109 lines
5.1 KiB
Markdown
109 lines
5.1 KiB
Markdown
**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.
|
||
|
||
{width="423" height="331"}
|
||
|
||
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**\
|
||
|
||
|
||
- -Do not work alone while working with high voltages or if you are
|
||
using electrically operated machinary like a drill.
|
||
- -Never leave high voltages on when you are not present.
|
||
- -Keep one hand in your pocket when probing high voltage circuits or
|
||
discharging capacitors.
|
||
- -Make sure all high voltage connections are adequately taped or
|
||
otherwise insulated to prevent accidental contact by you or
|
||
neighboring students.
|
||
- -After switching power off, discharge any capacitors that were in
|
||
the circuit. Do not trust supposedly discharged capacitors. Certain
|
||
types of capacitors can build up a residual charge after being
|
||
discharged. Use a shorting bar across the capacitor, and keep it
|
||
connected until ready for use.
|
||
- -If you use electrolytic capacitors, do not
|
||
- - put excessive voltage across them
|
||
- put ac across them
|
||
- connect them in reverse polarity
|
||
|
||
- -Take extreme care using tools that can cause short circuits if
|
||
accidental contact is made to other circuit elements. Only tools
|
||
with insulated handles should be used.
|
||
- -If a person comes in contact with a high voltage, immediately shut
|
||
off power. Do not attempt to remove a person in contact with a high
|
||
voltage unless you are insulated from them.
|
||
- -In the event of an electrical fire do not use water. The lab fire
|
||
extinguishers are specifically charged for electrical fires. Vacate
|
||
the lab and close the door. Do not breath toxic smoke or fumes. Ring
|
||
the fire alarm, if one is available.
|
||
- -Check wire current carrying capacity if you will be using high
|
||
currents. The lab power wiring can only handle 15 Amperes
|
||
continuously.
|
||
- -Make sure your leads are rated to withstand the voltages you are
|
||
using. This includes instrument leads. Common wire insulation is
|
||
rated for 600 Volts.
|
||
- -Avoid simultaneous touching of any metal chassis used as an
|
||
enclosure for your circuits and any pipes in the laboratory that may
|
||
make contact with the earth, such as a water pipe. Use a floating
|
||
voltmeter to measure the voltage from ground to the chassis to see
|
||
if a hazardous potential difference exists.
|
||
- -Make sure that the lab instruments are at ground potential by using
|
||
the ground terminal supplied on the instrument.
|