Electrical Safety

1
Electrical Safety

• Steven L. Shafer, M.D.Professor of
  AnesthesiaStanford UniversityAdjunct Professor
  of Biopharmaceutical SciencesUniversity of
  California at San Francisco

2
Ohms Law

• VI R
• Voltage (V) Current (I) ? Resistance (R)
• Variac example

3
What is 120 Volts?
120 volts is the root mean square voltage Actual
peak voltage is about 150 volts
4
Why are patients at risk?

• Placed on a metal table.
• Wet
• Surrounded by electrical devices.
• Connected to multiple electrical devices,
  typically by low resistance pads
• Cant move away from current
• Need to avoid the electric pickle
• Start hot dog

5
Pickle Physics

• Pickle is nearly saturated with Na Cl as part of
  pickling process
• Na Cl provides conducting path
• Na is reduced by electric current to Na0
• When the Na0 is oxidized to Na by oxygen, a
  reaction giving off light at the frequency of Na.
• Same color as Na lights on highways

6
Macroshock how much current?
Ventricular Fibrillation Threshold 80 mA
7
Macroshock how many volts?
How can the patient come in contact with 40 volts?
8
Basics of Power Distribution
ServiceEntrance
Hot
120 volts
Neutral
Examine power distribution in room
9
Basic Toaster
 
______________________________
/
\ Hot _______________________/____________
_/\/\/\/\/\/\/\ \

Neutral ____________________________________/\/\/
\/\/\/\/\
(filament) Ground
______________________

T o a s t e r
__________________________________
(Ground
connects to chassis)
10
Worst Case ScenarioHot shorts to chassis,
ground broken
(short circuit)
! ! ! ___!_______________
/ \
_____ _____Hot______/_______/\/\/\/\ \
// \\
Patient
__Neutral_____________/\/\/\/\ wants
gt lt
toast \ O / /
........... \_____/ /
T o a s t e r
__________/ _______________________________
___ /
Patient
Broken Ground ______________
/.\ is \
O.R. table -----towel-- / \ toast /
\__________/
/ \ /
/ \ /
Ground wire _/ \_ \ . . . . .
. . ground . . . . .. . . . . . .
10 amps
10 amps
Notice role of ground. How is the circuit
completed? Grounded vs. ungrounded demo
11
Grounded Patient Example
12
Safe ScenarioHot shorts to chassis, ground
intact
(short circuit)
! ! ! ___!_______________
/ \
_____ _____Hot______/_______/\/\/\/\ \
// \\
Patient
__Neutral_____________/\/\/\/\ wants
gt lt
toast \ . / /
........... \_____/ /
T o a s t e r
__________/ _______________________________
___ /
Fuse
______________
/.\ blows \
O.R. table -----towel-- / \ before /
\__________/
/ \ patient /
/ \ /
Ground wire _/
\_ \ . . . . . . . ground . . . . .. . . . . .
.
30 amps
30 amps
Notice role of ground. How is the circuit
completed?
13
Microshock hazard Neutral shorts, ground wire
intact

___________________ /
\ _____
_____Hot______/________/\/\/\/\ \
// \\
Patient __Neutral_____________/
\/\/\/\ wants gt lt
toast \ -
/ /
........... \_____/ / T
o a s t e r __________/
_________________________________
/ Short!!
Patient
/.\ is
\ O.R. table
-----towel-- / \ OK 0.0064 ?/ft
\__________/ / \
/
\ / Ground
wire _/ \_ \ . . . . . . . ground .
. . . .. . . . . . .
10 amps
5 amps
5 amps
VI R 5 A ? 0.064 ? 320 mV, even though the
chassisis properly grounded!
14
Line Isolation Transformer
  ___________________
_________________
Hot_____________
___________A
Isolation Neutral_________
Transformer ___________B
_________________
___________________
Ground_________________________________________Ir
relevant  
Cannot create a short by going to ground. 30 volt
transformer example
15
Line Isolation Monitor
  ___________________
_________________
Hot_____________
___________A
Isolation Neutral_________
Transformer ___________B
_________________
___________________
Ground_________________________________________Ir
relevant  

• Checks that resistance between A or B and ground
  is less than 75,000 ohms

16
Microshock

• Current delivered directly to heart
• 10 ?A will cause fibrillation
• The chassis voltage from a neutral-ground short
  carrying 5 amps, 320 mV, exceeds the microshock
  threshold. Since the line isolation monitor will
  sense this, the monitor is able to provide some
  protection against microshock.

17
Microshock

• Voltage to induce VF through intracardiac pacing
  electrode 5 mV
• Chassis voltage with neutral short to ground in
  conventional circuit 320 mV
• Chassis voltage with neutral short to ground with
  functioning line isolation monitor 0.120 mV
• If isolation transformer and line isolation
  monitor are working, and ground wires are intact,
  a chassis cannot reach a voltage that causes
  microshock hazard.

18
Electrocautery

• Frequencies exceed 500,000 Hz.
• No risk of VF, even if applied directly to heart
• Travels on surface, rather than through tissues
• Can complete circuit through inductance (e.g.
  antenna)
• Major concern is burns
• Thats the purpose of the device.
• Grounding plate is return electrode
• Patients should not be grounded!
• Plate dispenses current over wide area to prevent
  burns.
• Proper grounding plate placement and wiring is
  key to burn prevention.

19
Electrocautery Return Electrodes

• If the return electrode is broken, current will
  find alternative path back to electrocautery
  unit.
• ECG Electrodes
• Twitch Monitor (case of 3rd degree facial burn at
  VA)
• Be suspicious if standard settings are not
  producing an adequate spark
• Electrocautery example

20
Electrocautery Return Electrodes

• Place the return electrode close to surgical site
• If an ECG electrode is closer, it may still take
  some of the current
• Dont place over metal implants
• Function as internal lightening rods, focusing
  current on a small area near the implant, and the
  return electrode

21
Bipolar electrocautery

• No return electrode, current just flows between
  two poles
• Useful for laparoscopy, because current doesnt
  flow through bowel, risking burning the intestine

22
Summary

• Macroshock
• 80 mA can induce VF across an arm-arm circuit
• Hard with dry skin, easier with wet skin
• Isolation transformer makes it impossible to
  complete circuit to ground in OR, provided ground
  wires are attached.
• Microshock
• 10 ?A can induce VF on the heart
• Isolation monitor limits the ability of any
  grounded chassis to come up to adequate voltage
  to induce microshock
• Electrocautery
• Correct placement and function of return
  electrode prevents burns at site where current
  exits the body.