SYSC3203: Electrical Safety - Carleton · PDF fileSlow response. Can maintain continuous...

Copyright © by A. Adler, 2014 – 2016

SYSC3203:Electrical Safety

Slide 1.1

Electrical Safety

Electricity in the body

Physiological Effects of Electricity Variations among individuals

Frequency dependence

Chronaxie

Macroshock / Microshock

Electrical Faults in Equipment

Electrical Protection

Design considerations for this course



Slide 1.2

Biopotentials

Biopotentials arise from movement of ions in cells and organs. Measurement of biopotentials yeilds clinically and diagnostically significant information.

Important biopotentials

From Nerves Brain (EEG – electroencephalogram)

From Muscles Heart (ECG – electrocardiogram) Muscles (EMG – electromyogram)

From Retina Transepithelial potential (EOG – electrooculogram)



Slide 1.3

Anatomy of a nerve cell

Source: www.youtube.com/watch?v=G9rHAM0gIn8

Cell body(including nucleus)

Dendrites

Axon (up to ~ 1m length)

Myelin Sheath

~1 mm



Slide 1.4

Questions

Do nerves conduct only in one direction? What stops an AP from turning around and propagating

in both directions? Signalling in nerves can be compared to a Pulse Rate

Modulation Communications scheme. Compare and contrast.

Would you modify the video, to better explain the electrical activity? www.youtube.com/watch?v=G9rHAM0gIn8



Slide 1.5

Electro-Cardiogram (ECG)

Electro - Cardio - GraphElectrical Heart Writing

Measurements depend where on the bodysurface measurementsare performed

Therefore, we need standardplacements for ECG leads 3 leads RA, LA, LL 12 leads



Slide 1.6

Standard ECG Leads

The Einthoven limb leads (standard leads) are:

LA = Left ArmRA = Right ArmLL = Left Leg

V I= LA−RA

V II= LL−RA

V III= LL−LA

V IV III=V II

Six and 12 lead ECG positions

are also defined



Slide 1.7

ECG Vector Potential

Source: http://www.bem.fi/book/15/15x/animati/000.htm



Slide 1.8

ECG Vector Potential

Source: http://www.bem.fi/book/15/15x/animati/000.htm



Slide 1.9

Questions

Draw a heart, and label the important electrical and mechanical structures (nodes, chambers, arteries ...).

What is the function of the SA-node?

What is the function of the AV-node?

Why is the ECG as a vector field?

Indicate the electrode locations for the three lead ECG.

Draw the electrode configuration on Einthoven’s Triangle and explain the mathematical relationship between the electrodes.



Slide 1.10

Electro-Myogram (EMG)

Skeletal (voluntary muscle) is anchored by tendons to bones. Fast response. Fatigues with load.

Smooth (involuntary muscle) is found within the walls of organs and structures (e.g.: esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and skin). Slow response. Can maintain continuous force.

Cardiac muscle is found in heart. An involuntary muscle but is similar to skeletal muscle. Fast response, but does not fatigue easily.

Myoelectric (Myo = muscle) signals are the signals captured in the EMG (electromyogram). A voluntary (or induced contraction) creates an electrical signal composed of the action potentials travelling across the muscle fibers



Slide 1.11

Anatomy of Muscles

Source:wikipedia



Slide 1.12

Activation of Muscles

Each somatic (voluntary) neuron activates a group of muscle fibres.

Motor Unit: one somatic neuron and the group of fibres it activates

Every action potential initiated by a particular somatic neuron creates a motor unit action potential (MUAP), and for the same somatic neuron, all its MUAPs will look the same.

The SFAPs propagate along the muscle fibers and are summed by the electrode. Close fibers will contribute more to the sum than distant ones.

Source: signal.uu.se



Slide 1.13

Muscle Recruitment

To increase the contraction strength:

1. Increase the firing rate of MUs (temporal recruiting)

2. Activate more MUs (spatial recruiting) Large muscle: 100s of fibres/nerve Small muscle: 10s of fibres/nerve (fine motor control

comes from here) at max rate, MUAPs fuse together to form tetanus.



Slide 1.14

Questions

• What is a motor unit? What is a muscle fibre?• Explain the difference between spatial and temporal recruitment

• The normal ECG ‘always’ has the same structured shape and is easily recognizable, why is this not true of the EMG?

Vs



Slide 1.15

Physiological Effects of Electricity

Threshold of perception 1 mA

Let go current 6 mA

Maximum current allowing voluntary release

Respiratory paralysis 22 mA

Ventricular Fibrillation 75-400 mA

Sustained Myocardial 1A –6 A contraction

Burns 10 A

Usually on skin (from high skin resistance)

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.



Slide 1.16

Variation among individuals

Body Weight – thresholds are roughly proportional Points of entry – affected by the current path (and

amount of I going through heart)



Slide 1.17

Variation with frequency

Let-go current versus frequency. Percentile values indicate variability of let-go current among individuals. Let-go currents for women are about two-thirds the values for men. (Dalziel (1973) "Electric Shock”)

Maximum is around 50-60Hz



Slide 1.18

Strength vs. Duration

A smaller current for a longer time can have the same effect as a larger, shorter one

(above a threshol)

Fibrillation current versus shock duration. Thresholds for ventricular fibrillation in animals for 60 Hz AC current. Duration of current (0.2 to 5 s) and weight of animal body were varied. (Geddes, 1973)



Slide 1.19

Questions

What happens at the threshold of perception? Why is threshold for ventricular fibrillation so

much lower than for sustained myocardial contraction?

The Let-go current for men is larger than for women. What does this say about men and women☺?

Why is threshold higher for larger people? What is the threshold of pain?



Slide 1.20

Macroshock / Microshock

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 Weibell, 1974 "Electrical Safety in the Hospital")

A B



Slide 1.21

Macroshock

Macroshock Shock from electrical connections on the skin. Worrisome current levels are > 1mA (depending on

frequency). Typically due to an electrical fault, circuit flaw Any effect/device that reduces resistance of skin is a

potential hazard (Skin resistance is 15kΩ - 1MΩ, internal resistance is lower ≈100 Ω)

Skin resistance varies with Injury Water (wet) Oil



Slide 1.22

Microshock

Microshock Shock from connections on / near the heart Much smaller currents are dangerous. Even a few

µA directly to heart is dangerous Microshock hazard is due to leakage current in

designs Hazard from

Cardiac catheter IV lines with instruments Cardiac electrodes Pacemakers



Slide 1.23

Macroshock & Protection

Macroshock due to a ground fault from hot line to equipment cases for

ungrounded cases grounded chassis.



Slide 1.24

Shock prevention

Protection strategies for shock Reliable grounding Double-insulated equipment Isolation in design

Isolation transformers Isolation amplifiers

Optoisolators, Capacitive/Transformer isolation

Low voltage design/operation Low leakage current design



Slide 1.25

Classes of equipment

Class I

has a protective earth. In the event of a fault that would otherwise cause an exposed conductive part to become live, protective earth comes into effect.

Class II

double insulation or reinforced insulation Class III

protection relies on no voltages higher than safety extra low voltage (SELV) are present. (25V ac or 60V dc). In practice battery operated or supplied by a SELV transformer.

Source:www.ebme.co.uk/articles/electrical-safety/333-classes-and-types-of-medical-electrical-equipment



Slide 1.26

Electrical Safety Tests

Normal Condition Single Fault Condition (ie. Ground fault) Insulation Tests Earth Leakage Current Enclosure Leakage Current Patient Leakage Current Patient Auxiliary Current Mains on applied parts

www.slideshare.net/ishoney/medical-electrical-safety



Slide 1.27

Isolation

Isolation amplifiers: break ground loops, eliminate source ground connections, isolation protection to patient and equipment

Isolation amplifiers technologies: transformer isolation, capacitor isolation opto-isolation.

Nagel, J. H. “Biopotential Amplifiers.”The Biomedical Engineering Handbook: Second Edition



Slide 1.28

Electrical IsolationIsolation amplifiers

General model for an isolation amplifier

Transformer isolation amplifier (Analog Devices, Inc., AD202).

Isolation barrier

+

+ 15 V DC

o

Powerreturn

25 kHz 7.5 V

+ISOOut

SIG

ISOOut

+ 7.5 V

In com

In

In +

FB

± 5 VF.S.

Oscillator

25 kHz

SignalMod

Rect andfilter

Power

Demod

± 5 VF.S.

AD202

Hi

Lo

(a)

CM

CM

CMRR

SIG ISO

ISO

Error

Isolationbarrier

IsolationCapacitanceand resistance

+

+

Input common Outputcommon

RFISO

IMRR*

~~

~

~

~

Error



Slide 1.29

Electrical Isolation

Simplified equivalent circuit for an optical isolator

Capacitively coupled isolation amplifier

Frequency-to-voltage converter(phase-locked loop)

OscQ

Q

± 15 V (Receiver)± 15 V (Driver) Isolationbarrier

3 pF

3 pF

Freqcontrol

Analog signal out, o

Analogsignal in, i

Isolation barrier

Inputcontrol

Outputcontrol

V

+V+o

+

o = i

RK

RG

CR3 CR1 CR2

i2ii

i

i1

RG

AI AIIi3

i2

o

RK = 1M

+

++

~

1 2



Slide 1.30

Optocoupler

Optocoupler sharp-world.com/products/device/lineup/data/p

df/datasheet/pc1231xnsz_e.pdf

http://sharp-world.com/products/device/lineup/data/pdf/datasheet/pc1231xnsz_e.pdf

http://sharp-world.com/products/device/lineup/data/pdf/datasheet/pc1231xnsz_e.pdf



Slide 1.31

This course

Practically, we need guidelines for projects I want to say “I told you so” if I have to visit you

in hospital☺. Rules

In IV instruments for projects For student’s research building own circuits

Use batteries instead of plugging in.



Slide 1.32

Questions

Why are isolation amplifiers required for biomedical designs?

Name some isolation techniques used to design isolation amplifiers?

What is the difference between macroshock and microshock?

Imagine you're listening to a radio in the bath and you drop it in. How does the third ground wire help protect you? Draw the current pathways.

Midterm BIOM5100: 2007A

2B. One practical clinical problem is that electrodes tend to become detached as the patient moves and changes posture. In order to detect this, some manufacturers apply a 10mA current through the electrodes. When the impedance to current flow increases greatly, the electrode is detected as disconnected. What type of electrical shock hazard does this possibly represent and why?



Slide 1.33

Q1 from 2015 Midterm

(Electrical Safety) Electrical safety is obviously an important factor in biomedical electronics. In class we studied the optoisolator

(a) Define the terms “let-go current” and “threshold of perception”, and explain the difference between them.

(b) The optoisolator above has an isolation voltage of 6.2 kV. Unfortunately, lightening strikes the circuit, applying 500 kV to terminal C, while terminal D is at ground. The terminals A and B are connected to a circuit which is connected to the patient. Where does (and doesn’t) the current flow?

(c) In the previous question, explain whether the patient is protected



Slide 1.34

Q1 from 2015 final

Background: As you know, health and weight loss products sell!

After graduation, your first job asks you to design a bathroom scale which also measures heart rate, and gives feedback via a vibrating motor.

As shown in the diagram below, the customer will stand on the scale. Their weight will deform strain gauges attached to two internal posts; this signal drives a dial. At the same time, stainless steel electrodes touch their feet from which their ECG is measured

A/DConverterAmplifier Filter

Electrode

AmplifierStrain Gauge

VibratingMotor

ScaleDisplay



Slide 1.35

Q1 from 2015 final

1. (30 points) Electrodes and Electrical Safety

(a) ...

(b) What kind of shock risk is possible in this design? Sketch current pathways in the body and indicate if they can travel through the heart.

(c) In a bathroom, there is an obvious risk: the electronics can get wet. What risk does this pose for the user? Describe one electrical isolation technology that can protect against this risk.

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