ac isolation current Isolation current is the current that passes through patient leads to ground if and when line voltage appears on the patient. This should also be limited to 50μA 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 76 38

39 And this concludes 6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 77 39