Biomedical Instrumentation (BME420 ) Chapter 8: Electrical Safety

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Biomedical Instrumentation (BME420 )

Chapter 8: Electrical Safety

John G. Webster

4

th

Edition

Dr. Qasem Qananwah

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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.

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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.

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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.

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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?

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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.

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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.

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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.

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fig_14_01

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Touch Current Effects

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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Touch Current Effects

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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Touch Current Effects

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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Protection: Power Distribution

Grounding System

Isolated power distribution syytem

Ground fault circuit Interrupters (GFCI)

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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.

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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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fig_14_14

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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.

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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 (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.

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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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This Week

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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

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Equipment design

Electrical safety analyzers / Testing electrical systems

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Safety in Clinical Environment

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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,

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Radiation hazards

Use of radioactive materials, radiation devices (MRI, CT,

PET), exposure control

Electrical Safety

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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

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Lack of experience

Improper (lack of) use of manuals

Device failure

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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

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current faces; skin : largest.

Electricity can have one of three effects:

Electrical stimulation of excitable tissue (muscles, nerve)

Resistive heating of tissue

Physiological Effects of Electricity

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

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Physiological effects of electricity. Threshold or estimated mean values are given for each effect in a

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Physiological Effects of

Electricity

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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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Important Susceptibility Parameters

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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Important Susceptibility Parameters

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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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Important Susceptibility Parameters

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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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Important Susceptibility Parameters

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

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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).

Important Susceptibility Parameters

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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Distribution of

Electrical Power

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(230 V)

Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz

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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 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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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 μ A).

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Isolated Power Distribution

Not grounded !

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is used with such system that continuously monitors for the first ground fault, during which case it simply informs the operators to fix the problem. The single ground fault

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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.

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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

Macroshock Hazards

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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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Macroshock Hazards

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Microshock Hazards

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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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Microshock Hazards

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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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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.

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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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Microshock Via

Ground Potentials

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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).

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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

Grounding system

Isolated power-distribution system some combination of these two

Ground-fault circuit interrupters (GFCI)

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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.

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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

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Ground – Fault

Circuit Interrupters (GFCI)

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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GFCI

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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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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

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Electrical Isolation

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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Transformer Isolation Amplifiers

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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Optical Isolation Amplifier

(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

Wiring / Receptacle Testing

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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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Electrical Safety Analyzers

Testing Electrical Appliances

Ground-pin-to-chassis resistance: Should be <0.15

Ω the appliance during the life of

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Ground-pin-to-chassis resistance test

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Electrical Safety Analyzers

Testing Electrical Appliances

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

I

< 500 μ A for facility Ðowned housekeeping and maintenance appliances

> 300 μ A for appliances intended for use in the patient vicinity

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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 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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Leakage current Testers

Test for leakage current from patient leads to ground

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Department of Biomedical Systems and Informatics Engineering

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Leakage Current testers

Test for leakage current between patient leads

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BME 420 Department of Biomedical Systems and Informatics Engineering

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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

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Department of Biomedical Systems and Informatics Engineering

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And this concludes…

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BME 420 Department of Biomedical Systems and Informatics Engineering

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