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Electrical Safety Precautions and Basic Equipment
 

Electrical Safety Precautions and Basic Equipment

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The electricity that powers machines, tools, and other experimental apparatus must be handled with care and attention. Bodily contact with high voltage and current can cause muscular spasms, burns, cardiac arrest, and even death. Even small amounts of current passed through the body can cause electrocution. 10 milliamps can induce muscle contractions, loss of muscular control, and the inability to let go. 10 microamps through the heart can cause ventricular fibrillation. Laboratory experiments typically use equipment that complies with International Safety Standards. The Underwriters Laboratory UL label, for example, certifies that equipment meets these standards, which prevents certain types of hazardous exposure. However, electrical inputs and outputs, or customized equipment still pose a hazard. This video will present electrical safety precautions and introduce common electrical equipment used in many types of laboratory experiments.

When using electrical equipment, wear long pants, closed-toe shoes, and appropriate personal protective equipment. Avoid loose clothing, and remove any dangling or metal accessories that can accidentally contact electricity. In the United States, single-phase AC power from a wall outlet is 120 Volts. Outlets for three-phase AC power can provide up to 480 volts and over 10 amps. So power sources must be handled with respect. A clean lab environment is important for mitigating hazards. Avoid loose or frayed wires, cables, and connections. Know how to turn off all equipment, power supplies, and circuit breakers. Make sure that at least two people work on an experiment that has accessible DC power greater than 50 volts. Use the same precautions with single-phase or three-phase AC mains power. Assume any exposed metal carries live electricity unless verified. Before changing a setup, turn off or unplug power sources used in the experiment. Proper grounding of equipment ensures the chassis is at earth-ground potential, which prevents electrical shock. Always plug equipment into AC outlets with the power cord intended for it. Equipment that is hotter than expected is both a hazard and a symptom of a problem that should be addressed. Finally, turn off all equipment after an experiment is finished, and turn off unused equipment before leaving the lab. Now that basic safety precautions have been presented, the operation of some common electrical equipment will be demonstrated in the laboratory.

A function generator produces signals for other equipment needing an excitation or a drive voltage. The most common periodic outputs are sinusoidal, triangular, sawtooth, and square waves, which may be adjusted in amplitude, frequency, and DC offset. The output of the function generator is connected to the circuit or equipment using cables. Typically a BNC connector is used at one end, and alligator clips at the other end for easy connection to a circuit. A DC power supply provides voltage or a current to operate other electrical equipment. The adjustable output of a typical low-voltage laboratory supply ranges between 0 and 36 volts. Most single-output DC power supplies have three terminals: plus, minus, and ground. The plus terminal is connected to the higher voltage input of downstream equipment. The minus terminal is connected to the lower voltage input. The output is the voltage or current between the plus and minus terminals, which are electrically isolated from the ground. The ground terminal is a fixed earth-ground reference that is zero volts. Other common power sources include single-phase AC power from a standard wall outlet, or three-phase AC power. Single-phase power has one hot line and one neutral line for carrying current and delivers 120 volts. Three-phase power delivers higher voltages via three hot lines, with AC voltage on each line equal in frequency and magnitude, and 120 degrees out of phase from one another. The result can supply 208, 230, and 480 volts, with correspondingly greater power. Handling three-phase power requires special training and safety precautions. Next, a variable autotransformer, also known as a Variac, is used to either step up or down AC voltage. This is useful in applications requiring non-standard voltages or where the voltage must be varied. A knob varies the output voltage between zero and 100% of its maximum value. Note that the Variac does not provide electrical isolation, so avoid touching the output at any setting. An oscilloscope displays the voltages of time-varying signals, and is used to study the behavior of circuits. Oscilloscopes may have multiple channels, each displaying a single waveform. The two main types of probes used with this instrument are the conventional grounded probe and the differential probe. Here a regular grounded probe is connected to channel one. The grounded probe is usually rated to tolerate several hundred volts and measures voltage between the probe tip and its ground lead. The ground lead is tied to earth ground at the chassis of the oscilloscope. It is important to connect the ground lead only to a point in the circuit that is also grounded. Touching a ground lead to any other point will cause a short-circuit to ground. Now connect channel one of the oscilloscope to the output of the function generator, then turn it on. Adjust the time-scale of the oscilloscope with the seconds per division knob, and adjust the voltage scale with the volts per division knob. The trigger level is the voltage that a signal crosses to cause synchronization of the oscilloscope. Proper triggering minimizes noise in the display. Adjust the trigger knob to set the trigger level manually, or press set level to 50% to set it automatically. Finally, the multimeter is a versatile handheld, or bench-top instrument, for measuring voltage, current, resistance, and other electrical quantities. To measure voltage, insert the red probe into the contact labeled V Ohms and the black probe in the contact labeled COM for common. Turn on the DC power supply and set it to output 20 volts. Measure across the two output terminals by touching the red probe to the plus terminal and the black probe to the minus terminal. The multimeter reads 20 volts.

Many experiments require the measurement of electrical quantities, and use basic instruments to provide this data. The study of polar dielectric liquid bridges requires a high-intensity electric field between two beakers of fluid. The beakers are initially in contact, and then are slowly pulled apart to form the bridge. In this application, a high-voltage DC power supply generates 1,500 volts, which requires great care for safe handling. To develop ways to control neural stem cell migration for therapeutic treatments, researchers studied their movement under the influence of an electric field. An experimental chamber used a DC power supply to generate the required controlled electric field. An ammeter measured the current and a multimeter measured the voltage across the test chamber, which was used to calculate electric field strength.

You've just watched JoVE's introduction to electrical safety and basic electronic equipment. You should now understand how to work safely with electricity and how to use some basic electrical test equipment. Thanks for watching!

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