Electrical Safety in the Laboratory Joshua D Horvath

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: 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) Alternating current (AC)

Electrical Components • Capacitors • Resistors • Integrated circuits

Measuring Voltage Connect Probe + 5 V -

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 + 5 V -

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 m. A Perception 5 m. A Slight shock felt; not painful but disturbing 6 -30 m. A Painful shock; "let-go" range 50 -150 m. A Extreme pain, respiratory arrest, severe muscular contraction 1, 000 -4, 300 m. A Ventricular fibrillation 10, 000+ m. A Cardiac arrest, severe burns and probable death Laboratory wall outlets provide 20, 000 m. A !!

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 20 A 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 (1/2 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 10 16 14 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.