5. Electrical Safety(1).pdf
Transcript of 5. Electrical Safety(1).pdf
BG 3105 Biomedical Instrumentation
Electrical Safety
Asst Prof Manojit Pramanik School of Chemical and Biomedical Engineering
Nanyang Technological University
[email protected] Office: N1.3-B2-11
Electrical Safety
1 Introduction 2 Electrical safety and medical applications 2.1 Electrical shock 2.2 Protection against shock 2.3 Safety tester
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1. Introduction
• Electricity is the main power source for: Lighting, equipment, life support
• Electrical safety is the limitation/elimination of hazardous condition
Electrical shock Explosion Fire Damage to equipment and buildings.
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• Electrical shock refers to both macroshock and microshock.
• Electrical shock may occur to patients, staff and visitors to hospitals.
• Shock results from improperly wired or maintained electrical equipment or power systems.
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1. Introduction
2. Electrical safety and medical applications
• Electrical current passing through the human body has three primary effects Injury to tissues Uncontrollable muscle contraction unconsciousness
• Electrical shock is measured in terms of current
intensity at specified frequencies.
• The frequency of the current is also important when considering the shock phenomenon.
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Effects of Electrical Shock
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Current intensity – 1 s contact
Effect
1 mA Threshold of perception.
5 mA Accepted as maximum harmless current intensity.
10-20 mA “Let-go” current before sustained muscular contraction.
50 mA Pain. Possible fainting, exhaustion, mechanical injury; heart and respiratory functions continue.
100-300 mA Ventricular fibrillation will start, but respiratory center remains intact.
6 A Sustained myocardial contraction followed by normal heart rhythm. Temporary respiratory paralysis. Burns if current density is high.
Effects of 60-Hz electric shock (current) through the body of an average human
2.1 Electrical Shock
• Macroshock Macroshock is a shock due to touching H (hot) and
N (neutral) wires with two limbs. The current which may cause ventricular fibrillation
is 50~250 mA for 50 Hz voltage.
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230 V 50 Hz
Electric device
Fault
Chassis
Water pipe
• The hot wire is shorted to chassis • Chassis is not grounded • A man touches the chassis with his foot
grounded
• The shorted component results in a macroshock when a patient simultaneously touches the chassis and a grounded object.
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Rbody
The equivalent circuit is shown
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Electric device
Fault 230 V 50 Hz
Protection: If the chassis is grounded, then when a fault occurs, the most current flows safely to ground.
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The equivalent circuit is shown
Chassis grounded
• Microshock Microshock is shock due to current directly passing
through the heart. It is caused by the leakage current from needle and
catheter inserted inside the heart. Microshock current of 10~100 µA can cause
ventricular fibrillation.
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Catheter
Right atrium
Superior vena cava
Ventricular fibrillation is a condition in which there is uncoordinated contraction of the cardiac muscle of the ventricles in the heart, making them quiver rather than contract properly.
Leakage Current
• Leakage current is defined as the low-value electrical current (µA) that inherently flows (leaks) from the energized electrical portion of an appliance or instrument to the metal chassis.
• All electrically operated equipment has some leakage current. • This current is not a result of a fault but is a natural consequence
of electrical wiring and components. • Typically capacitive leakage current is the main contributor
compared to resistive leakage current.
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Chassis Ground
• A catheter is inserted inside heart of a patient • There may be leakage currents due to stray capacitors
(very small) between chassis and power lines • The chassis is grounded
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Chassis
Catheter
Ground
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As the chassis is grounded, most of the currents will return through the ground wire and safe.
Equivalent circuit H
N
G
catheter Ileak
R – resistance between heart and foot (500Ω)
Chassis
Catheter
Ground broken
But, if the ground wire is broken, then the leakage current will pass through the patient
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Equivalent circuit
If the current flowing through the patient is higher than 10 µA, then ventricular fibrillation may occur.
H
N
G
catheter Ileak
2.2 Protection against shock
• Two fundamental methods are: The patient can be completely isolated and insulated
from all grounded objectives and all sources of electric current.
All conductive surfaces within reach of patient can be maintained at the same potential.
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Isolation Transformer (Method 1)
• Equipment are connected after the isolation transformer • So that protection can be provided
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IH
IN
IH=IN
• Example 1:
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The equivalent circuit is as follows
Body resistance
H
N
I H
Isolated output
H
N
I H
Isolated output
The isolation transformer provides protection against macroshock
This is because that the person touches the hot lead, but 𝐼𝐻 = 0 as both hot and neutral leads are isolated.
No return path !
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• Example 2: If an equipment is connected to the isolated power and chassis is grounded I H
Isolated transformer I N
Equipement
No fault
Clearly, the hot lead current 𝐼𝐻 is equal to 𝐼𝑁
𝑰𝑯 = 𝑰𝑵
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Suppose the hot lead is fault to the chassis
𝐶 is the stray capacitor (very small) formed between the neutral lead and the ground wire
I H
Isolated transformer
I N
Equipment
Fault
I Leak
C
V 0 V i
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Then, the leakage current is
- 𝑽𝟎 is the voltage (isolated power)
- 𝝎 is angular frequency
- 𝑹𝒈 is a wire resistance
Since 𝐶 is very small, 𝑰𝑳𝑳𝑳𝑳 is very small.
𝑰𝑳𝑳𝑳𝑳 =𝑽𝟎
𝟏𝝎𝝎
𝟐+ 𝑹𝒈𝟐
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should be very small
I H
Isolated
transformer
Fault
C
V 0
V i
IN
ILeak
This is considered safe even when a person touches faulty wire
𝑰𝑳𝑳𝑳𝑳 =𝑽𝟎
𝟏𝝎𝝎
𝟐+ 𝑹𝒃𝒃𝒃𝒃𝟐
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For the case of the isolated transformer faulty, for example, the neutral line is shorted, then, no more isolated power
I H
Isolated transformer
I N Equipment
shorted
Line isolation monitor (LIM)
• The LIM is a device that continuously monitors the isolated power lines from fault.
• It is done by detecting the impedance between the power lines to ground wire.
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• How to operate: Consider one branch of LIM. It detects impedance between Hot wire to ground
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I H
Isolated transformer
I N Equ.
LIM
R
C
V o ILIM
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Clearly, the current 𝑰𝑳𝑰𝑳 is determined by
This current should be very small if no faulty to the ground occurs
R Vo Neutral
C
The equivalent circuit is
𝑰𝑳𝑰𝑳 =𝑽𝟎
𝑹𝟐 + 𝟏𝝎𝝎
𝟐
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But, if the neutral line is shorted to the ground
I H
Isolated transformer
I N Equ.
LIM
R
C
I LIM V o
Fault
The equivalent circuit becomes
R Vo Neutral
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In this case, the ammeter indicates a current showing a faulty.
Thus,
Note: Similarly, for the hot lead shorted, another part of LIM can detect it
𝑰𝑳𝑰𝑳 =𝑽𝟎𝑹
Safety ground (Method 2)
• Safety ground is a method to maintain the same potential Equipment is grounded Metal bed is grounded Regular checking of ground wires
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• Example 1: Suppose that the leakage current is 100 µA. If the ground wire resistance is 1.1Ω and a patient of 500-Ω resistance touches the instrument metal case, what is the body current?
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Example 1
Chassis
1.1Ω
• Answer: The equivalent circuit of Example 1 is
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Vo C, stray capacitor
500Ω, body resistor 1.1Ω
ILeak=100μA
I2
I1
I1 = 99.8 μA, I2 = 0.2 μA,
0.2 µA of leakage current flows through the patient, and 99.8 µA flows through the safety ground.
Thus, the patient is safe.
Ground-fault interrupter (GFI)
• A ground fault interrupter (GFI) protects against a shock.
• The GFI consists of: A ring magnetic material A sensing coil Hot and neutral coils with the same number of turns on a magnetic material
but they are in opposite directions A relay
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relay
• How to operate? When the system is in normal, 𝑰𝑯 = 𝑰𝑵 In this case, the magnetic flux in the coil 𝝓 = 𝟎 and the sensing coil does not have a voltage. Therefore, the sensing amplifier output is zero
voltage.
However, when the hot lead faults, or is touched by a person 𝑰𝑭 = 𝑰𝑯 − 𝑰𝑵, 𝑰𝑭 ≠ 𝟎
In this case, the net flux in the coil is not zero, i.e. 𝝓 ≠ 𝟎
A voltage is induced by the sensing coil. If 𝑰𝑭 ≥ 𝟐 𝒎𝒎, the relay actuates (interrupt power).
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2.3 Safety tester
• Receptacle Tester • The LIM and GFI are permanently attached to the power
lines for protection. • The receptacle tester is for inspecting equipment and
circuits by inserting in the power receptacle. • It can test:
Polarities reversals Shorts opens
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H N G
Tester
• A power receptacle tester
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Normal condition: LED1 ON, LED2 OFF, LED3 ON
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An H to G short will turn all LEDs OFF
H N G
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An open H lead will turn all LEDs OFF
open H N G
open
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If H to N are reversed, LED1 OFF, LED2 ON, LED3 ON
H N G
H
N
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LED state1 2 3
Possiblewiring defect
Normal ON OFF ONH to G short OFF OFF OFFH, N reversed OFF ON ON
This table summarizes LEDs states corresponding H lead fault.
Determination of the other LED states can also be done