UNIVERSITY of PENNSYLVANIA
Department of Electrical and Systems Engineering

ESE171: Principles of Digital Design Laboratory

Spring 2012


Lab Safety


Acquaint yourself with the location of the following safety items within the lab.
 
  1.  Fire extinguisher
  2.  First aid kit
  3.  Telephone and emergency numbers : Make sure that you have handy emergency phone numbers to call for assistance if necessary. The number for  emergencies is on campus is 511. (No need to dial 8 for this number). If any safety questions arise, consult the lab instructor or staff for guidance and instructions.


Observing proper safety precautions is important when working in the laboratory to prevent harm to yourself or others. The most common hazard is the electric shock which can be fatal if one is not careful.

Electric shock

Shock is caused by passing an electric current through the human body. The severity depends mainly on the amount of current and is less function of the applied voltage. The threshold of electric shock is about 1 mA which usually gives an unpleasant tingling. For currents above 10 mA, severe muscle pain occurs and the victim can't let go of the conductor due to muscle spasm. Current between 100 mA and 200 mA (60 Hz AC) causes ventricular fibrillation of the heart and is most likely to be lethal.

What is the voltage required for a fatal current to flow? This depends on the skin resistance. Wet skin can have a resistance as low as 150 Ohm and dry skin may have a resistance of 15 kohm. Arms and legs have a resistance of about 100 Ohm and the trunk 200 Ohm. This implies that 110 V can cause about 160 mA to flow in the body if the skin is wet and thus be fatal. In addition skin resistance falls quickly at the point of contact, so it is important to break the contact as quickly as possible to prevent the current from rising to lethal levels.

Equipment grounding

Electric instruments and appliances have equipment cases that are electrically insulated from the wires that carry the power. The isolation is provided by the insulation of the wires as shown in the figure a below. However, if the wire insulation gets damaged and makes contact to the case, the case will be at the high voltage supplied by the wires. If the user touches the instrument he or she will feel the high voltage. If, while standing on a wet floor, a user simultaneously comes in contact with the instrument case and a pipe or faucet connected to ground, a sizable current can flow through him or her, as shown in Figure b. However, if the case is connected to the ground by use of a third (ground) wire, the current will flow from the hot wire directly to the ground and bypass the user as illustrated in figure c.

shockimage
Equipment with a three wire cord is thus much safer to use. The ground wire (3rd wire) which is connected to metal case, is also connected to the earth ground (usually a pipe or bar in the ground) through the wall plug outlet.

Safety Precautions
 



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