Electrical Safety in the
Laboratory
Joshua D. Horvath
Andrew J. Gellman
Carnegie Mellon University
Department of Chemical Engineering
February 18, 2003
Accidents
•
Electrical shock from laser
power supply
•
Electrical shock from
electrophoresis power
supply
Introduction
Relationship between power,
current, voltage and resistance:
V  I R
P  I V
P  I R
2
P = power (watts)
I = current (amperes)
V = voltage (volts)
R = resistance (ohms)
Example:
Heating tape uses 5 amps at 120 volts.
Power dissipated by this device is 600 watts.
AC vs. DC Electricity
Direct current (DC)
Potential (volts)
2
1
0
-1
0
300
600
900
Time
Alternating current (AC)
Electrical Components
•
Capacitors
•
Resistors
•
Integrated circuits
Measuring Voltage
Connect
Probe
Connect
Probe
+ 5V -
Safety Devices
High Voltage Probe
•
•
•
•
Connects to normal multimeter.
Reduces voltage by factor of 1000.
Should only be used with training
or assistance.
Dangerous if used improperly. You
could be KILLED.
Measuring Current (DC)
Connect
Probe
Connect
Probe
+ 5V -
Safety Devices
Clamp-on Ammeter
•
•
•
Measures AC current through a
single conductor.
Does not work for DC current.
If both wires (positive and
negative) pass through the clamp,
no current reading is possible.
Lab Equipment
Power requirements (input)
Equipment
Volts Amps Watts
PC
120
6.0
720
Monitor
120
1.5
180
Turbopump
120
4.0
480
Cryopump
240
12.0
2,880
Heating tape (big) 120
6.92
830
FT-IR
120
5.0
600
Heat gun
120
14.0
1680
Lethal Electricity
Chart below assumes 1 second hand to foot
exposure to 120 volt, 60 cycle AC power supply.
Current
Reaction
1 mA
Perception
5 mA
Slight shock felt; not
painful but disturbing
6-30 mA
Painful shock; "let-go"
range
50-150 mA
Extreme pain, respiratory
arrest, severe muscular
contraction
1,000-4,300 mA Ventricular fibrillation
10,000+ mA
Cardiac arrest, severe
burns and probable death
Laboratory wall outlets provide 20,000 mA !!
Safety Devices
Fuses
•
•
Protect electrical wiring from
carrying too much current.
Protect malfunctioning equipment
from drawing too much current.
New fuse
•
•
•
Blown fuse
Conductor melts if too much
current flows through fuse.
Can only be used once.
Always use the correct fuse.
Safety Devices
Circuit Breakers
•
•
•
Protect electrical wiring from
carrying too much current.
Mechanical switch is driven by an
electromagnet.
Can only be reset many times.
Safety Devices
Circuit Breakers (in action)
Safety Devices
Circuit Breakers
•
•
How much equipment can one 20A
circuit breaker handle?
Not enough to bake an entire
vacuum chamber.
Device
Current
Heat tape (1 inch by 8 feet)
6.92 A
Heat tape (1 inch by 8 feet)
6.92 A
Heat tape (1/2 inch by 5 feet)
2.58 A
Heat tape (1/2 inch by 10 feet)
4.33 A
Infrared lamp (inside chamber)
2.75 A
Total
23.5 A
Safety Devices
Circuit Breakers
•
•
One circuit breaker may cover
many electrical receptacles in a
lab.
Receptacles in some labs are
labeled with a number that
corresponds to a circuit breaker.
Preventing Accidents
•
Avoid contact with energized circuits.
•
Disconnect the power source before
servicing or repairing electrical
equipment.
•
Work with only one hand, keeping the
other hand behind you or in a pocket.
•
If water or a chemical is spilled onto
equipment, shut off power at the main
switch or circuit breaker and unplug the
equipment.
•
If an individual comes in contact with a
live electrical conductor, do not touch
the equipment, cord or person.
•
Disconnect the power source from the
circuit breaker or pull out the plug using
a leather belt .
Preventing Accidents
Extension Cords
•
•
•
Do not plug one extension cord
into another extension cord.
Do not exceed safety ratings.
If cord is warm to touch, it is
probably overloaded.
Amperage
0-5
5-7
Cord length
(feet)
25
50
75
100
150
200
7-12
12-16
16-20
Wire gauges
18
18
16
14
12
12
18
16
14
12
12
10
16
14
12
10
14
12
10
12
10
Sizes smaller than wire gauge 10 are not normally available as flexible wire.
Preventing Accidents
Electrical Fires
•
•
•
•
Unplug equipment first.
Use dry chemical fire
extinguisher.
If fire is large, unplug
equipment and call fire
dept.
Never use water!!
Conclusions
•
Be careful with electricity.
•
Ask someone for help if
you are unsure.