5. electrical safety(1).pdf

BG 3105 Biomedical Instrumentation

Electrical Safety

Asst Prof Manojit Pramanik School of Chemical and Biomedical Engineering

Nanyang Technological University

manojit@ntu.edu.sg 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

Biomedical Instrumentation - wk 5 2

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.

Biomedical Instrumentation - wk 5 3

• 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.

Biomedical Instrumentation - wk 5 4

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.

Biomedical Instrumentation - wk 5 5

Effects of Electrical Shock

Biomedical Instrumentation - wk 5 6

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.

Biomedical Instrumentation - wk 5 7

Biomedical Instrumentation - wk 5 8

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.

Biomedical Instrumentation - wk 5 9

Rbody

The equivalent circuit is shown

Biomedical Instrumentation - wk 5 10

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.

Biomedical Instrumentation - wk 5 11

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.

Biomedical Instrumentation - wk 5 12

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.

Biomedical Instrumentation - wk 5 13

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

Biomedical Instrumentation - wk 5 14

Chassis

Catheter

Ground

Biomedical Instrumentation - wk 5 15

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

Biomedical Instrumentation - wk 5 16

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.

Biomedical Instrumentation - wk 5 17

Isolation Transformer (Method 1)

• Equipment are connected after the isolation transformer • So that protection can be provided

Biomedical Instrumentation - wk 5 18

IH

IN

IH=IN

• Example 1:

Biomedical Instrumentation - wk 5 19

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 !

Biomedical Instrumentation - wk 5 20

• 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

Biomedical Instrumentation - wk 5 21

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

Biomedical Instrumentation - wk 5 22

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

Biomedical Instrumentation - wk 5 23

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

Biomedical Instrumentation - wk 5 24

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.

Biomedical Instrumentation - wk 5 25

• How to operate: Consider one branch of LIM. It detects impedance between Hot wire to ground

Biomedical Instrumentation - wk 5 26

I H

Isolated transformer

I N Equ.

LIM

R

C

V o ILIM

Biomedical Instrumentation - wk 5 27

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

Biomedical Instrumentation - wk 5 28

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

Biomedical Instrumentation - wk 5 29

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

Biomedical Instrumentation - wk 5 30

• 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?

Biomedical Instrumentation - wk 5 31

Example 1

Chassis

1.1Ω

• Answer: The equivalent circuit of Example 1 is

Biomedical Instrumentation - wk 5 32

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

Biomedical Instrumentation - wk 5 33

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).

Biomedical Instrumentation - wk 5 34

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

Biomedical Instrumentation - wk 5 35

H N G

Tester

• A power receptacle tester

Biomedical Instrumentation - wk 5 36

Normal condition: LED1 ON, LED2 OFF, LED3 ON

Biomedical Instrumentation - wk 5 37

An H to G short will turn all LEDs OFF

H N G

Biomedical Instrumentation - wk 5 38

An open H lead will turn all LEDs OFF

open H N G

open

Biomedical Instrumentation - wk 5 39

If H to N are reversed, LED1 OFF, LED2 ON, LED3 ON

H N G

H

N

Biomedical Instrumentation - wk 5 40

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