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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