Lab Safety

Lab Safety

Acquaint yourself with the location of the following safety items within the lab. a. fire extinguisher b. first aid kit c. telephone and emergency numbers : Make sure that you have handy emergency phone numbers to call for assistance if necessary. The number for emergencies is 3003. (No need to dial 9 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. 2

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.

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Safety Facts • The threshold of electric shock is about 1 mA which usually gives an unpleasant tingling. • For less than 10 mA at the skin level, the person merely feels a "funny" sensation • For currents above 10 mA, the person freezes to the circuit and is unable to let go • For currents of 100 mA to 1 A, the likelihood of sudden death is very high • More than 1 A, the heart experiences a single contraction, and internal heating is significant.

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Factors affecting human Safety • • • • • •

Voltage level Current flowing in person Resistance of body Frequency of source Duration of shock Pathway of current

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Effect of Voltage What is the voltage required for a fatal current to flow? This depends on the skin resistance.

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.

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• The higher the voltage the higher the current! • 100-400 V ac is the most lethal voltage – High enough to cause significant current flow in the body – Can cause muscles to contract tightly on the energized equipment. • At higher voltages, fierce involuntary muscle contractions may throw the victim away from the hazard.

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Effect of Current • High current causes heating damage to tissues. • 10 µ A passing directly through the heart can cause cardiac arrest. Heart muscle fibers beat out of sync, so no blood is pumped • The spinal cord may also be affected, altering respiration control. 100-1000 mA is sufficient to induce respiratory arrest and/or cardiac arrest. • Thermal heating of tissues increases with the square of the current (I2R).

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Effects of Electric Current in the Human Body Current

Reaction

1 Milliampere

Perception level. Just a faint tingle.

5 Milliamperes

Slight shock felt; not painful but disturbing.

6-25 Milliamperes (women)

Painful shock, muscular control is lost.

9-30 Milliamperes (men)

This is called the freezing current or "let-go" range.

50-150 Milliamperes

Extreme pain, respiratory arrest, severe muscular contractions.* Individual cannot let go. Death is possible.

1,000-4,300 Milliamperes

Ventricular fibrillation. (The rhythmic pumping action of the heart ceases.) Muscular contraction and nerve damage occur. Death is most likely.

10,000Milliamperes

Cardiac arrest, severe burns and probable death.

Average individual can let go. However, strong involuntary reactions to shocks in this range can lead to injuries.

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Effect of Body Resistance • Wet skin can have a resistance as low as 150 Ohm. • Dry skin may have a resistance of 15 kohm. • Arms and legs have a resistance of about 100 Ohm and the trunk 200 Ohm. • Palm resistance can range from 100 Ω to 1 MΩ . • Nerves, arteries and muscle are low in resistance. • Bone, fat and tendon are relatively high in resistance. • Across the chest of an average adult, the resistance is about 70-100 Ω .

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Effect of Source Frequency • 50-60 Hz current has a much greater ability to cause ventricular fibrillation than D.C. current. • At 50-60 Hz, involuntary muscle contractions may be so severe that the individual cannot let go of the power source. • As the frequency gets above about 500 kHz, little energy passes through the internal organs.

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Effect of Duration • The longer the duration, the more severe the internal heating of tissues. • With 110-240 V, the individual is incapable of letting go.

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Effect of Pathway • If the current passes through the brain or heart, (e.g. head to arm, arm to arm) the likelihood of a lethal result increases significantly.

Use one hand! 13

Equipment grounding • Grounding is very important. Improper grounding can be the source of errors, noise and a lot of trouble. Please consult the section of "Circuit Ground and Grounding Practice". Here we will focus on equipment grounding as a protection against electrical shocks. • 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. 14

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 the following 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. 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. 15

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Neutral vs Ground High voltage Side

Load

Cable resistance

Neutral Lead

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Neutral vs Ground

Load

Voltage across cable resistance

V =I R V = 100 × 1 = 100 V

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Grounded vs. Grounding • The terms grounded and grounding are very similar, but their meanings are quite different. • In any electrical circuit, there are two wires needed to complete any circuit. One is called the "hot wire" and the other is called "neutral" or "grounded". Sometimes the neutral wire is referred to as a grounded wire. It is most correctly referred to as a "grounded neutral conductor," but most times referred to as "the neutral" or "the ground wire".

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Since the neutral or grounded wire is a necessary part of the electrical path, grounded wires carry electrical current under normal operating conditions. A grounded wire is required by the National Electrical Code to be white or gray in color on the customer side of the meter. Grounded wires on the utility side of the system do not generally have insulation.

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A "grounding" wire on the other hand is a safety wire that has intentionally been connected to earth. The grounding wire does not carry electricity under normal circuit operations. It's purpose is to carry electrical current only under short circuit or other conditions that would be potentially dangerous. Grounding wires serve as an alternate path for the current to flow back to the source, rather than go through anyone touching a dangerous appliance or electrical box.

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• Confusion arises because it is commonly referred to as a ground wire even though it is more correctly called a "grounding" wire. Some people will refer to this wire as the "case ground" since this wire is typically connected to the cases or outer parts of electrical boxes and appliances and tools. • The grounding wire is required by the National Electrical Code to be a bare wire, or if insulated, a green or green with yellow colored insulation.

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3-prong Receptacles

Neutral Lead (N)

High voltage (H)

Ground (G)

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High voltage (H)

Neutral (N) Even if grounded Direct or indirect conduction

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High voltage (H)

Neutral (N)

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Safety Precautions • Think safety when voltage exceeds 12 V • Don’t work on energized circuits • Wear insulating shoes (where static electricity is not a concern) • Use one hand when working on energized circuits • Learn CPR • Do not work alone while working with high voltages or if you are using electrically operated machinery 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. 27

• 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 • 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.

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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 29