th
Dr. Qasem Qananwah
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• 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.
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• 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.
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• 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.
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• 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?
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• 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.
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• 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.
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Leakage Currents: Caused by stray capacitances, which are always present between conducting surfaces.
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From John G. Webster (ed.), Medical instrumentation application and design, 4th ed., John Wiley & Sons, 2010. This material is reproduced with permission of John Wiley & Sons, Inc.
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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.
BME 420 Department of Biomedical Systems and Informatics Engineering fig_14_01
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effect
Un sensed
Muscle contraction. Let-go possible.
Pain. Increasing probability of let-go impossibility
Passing let-go threshold. Minor effects on breathing & blood circulation.
Let-go impossible. Increased heart beat & blood pressure
(BP). Arrhythmia. Breathing irregularities.
Current
(mA)
0 – 0.6
0.6 – 6
6 – 15
15 – 25
25 - 50 range
A1
A2
B1
B2
B3
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effect Current
(mA)
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 C1
50 – 80
80 -120
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.
120-800 range
C1
C2
D
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effect Current
(mA)
800-2000 Often VF even if shock interval is less than (0.1) second.
Thermal effects appear if shock lasts for more than (10) second.
Like E concerning heart. Increasing probability of burns of muscles and limbs if shock lasts for more than (5) second.
More than 2000 mA range
E
F
Continuous cardiac contraction. Temporary breathing paralyzed.
Burns
More than 5000 mA
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Figure 14.6
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
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Figure 14.10 Macroshock due to a ground fault from hot line to equipment cases for (a) ungrounded cases and (b) grounded chassis.
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Figure 14.11 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.
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• Grounding System
• Isolated power distribution syytem
• Ground fault circuit Interrupters (GFCI)
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• 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.
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Figure 14.14 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.
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• 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.
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Figure 14.9 Power-isolation-transformer system with a line-isolation monitor to detect ground faults.
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• 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.
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•
• 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.
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Figure 14.15
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: 223–248.]
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• 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
• Electrical safety analyzers / Testing electrical systems
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• 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,
Department of Biomedical Systems and Informatics Engineering
• Radiation hazards
• Use of radioactive materials, radiation devices (MRI, CT,
PET), exposure control
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• 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
• Improper (lack of) use of manuals
• Device failure
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• 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
6/2/2014 BME 420 Department of Biomedical Systems and Informatics Engineering current faces; skin : largest.
• Electricity can have one of three effects:
• Electrical stimulation of excitable tissue (muscles, nerve)
•
• Resistive heating of tissue
voltages
The real physiological effect depends on the actual path of the current
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Dry skin impedance:93 k Ω / cm 2
Electrode gel on skin: 10.8 k Ω / cm 2
Penetrated skin: 200 Ω / cm 2
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Physiological effects of electricity. Threshold or estimated mean values are given for each effect in a
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• 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.
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• Threshold and let-go current variability
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Distributions of perception thresholds and let-go currents These data depend on surface area of
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• 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.
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• 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
Thresholds for ventricular fibrillation in animals
130 mA for 24 kg dogs.
5 s) and weight of animal body were varied.
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• Points of entry
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• 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 k Ω ~ 2 M Ω ) is bypassed, greatly reducing the amount of current req’d to cause fibrillation. Even smallest currents ( 80 ~ 600 μ A), causing microshock , may result in fibrillation. Safety limit for microshocks is 10 μ A.
• The precise point of entry, even externally is very important: If
BME 420 Department of Biomedical Systems and Informatics Engineering fibrillation is greatly reduced even at high currents (e.g. the current req’d for fibrillation through Lead I (LA -RA) electrodes is higher than for Leads II (LL-RA) and III (LL-LA).
• Points of entry
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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.
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(230 V)
Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz
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• 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 current to flow to the ground is subject to microshock.
• The additional ground line provides a good line of defense! (how / why?)
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• 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 μ A).
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Not grounded !
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• 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 k Ω to 1 M Ω 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 200 Ω for each limb, and 100 Ω for the trunk, thus internal body resistance between any two limbs is about 500 Ω (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 electrode gel, electronic thermometers placed in ears, mouth, rectum, intravenous catheters, etc.
• A third wire, grounded to earth, can greatly reduce the effect of macroshock, as the resistance of that path would
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• 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.
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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.
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• 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.
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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.
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If the patient resistance is less then 50kΩ, this would cause an above safe current of 10μA
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• 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).
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• 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)
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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.
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• 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
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• Disconnects source of electric current when a ground fault greater than about 6 mA occurs!
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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
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The National Electric Code (NEC - 1996) 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!
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• 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
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
CM
CMRR
Error
~

SIG
~

ISO
IMRR *
~
Error
Isolation barrier
-
+
R
F
-
+

ISO
Isolation
Capacitance and resistance

CM
~
Input common
~

ISO
Output common
 o =

SIG ±

CM
CMRR
±

ISO
IMRR
Gain
(a) *IMRR in v/v
• Main features of an isolation amplifier:
• High ohmic isolation between input and output (>10M Ω )
• High isolation mode voltage (>1000V)
• High common mode rejection ration (>100 dB)
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FB
In
In +
-

SIG
In com
+

ISO
Out
-

ISO
Out
-
-
+
± 5 V
F.S.
+ 7.5 V
7.5 V
Mod
Signal
AD202
Demod
Rect and filter
25 kHz
Power
25 kHz
Oscillator
± 5 V
F.S.
Hi
Lo
 o
+ 15 V DC
Power return
(b)
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(c)
R
G
+
-
~
 i i
1
CR
3 i i

1
+
 o
CR
1
-
AI
+ i
3
Input control
Isolation barrier
CR
2
R
K
= 1M
W i
2

2
+ V
 o
=  i
R
K
R
G i
2
-
AII
+
Output control
-
V
+
-
 o
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Electrical Safety Analyzers
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• 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)
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• Ground-pin-to-chassis resistance: Should be <0.15
Ω during the life of the appliance
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Ground-pin-to-chassis resistance test
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Electrical Safety Analyzers
• Chassis leakage current: The leakage current should not exceed 500 μ A with single fault for devices not intended for patient contact, and not exceed 300 μ A for those that are intended for patient contact.
Appliance power switch
(use both OFF and ON positions)
Open switch for appliances not intended to contact a patient
Grounding-contact switch (use in
OPEN position)
Polarity- reversing switch (use both positions) Appliance
H (black)
120 V
N (white)
H
N
G
To exposed conductive surface or if none, then 10 by
20 cm metal foil in contact with the exposed surface
G (green)
Building ground
This connection is at service entrance or on supply side of
Insulating surface
I
Current meter
H = hot
N = neutral (grounded)
G = grounding conductor
Test circuit
I < 500 μ A for facility Ðowned housekeeping and maintenance appliances
I > 300 μ A for appliances intended for use in the patient vicinity
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Electrical Safety Analyzers
• 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 50 μ A for non-isolated leads and to 10 μ A 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.
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Test for leakage current from patient leads to ground
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Test for leakage current between patient leads
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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
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