109 lines
5.1 KiB
Markdown
109 lines
5.1 KiB
Markdown
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**Lab Safety**
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**Acquaint yourself with the location of the following safety items
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within the lab.**
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1. Fire extinguisher
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2. First aid kit
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3. Telephone and emergency numbers : Make sure that you have handy
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emergency phone numbers to call for assistance if necessary. The
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number for emergencies is on campus is **511**. (No need to dial 8
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for this number). If any safety questions arise, consult the lab
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instructor or staff for guidance and instructions.
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\
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Observing proper safety precautions is important when working in the
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laboratory to prevent harm to yourself or others. The most common hazard
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is the electric shock which can be fatal if one is not careful.
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### Electric shock
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Shock is caused by passing an electric current through the human body.
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The severity depends mainly on the amount of current and is less
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function of the applied voltage. The threshold of electric shock is
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about 1 mA which usually gives an unpleasant tingling. For currents
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above 10 mA, severe muscle pain occurs and the victim can\'t let go of
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the conductor due to muscle spasm. Current between 100 mA and 200 mA (60
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Hz AC) causes ventricular fibrillation of the heart and is most likely
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to be lethal.
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What is the voltage required for a fatal current to flow? This depends
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on the skin resistance. Wet skin can have a resistance as low as 150 Ohm
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and dry skin may have a resistance of 15 kohm. Arms and legs have a
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resistance of about 100 Ohm and the trunk 200 Ohm. This implies that 110
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V can cause about 160 mA to flow in the body if the skin is wet and thus
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be fatal. In addition skin resistance falls quickly at the point of
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contact, so it is important to break the contact as quickly as possible
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to prevent the current from rising to lethal levels.
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Equipment grounding
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Electric instruments and appliances have equipment cases that are
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electrically insulated from the wires that carry the power. The
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isolation is provided by the insulation of the wires as shown in the
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figure a below. However, if the wire insulation gets damaged and makes
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contact to the case, the case will be at the high voltage supplied by
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the wires. If the user touches the instrument he or she will feel the
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high voltage. If, while standing on a wet floor, a user simultaneously
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comes in contact with the instrument case and a pipe or faucet connected
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to ground, a sizable current can flow through him or her, as shown in
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Figure b. However, if the case is connected to the ground by use of a
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third (ground) wire, the current will flow from the hot wire directly to
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the ground and bypass the user as illustrated in figure c.
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{width="423" height="331"}
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Equipment with a three wire cord is thus much safer to use. The ground
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wire (3rd wire) which is connected to metal case, is also connected to
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the earth ground (usually a pipe or bar in the ground) through the wall
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plug outlet.
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**Safety Precautions**\
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- -Do not work alone while working with high voltages or if you are
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using electrically operated machinary like a drill.
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- -Never leave high voltages on when you are not present.
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- -Keep one hand in your pocket when probing high voltage circuits or
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discharging capacitors.
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- -Make sure all high voltage connections are adequately taped or
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otherwise insulated to prevent accidental contact by you or
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neighboring students.
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- -After switching power off, discharge any capacitors that were in
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the circuit. Do not trust supposedly discharged capacitors. Certain
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types of capacitors can build up a residual charge after being
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discharged. Use a shorting bar across the capacitor, and keep it
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connected until ready for use.
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- -If you use electrolytic capacitors, do not
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- - put excessive voltage across them
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- put ac across them
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- connect them in reverse polarity
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- -Take extreme care using tools that can cause short circuits if
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accidental contact is made to other circuit elements. Only tools
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with insulated handles should be used.
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- -If a person comes in contact with a high voltage, immediately shut
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off power. Do not attempt to remove a person in contact with a high
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voltage unless you are insulated from them.
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- -In the event of an electrical fire do not use water. The lab fire
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extinguishers are specifically charged for electrical fires. Vacate
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the lab and close the door. Do not breath toxic smoke or fumes. Ring
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the fire alarm, if one is available.
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- -Check wire current carrying capacity if you will be using high
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currents. The lab power wiring can only handle 15 Amperes
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continuously.
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- -Make sure your leads are rated to withstand the voltages you are
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using. This includes instrument leads. Common wire insulation is
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rated for 600 Volts.
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- -Avoid simultaneous touching of any metal chassis used as an
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enclosure for your circuits and any pipes in the laboratory that may
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make contact with the earth, such as a water pipe. Use a floating
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voltmeter to measure the voltage from ground to the chassis to see
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if a hazardous potential difference exists.
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- -Make sure that the lab instruments are at ground potential by using
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the ground terminal supplied on the instrument.
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