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Subelement T0
Electrical safety: AC and DC power circuits; antenna installation; RF hazards
Section T0A
Power circuits and hazards: hazardous voltages; fuses and circuit breakers; grounding; lightning protection; battery safety; electrical code compliance
Which of the following is a safety hazard of a 12-volt storage battery?
  • Touching both terminals with the hands can cause electrical shock
  • Correct Answer
    Shorting the terminals can cause burns, fire, or an explosion
  • RF emissions from the battery
  • All of these choices are correct

Shorting the terminals of a 12-volt storage battery with a good conductor can result in a very high current flow. Because P = I X E, high I (Current) means high P (Power), which can be exhibited in burns, fire, or even explosion due to rapid battery discharge.

Touching both battery terminals with only your hands, even if they are wet, because of the high resistance in your hands, will result in a low power output because P = V2/R, with dissipated power therefore inversely related to the resistance.

The only way a battery will release poisonous gas is if it is severely overcharged, and blows a seal. RF radiation won't do that, but could cause heat and other effects.

Finally, since two of the answers are incorrect, they cannot all be correct.

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Tags: arrl chapter 9 arrl module 18

How does current flowing through the body cause a health hazard?
  • By heating tissue
  • It disrupts the electrical functions of cells
  • It causes involuntary muscle contractions
  • Correct Answer
    All of these choices are correct

Your nervous system functions using electrical signals. Further, your body acts as a resistor. These two factors are the basis for electricity causing a health hazard. Because your nervous system uses electrical impulses, electricity can disrupt the cells' normal functions, cause muscles to contract or even disrupt the electrical signals to your heart causing it to beat irregularly or stop. Since your body acts as a resistor, an electrical current through the body will dissipate heat. As the current increases, the heat dissipated also increases and may transfer enough heat energy to damage tissue.

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Tags: electrical current safety arrl chapter 9 arrl module 18

What is connected to the green wire in a three-wire electrical AC plug?
  • Neutral
  • Hot
  • Correct Answer
    Safety ground
  • The white wire

Standard electrical cable contains three wires. The black wire is the 'hot' wire or source. The white wire is the return. The third wire is the safety ground and may be either bare (no sheath) or green. If there is a green wire, it's the safety ground. Normally, current does not flow in the safety ground. It's there to prevent accidental shocks. If you need a last second reminder, just remember that green usually means ground.

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Tags: safety grounding arrl chapter 9 arrl module 18

What is the purpose of a fuse in an electrical circuit?
  • To prevent power supply ripple from damaging a circuit
  • Correct Answer
    To interrupt power in case of overload
  • To limit current to prevent shocks
  • All of these choices are correct

A "ripple" would be an up and down type of effect, which is not what a fuse necessarily prevents.

A fuse doesn't prevent a person from getting shocked or even mortally harmed depending on the circuit.

When a circuit draws too much current, it is described as 'overloaded. This condition may damage the equipment or pose a shock hazard. The method to respond to an overload condition is to use a fuse. The fuse is designed to vaporize a small wire if the current becomes too large, thereby creating an open circuit. The fuse is always placed in series with the source, or 'hot,' wire.

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Tags: electrical components electronics safety arrl chapter 3 arrl module 6

Why is it unwise to install a 20-ampere fuse in the place of a 5-ampere fuse?
  • The larger fuse would be likely to blow because it is rated for higher current
  • The power supply ripple would greatly increase
  • Correct Answer
    Excessive current could cause a fire
  • All of these choices are correct

This would completely eliminate the purpose of having a fuse in the first place. Clearly with the intended 5-amp fuse for the circuit, that would be the maximum current that would be safe. A 20 amp fuse would not stop the circuit from overheating in the scenario.

Fuses are selected based on the capability of the circuit they protect to handle a specific amount of current. If the current exceeds the current rating of the circuit, it can cause it to dissipate too much heat and start things on fire. A rather vivid demonstration of this is sometimes used in college physics classes where a 12 gauge copper wire is directly connected across the terminals to a 12 v car battery. Because the wire is unprotected and the car battery can supply hundreds of amps, so much heat will be generated in the copper wire that it glows bright white and melts. (This is NOT recommended to try, because it splatters very hot, molten copper.)

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Tags: safety electrical components arrl chapter 3 arrl module 6

What is a good way to guard against electrical shock at your station?
  • Use three-wire cords and plugs for all AC powered equipment
  • Connect all AC powered station equipment to a common safety ground
  • Use a circuit protected by a ground-fault interrupter
  • Correct Answer
    All of these choices are correct

The key to avoiding electrical shock is to use properly connected electrical wires and good safety grounds. A three wire ground includes the safety ground and should be used for all equipment. Two wire cords should be avoided because they lack the ground wire. The use of a common ground ensures proper grounding of the equipment and avoids 'ground loops,' which may circulate currents and pose a shock hazard.

A mechanical interlock, more specifically a mechanical safety interlock, turns off power when an equipment access door is opened or a cover is removed.

Finally, a GFI (ground-fault interrupter) will sense cases where current is flowing in the safety ground wire and disconnect the power.

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Tags: safety grounding arrl chapter 9 arrl module 18

Which of these precautions should be taken when installing devices for lightning protection in a coaxial cable feed line?
  • Include a parallel bypass switch for each protector so that it can be switched out of the circuit when running high power
  • Include a series switch in the ground line of each protector to prevent RF overload from inadvertently damaging the protector
  • Keep the ground wires from each protector separate and connected to station ground
  • Correct Answer
    Ground all of the protectors to a common plate which is in turn connected to an external ground

Installing switches is a way to bypass the ground system, which defeats the purpose of having a ground system in the first place. Connecting the antenna ground to station ground will cause nothing but problems for your station equipment. The correct answer is to connect the protectors to a common plate, giving them a heavy duty connection point that channels the current to an external ground wire. Be sure to observe local building codes for correct grounding. Also note that any ground wires should be large, short and follow a direct path. Avoid sharp turns and wires longer than necessary.

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Tags: grounding safety feedline arrl chapter 9 arrl module 18

What safety equipment should always be included in home-built equipment that is powered from 120V AC power circuits?
  • Correct Answer
    A fuse or circuit breaker in series with the AC hot conductor
  • An AC voltmeter across the incoming power source
  • An inductor in series with the AC power source
  • A capacitor across the AC power source

Refer to question A04 for a discussion on fuses. As noted there, the fuse should always be in series with the source, or 'hot' conductor.

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Tags: ac power safety arrl chapter 9 arrl module 18

What kind of hazard is presented by a conventional 12-volt storage battery?
  • It emits ozone which can be harmful to the atmosphere
  • Shock hazard due to high voltage
  • Correct Answer
    Explosive gas can collect if not properly vented
  • All of these choices are correct

12 volt storage batteries use lead and sulfuric acid. When the battery is charged, Hydrogen gas is generated. Hydrogen is extremely flammable and in the right combination of oxygen will burn so fast that it explodes. Obviously, the 12 volt battery is not high voltage and it won't generate ozone.

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Tags: dc power safety batteries arrl chapter 5 arrl module 12

What can happen if a lead-acid storage battery is charged or discharged too quickly?
  • Correct Answer
    The battery could overheat and give off flammable gas or explode
  • The voltage can become reversed
  • The memory effect will reduce the capacity of the battery
  • All of these choices are correct

See answer A09 for more details on hydrogen. Fast charging or discharging generates hydrogen faster which makes it easier to create concentrations capable of exploding. Also, the chemical processes that occur when charging or discharging are exothermic (heat generating), so it is possible to overheat and damage the battery. Note that the battery terminal voltage will not be reversed and there is no 'memory effect' for lead acid batteries like there is for the nickel-cadmium batteries used in some consumer electronics.

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Tags: batteries safety arrl chapter 5 arrl module 12

What kind of hazard might exist in a power supply when it is turned off and disconnected?
  • Static electricity could damage the grounding system
  • Circulating currents inside the transformer might cause damage
  • The fuse might blow if you remove the cover
  • Correct Answer
    You might receive an electric shock from the charged stored in large capacitors

Power supplies may use large capacitors to help provide smooth output voltages. These capacitors can store a significant amount of energy and may store it at large enough voltages (more than 30 volts) to pose a shock hazard. The charge could last quite a few minutes, depending on the power supply capacitors. If there is a charge on the capacitor, you may inadvertently become the low resistance drain, so it's a good idea to check the circuit with a voltmeter before you expose yourself to a potential shock hazard. Even without the shock hazard (lower voltage), it's a good idea to drain the capacitors to prevent damage to sensitive circuits. A resistor between 1 and 10 Megaohms generally works well for something like this.

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Tags: dc power capacitance safety arrl chapter 9 arrl module 18

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