Transcript Training
Principles of Electrical Safety
Franklin L. Scamman
Electricity - What Is It?
Flow of electrons along a conductor
Current (I)
Ampere (A)
18
1 A = 1 coulomb/sec = 6.242 x 10
electrons/sec
Think FLOW (Cardiac output)
Voltage (E)
Volt (V)
1 V = 1 joule/coulomb
Potential energy with each charge
Think PRESSURE
Resistance (R)
Ohm (W)
1 W = 1 volt/ampere
Opposition to flow
Think RESISTANCE
Ohm’s Law
E = IR or R = E/I
R = MAP-CVP/CO
Power (P)
Watt (W)
1 W = 1 joule/sec
Work done per unit time
P = EI= I2R
Energy
Kinetic
– One-half mass X velocity squared
Potential
–
–
–
–
Watt Hours
Joules
Car battery – 12 volts times amp hours
AA battery – about 2.2 amp hours
Electrical Circuit
120 V 0.5 AMP “HOT”
FLOW OF ELECTRONS
- POLE (CATHODE)
60 WATT
BULB
+ POLE (ANODE)
0 V 0.5 AMP “NEUTRAL”
Capacitance
Ability of a charge to induce a equal but
opposite charge in nearby conductors
Capacitor
- POLE
----------+++++++
+ POLE
Induction
Ability of a magnetic field to induce current
in a wire
The wire must cross the lines of flux of the
magnetic field
All energized circuits are surrounded by a
magnetic field
With AC circuits, the field constantly
expands and collapses, producing a moving
magnetic field
Transformer
Uses principle of induction
Functions in AC circuits only
Magnetic field from primary winding
induces current in secondary winding
No physical contact between the two
windings
Transformer
VOLTAGE
SOURCE
IRON
CORE
PRIMARY
WINDING
SECONDARY
WINDING
Grounded Power
Common household electrical supply
Supplied through “hot” and “neutral” lines
Neutral connected to earth ground both at
source and end user
Earth ground provides an easy reservoir for
excess electrical charge
Grounded Power
HOT 120 volts
Power
Suppily
NEUTRAL 0 volts
GROUND
Isolated Power
Isolated power - requires isolation
transformer
Lacks connection to earth ground
Requires line isolation monitor to detect
faults (short circuits)
Originally installed to reduce
explosions/fires from anesthetics
Isolated Power
ISOLATION TRANSFORMER
HOT 120 volts
Power
Suppily
NEUTRAL 0 volts
GROUND
No connection between circuit and ground
Advantages of Isolated Power
Reduced spark hazard
Reduced possibility of electrocution
Short circuits do not shut off power
Line Isolation Monitor
Monitors isolation of circuit from ground
Alarms when a “first fault” occurs
Threshold for alarms 2 or 5 mA
LIM Function
Connects isolated circuit directly to ground
and measures potential current flow
Alternates connecting each lead of the
isolated circuit to ground
Contains high resistance to prevent
appreciable current flow
LIM Circuit
ISOLATION
TRANSFORMER
BLOOD
WARMER
LIM
First Fault
ISOLATION
TRANSFORMER
BLOOD
WARMER
LIM
Response to LIM Alarms
Find device that has excess leakage current
Unplug devices one at a time
LIM may alarm without a specific first fault
Leakage Current
All electrical appliances/monitors have
leakage current
Leakage current arises principally from
capacitance coupling in appliances
The ground wire is critical for detecting
leakage current
Leakage current is additive in a circuit
Maximum leakage for each device 100 µA
Leakage to Ground
ISOLATION
TRANSFORMER
BLOOD
WARMER
LIM
Physiology of Electrical
Current
Nerve excitation
Muscle contraction
Ventricular fibrillation
Threshold of Perception
Let Go Current
Factors In Fibrillation
Location of current flow
Duration of current flow
Density of current flow
Alternating verses direct current
Frequency of AC current
Fibrillation Current vs. Weight
Current necessary to achieve fibrillation
Fibrillation Potential
0
100
200
300
400
Frequency (Hz)
500
Effect of Current
Intensity (amps)
0.001
0.005
0.010-0.020
0.050
0.1 to 2
6 or more
1 second application
Threshold of perception; tingling
Maximum “ harmless” current
intensity
Skeletal muscle spasm; unable to let
go
Pain; mechanical injury; possible
unsciousness; transient spastic
interruption of respirations
Ventricular fibrillation
Sustained myocardial contraction
followed by normal sinus rhythm;
temporary respiratory paralysis;
possible thermal injury if contact area
is small
In Vivo Resistance
Electrical contact
Dry skin
Resistance (ohms)
100,000
Moist skin
1000
Electrode Paste
500
Implanted cardiac electrode
10
Macroshock
Application of current to the exterior of the
body
Fibrillation threshold around 100 mA (0.1
A) for a 1 second application to dry skin
Isolated power system provides protection
against macroshock (2-5 mA limit)
Macroshock
How We Get Electrocuted
Isolated Power Circuit
Microshock
Application of current directly to the heart
Route for current flow may be a pacemaker
lead or a saline filled CVP/PA catheter
Fibrillation threshold about 50 µA (2x103
less than macroshock)
Isolated power system does not protect
against microshock
Microshock
Ground Fault Interrupter (GFI)
Required in wet locations
Monitors outflow and return of current in a
circuit
Disconnects circuit if a flow imbalance
occurs
Can detect 5 mA differences and disconnect
the circuit in milliseconds
GFI
Electrical Surgical Unit Burns
Electrical Surgical Unit Burns
History of Electrical Safety
History of Electrical Safety
History of Electrical Safety
History of Electrical Safety