sysc3203: electrical safety - carleton · pdf fileslow response. can maintain continuous...
TRANSCRIPT
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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)
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
Slide 1.7
ECG Vector Potential
Source: http://www.bem.fi/book/15/15x/animati/000.htm
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
Slide 1.8
ECG Vector Potential
Source: http://www.bem.fi/book/15/15x/animati/000.htm
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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.
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
Slide 1.11
Anatomy of Muscles
Source:wikipedia
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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.
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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.
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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)
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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)
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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?
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
Slide 1.23
Macroshock & Protection
Macroshock due to a ground fault from hot line to equipment cases for
ungrounded cases grounded chassis.
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203: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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
Slide 1.30
Optocoupler
Optocoupler sharp-world.com/products/device/lineup/data/p
df/datasheet/pc1231xnsz_e.pdf
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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.
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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?
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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
Copyright © by A. Adler, 2014 – 2016
SYSC3203:Electrical Safety
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.