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ON THE USE OF A REALLIFE ACCIDENT CASE ON THE USE OF A REALLIFE ACCIDENT CASE TO EFFECTIVELY TEACH THE SUBJECT OF TO EFFECTIVELY TEACH THE SUBJECT OF ELECTRICAL SAFETY IN THE ENGINEERING ELECTRICAL SAFETY IN THE ENGINEERING
CURRICULUMCURRICULUM
Navarun Gupta, Vaibhav Bhatnagar, Navarun Gupta, Vaibhav Bhatnagar, Mark Mattie, and Lawrence V. HmurcikMark Mattie, and Lawrence V. Hmurcik
Electrical Engineering DepartmentElectrical Engineering Department
University of BridgeportUniversity of BridgeportBridgeport, CT 06604Bridgeport, CT 06604
Industrial Voltages and Resistance Associated Industrial Voltages and Resistance Associated With Dry SkinWith Dry Skin
Industrial voltages Industrial voltages 110v, 220v, 440v, 110v, 220v, 440v,
ResistanceResistance
An approximate value of dry skin resistance An approximate value of dry skin resistance R=60,000/VR=60,000/V Example: If the voltage is 120 DC or 120 volts AC RMS, Example: If the voltage is 120 DC or 120 volts AC RMS,
then the skin resistance then the skin resistance
R=60,000/120=500ohmsR=60,000/120=500ohmsCore R=500 ohms (constant)Core R=500 ohms (constant)
LOTO ( Locked OutTagged Out)LOTO ( Locked OutTagged Out)
All electric Circuits, circuit boxes, circuit All electric Circuits, circuit boxes, circuit machinery, etc. that are potentially dangerous machinery, etc. that are potentially dangerous
must bemust be Locked OutTagged OutLocked OutTagged Out
Figure 1Figure 1
VFD (Variable Frequency Drive) VFD (Variable Frequency Drive)
Shows the open electric CabinetShows the open electric Cabinet
Left of fuses is the clock wire that Left of fuses is the clock wire that bypasses the safety interlock.bypasses the safety interlock.
Three line phase wires feed in from Three line phase wires feed in from the floorthe floor
Plastic acrylic block is opened to Plastic acrylic block is opened to expose 3 fuses limited to 125 amps expose 3 fuses limited to 125 amps per phase.per phase.
Figure 2Figure 2
Capacitor Bank lies on the floor next to open electric cabinet Capacitor Bank lies on the floor next to open electric cabinet and dead victim.and dead victim.
Hot lead of each capacitor is protected by a plastic cap( on top Hot lead of each capacitor is protected by a plastic cap( on top of the aluminum metal housing).of the aluminum metal housing).
Figure 3Figure 3
Automatic shutoff on ceiling is too far away to have prevented Automatic shutoff on ceiling is too far away to have prevented accident accident
The VFD requires a clock to synchronize its output frequency (i.e. The VFD requires a clock to synchronize its output frequency (i.e. chopping frequency).chopping frequency).
The frequency of choice, 60Hz is readily available from the line The frequency of choice, 60Hz is readily available from the line voltage found in America.voltage found in America.
From the output power of the 3phase circuit a single wire runs From the output power of the 3phase circuit a single wire runs from one of the line voltages through a limiting resistor and to the from one of the line voltages through a limiting resistor and to the clock.clock.
Clock requires a trivial amount of current.Clock requires a trivial amount of current.
Each phase supplies 125 amps and since the draw from the clock Each phase supplies 125 amps and since the draw from the clock chip is limited to 20mA.chip is limited to 20mA.
Variable Frequency Drive
Possible Ways Victim Was ElectrocutedPossible Ways Victim Was Electrocuted
John could have reached under the plastic acrylic plate? Current John could have reached under the plastic acrylic plate? Current would be touching 440 volts of line voltage (587 mA).would be touching 440 volts of line voltage (587 mA).
John could have died from the capacitor bank. Stand alone John could have died from the capacitor bank. Stand alone capacitor can kill.capacitor can kill.
John could have died from a back emf. He opens the cabinet door John could have died from a back emf. He opens the cabinet door and start testing and back emf voltage generated by the stopped and start testing and back emf voltage generated by the stopped motor.motor.
John could have died from the electric wire supplying the clock. John could have died from the electric wire supplying the clock. This appears to be the least suspect.This appears to be the least suspect.
Conclusion 1Conclusion 1
John was purple in color when the Police found him. In medicine John was purple in color when the Police found him. In medicine this is described as cyanotic.this is described as cyanotic.
If John received a large electric current (like 568 mA) he would If John received a large electric current (like 568 mA) he would have been dead in seconds from ventricular fibrillation. His red have been dead in seconds from ventricular fibrillation. His red blood would have remained red. His heart kept beating, his lungs blood would have remained red. His heart kept beating, his lungs were “off”, and after several minutes John died of asphyxiation.were “off”, and after several minutes John died of asphyxiation.
Vfib not possible here.Vfib not possible here.
Respiratory arrest is not a good candidate in John’s death. No Respiratory arrest is not a good candidate in John’s death. No burn marks on his head.burn marks on his head.
Conclusion 2Conclusion 2
The Capacitor bank is a very good suspect in this case. The The Capacitor bank is a very good suspect in this case. The current produced is a time varying exponential. current produced is a time varying exponential.
The peak current is 676 mA. This can easily induce ventricular The peak current is 676 mA. This can easily induce ventricular fibrillating. It takes 5 and 20 seconds to die from vfib.fibrillating. It takes 5 and 20 seconds to die from vfib.
No vfib possible with cyanosis.No vfib possible with cyanosis.
Conclusion 3Conclusion 3
The back emf from the motor is not a very good suspect here. The back emf from the motor is not a very good suspect here.
There is sufficient power. However, one must consider the time There is sufficient power. However, one must consider the time constant. constant.
In a capacitive system, power decays according to the time In a capacitive system, power decays according to the time constant RC.constant RC.
In an inductive system, power decays as L/R. In an inductive system, power decays as L/R.
So the inductance and Resistance is large.So the inductance and Resistance is large.
If the power feedback to load in the VFD, this was 2 million ohms. If the power feedback to load in the VFD, this was 2 million ohms.
John removed the capacitor bank before he died. This requires John removed the capacitor bank before he died. This requires several minutes. several minutes.
Conclusion 4Conclusion 4
The clock wire killed John. The clock wire killed John.
Basically, the clock was synchronized by the 60 Hz AC line Basically, the clock was synchronized by the 60 Hz AC line voltage. voltage.
When John opened up the VFD to service it, John killed all of the When John opened up the VFD to service it, John killed all of the power to the VFD via an automatic interlock, built in as part of the power to the VFD via an automatic interlock, built in as part of the machine. However, John did not kill the power of the extra wire machine. However, John did not kill the power of the extra wire run by the company electrician.run by the company electrician.
To have put this extra wire into the VFD did not of itself violate the To have put this extra wire into the VFD did not of itself violate the known laws of electric safety, so long as the electrician was willing known laws of electric safety, so long as the electrician was willing to padlock the VFD and place on it a warning label detailing the to padlock the VFD and place on it a warning label detailing the new danger (LOTO). new danger (LOTO).
ReferencesReferences[1] Electric Impedance Tomography, J. G. Webster, Hilger Press (1990). ISBN [1] Electric Impedance Tomography, J. G. Webster, Hilger Press (1990). ISBN
0852473041.0852473041.[2] “Reevaluation of Lethal Electric Currents”, Charles F. Dalziel, IEEE [2] “Reevaluation of Lethal Electric Currents”, Charles F. Dalziel, IEEE
Transactions on Industrial and Industrial Applications, VOL IGA4, 467 Transactions on Industrial and Industrial Applications, VOL IGA4, 467 (1968). (1968).
[3] Handbook of Electrical Hazards and Accidents, Leslie A. Geddes, CRC [3] Handbook of Electrical Hazards and Accidents, Leslie A. Geddes, CRC Press (1995). ISBN 0849394317.Press (1995). ISBN 0849394317.
[4] National Electrical Code (NEC 2005), National Fire Protection Association, [4] National Electrical Code (NEC 2005), National Fire Protection Association, One Batterymarch Park, Quincy MA 02169, ISBN 0877656231.One Batterymarch Park, Quincy MA 02169, ISBN 0877656231.
[5] Medical and Bioengineering Aspects of Electrical Injuries, Raymond M. Fish [5] Medical and Bioengineering Aspects of Electrical Injuries, Raymond M. Fish and Leslie A. Geddes, Lawyers and Judges Publishing Company (2003), and Leslie A. Geddes, Lawyers and Judges Publishing Company (2003), ISBN 1930056087.ISBN 1930056087.
[6] “Behind the scenes with VFD’s”, Electric Construction and Maintenance [6] “Behind the scenes with VFD’s”, Electric Construction and Maintenance magazine (EC&M) September 2006, pages 1821. See also “Instruction magazine (EC&M) September 2006, pages 1821. See also “Instruction Manual for the Danfoss VLT 5000 Series, Adjustable Frequency Drive”, Manual for the Danfoss VLT 5000 Series, Adjustable Frequency Drive”, Danfoss Electronics Drives, 2995 East Rockford, IL 61109. Danfoss Electronics Drives, 2995 East Rockford, IL 61109.
[7] Electromagnetic Fields, Roald K. Wangsness, Wiley (1986). ISBN [7] Electromagnetic Fields, Roald K. Wangsness, Wiley (1986). ISBN 0471811866. 0471811866.
[8] Bioengineering, Biomedical, Medical, and Clinical Engineering, A. Terry [8] Bioengineering, Biomedical, Medical, and Clinical Engineering, A. Terry Bahill, PrenticeHall (1981), ISBN 0130763802.Bahill, PrenticeHall (1981), ISBN 0130763802.