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SourceMaterialTomerge/tsysShopmanual/safety.html
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SourceMaterialTomerge/tsysShopmanual/safety.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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"http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<title>ESE171: Lab Safety</title>
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</head>
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<body>
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<p style="margin-top: 0; margin-bottom: 0;"> </p>
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<center>
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<i><font size=+2><a href="http://www.seas.upenn.edu"><b>UNIVERSITY of PENNSYLVANIA</b></a></font></i>
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<br>
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<font size=+2><a href="http://www.ese.upenn.edu"><b>Department of Electrical and Systems Engineering</b></a></font>
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<p style="margin-bottom: 0"><font size=+2><b>ESE171: Principles of Digital Design Laboratory</b></font>
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<p style="margin-top: 0; margin-bottom: 0;"> </p>
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<p style="margin-top: 0; margin-bottom: 0;"><font size=+2>Spring 2015</font>
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<p style="margin-top: 0; margin-bottom: 0;"> </p>
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<hr>
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<p style="margin-top: 0; margin-bottom: 0;"> </p>
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<p style="margin-top: 0; margin-bottom: 0;"><font size=+2><b>Lab Safety</b></font></p>
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</center>
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<br>
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<b>Acquaint yourself with the location of the following safety items within
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the lab.</b>
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<br>
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<ol>
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<li>
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Fire extinguisher</li>
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<li>
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First aid kit</li>
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<li>
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Telephone and emergency numbers : Make sure that you have handy emergency
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phone numbers to call for assistance if necessary. The number for
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emergencies is on campus is <b>511</b>. (No need to dial 8 for this number).
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If any safety questions arise, consult the lab instructor or staff for
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guidance and instructions.</li>
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</ol>
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<p style="margin-bottom: 0;"><br>Observing proper safety precautions is important when working in
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the 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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<h3>
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<font size=+3>Electric shock</font></h3>
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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 function
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of the applied voltage. The threshold of electric shock is about 1 mA which
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usually gives an unpleasant tingling. For currents above 10 mA, severe
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muscle pain occurs and the victim can't let go of the conductor due to
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muscle spasm. Current between 100 mA and 200 mA (60 Hz AC) causes ventricular
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fibrillation of the heart and is most likely to be lethal.
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<p style="margin-bottom: 0">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 resistance
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of about 100 Ohm and the trunk 200 Ohm. This implies that 110 V can cause
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about 160 mA to flow in the body if the skin is wet and thus be fatal.
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In addition skin resistance falls quickly at the point of contact, so it
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is important to break the contact as quickly as possible to prevent the
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current from rising to lethal levels.
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<p style="margin-top: 0;"> </p>
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<p style="margin-bottom: 0;"><font size=+2>Equipment grounding</font>
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<p style="margin-bottom: 0;">Electric instruments and appliances have equipment cases that are electrically
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insulated from the wires that carry the power. The isolation is provided
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by the insulation of the wires as shown in the figure a below. However,
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if the wire insulation gets damaged and makes contact to the case, the
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case will be at the high voltage supplied by the wires. If the user touches
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the instrument he or she will feel the high voltage. If, while standing
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on a wet floor, a user simultaneously comes in contact with the instrument
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case and a pipe or faucet connected to ground, a sizable current can flow
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through him or her, as shown in Figure b. However, if the case is connected
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to the ground by use of a third (ground) wire, the current will flow from
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the hot wire directly to the ground and bypass the user as illustrated
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in figure c.
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<p style="margin-bottom: 0;">
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<center><img SRC="shock.gif" ALT="shockimage" BORDER=0 height=331 width=423></center>
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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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<p style="margin-bottom: 0;"><b><font size=+3>Safety Precautions</font></b>
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<br>
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<ul style="margin-bottom: 0;">
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<li>
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-Do not work alone while working with high voltages or if you are using
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electrically operated machinary like a drill.</li>
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<li>
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-Never leave high voltages on when you are not present.</li>
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<li>
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-Keep one hand in your pocket when probing high voltage circuits or discharging
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capacitors.</li>
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<li>
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-Make sure all high voltage connections are adequately taped or otherwise
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insulated to prevent accidental contact by you or neighboring students.</li>
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<li>
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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 types
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of capacitors can build up a residual charge after being discharged.
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Use a shorting bar across the capacitor, and keep it connected until ready
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for use.</li>
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<li>
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-If you use electrolytic capacitors, do not</li>
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<li>
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<ul>
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<li>
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put excessive voltage across them</li>
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<li>
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put ac across them</li>
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<li>
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connect them in reverse polarity</li>
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</ul>
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</li>
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<li>
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-Take extreme care using tools that can cause short circuits if accidental
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contact is made to other circuit elements. Only tools with insulated handles
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should be used.</li>
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<li>
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-If a person comes in contact with a high voltage, immediately shut off
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power. Do not attempt to remove a person in contact with a high voltage
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unless you are insulated from them.</li>
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<li>
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-In the event of an electrical fire do not use water. The lab fire extinguishers
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are specifically charged for electrical fires. Vacate the lab and close
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the door. Do not breath toxic smoke or fumes. Ring the fire alarm, if one
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is available.</li>
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<li>
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-Check wire current carrying capacity if you will be using high currents.
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The lab power wiring can only handle 15 Amperes continuously.</li>
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<li>
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-Make sure your leads are rated to withstand the voltages you are using.
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This includes instrument leads. Common wire insulation is rated for
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600 Volts.</li>
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<li>
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-Avoid simultaneous touching of any metal chassis used as an enclosure
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for your circuits and any pipes in the laboratory that may make contact
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with the earth, such as a water pipe. Use a floating voltmeter to measure
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the voltage from ground to the chassis to see if a hazardous potential
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difference exists.</li>
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<li>
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-Make sure that the lab instruments are at ground potential by using the
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ground terminal supplied on the instrument.</li>
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</ul>
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<p style="margin-top: 0; margin-bottom: 0;"> </p>
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<hr>
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<br>
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Back to the <a href="../index.html">ESE171 homepage</a>
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</body>
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</html>
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