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Subelement T0
SAFETY
Section T0A
Power circuits and hazards: hazardous voltages, fuses and circuit breakers, grounding, electrical code compliance; Lightning protection; Battery safety
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 a nearby transmitter can cause the electrolyte to emit poison gas
  • All 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.

Furthermore, because the battery presents a DC voltage, it cannot present an RF signal on its own, so RF emissions from the battery are not a concern.

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

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Tags: arrl chapter 9 arrl module 22

What health hazard is presented by electrical current flowing through the body?
  • It may cause injury by heating tissue
  • It may disrupt the electrical functions of cells
  • It may cause involuntary muscle contractions
  • Correct Answer
    All 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 22

In the United States, what circuit does black wire insulation indicate in a three-wire 120 V cable?
  • Neutral
  • Correct Answer
    Hot
  • Equipment ground
  • Black insulation is never used

Remember "Black = Booboo." It's silly, but it worked for me early on remembering which was hot and which was common.

Black wires are supposed to be hot, or negative, when used properly. And, mishandling the black ones may lead to injury because the black wire carries the electricity from the source to the device/terminal/etc.
The white wire carries unused electricity away from the device back into the circuit.
And, ground wires provide a safe means for electricity to be discharged in the event of a short-circuit.

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

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 5

Why should a 5-ampere fuse never be replaced with a 20-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 these choices are correct

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 5

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
  • Install mechanical interlocks in high-voltage circuits
  • Correct Answer
    All 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. 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 22

Where should a lightning arrester be installed in a coaxial feed line?
  • At the output connector of a transceiver
  • At the antenna feed point
  • At the ac power service panel
  • Correct Answer
    On a grounded panel near where feed lines enter the building

A lightning arrestor is use to direct induced energy to ground. The most effective location is where the feedline enters the building. The arrestor should be mounted on a properly grounded panel.

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Where should a fuse or circuit breaker be installed in a 120V AC power circuit?
  • Correct Answer
    In series with the hot conductor only
  • In series with the hot and neutral conductors
  • In parallel with the hot conductor only
  • In parallel with the hot and neutral conductors

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 22

What should be done to all external ground rods or earth connections?
  • Waterproof them with silicone caulk or electrical tape
  • Keep them as far apart as possible
  • Correct Answer
    Bond them together with heavy wire or conductive strap
  • Tune them for resonance on the lowest frequency of operation

Bonding external ground rods or earth connections is a best practice. The reference to heavy wire or a conductive strap is to make them useful for high current situations, such as a lightning strike, and to minimize resistance between them so they all participate in connecting to ground without having high resistance.

Grounds do not need to be waterproofed or taped, except, perhaps, to keep connections from oxidizing. So, that isn't a good answer.

Grounds are generally not tuned, except, perhaps, if the actual ground is far from the equipment and you need to make sure the ground presents a low-impedance at RF frequencies - but you wouldn't want the connections to be resonant.

The purpose of the ground rods is to make an electrical connection with ground, so there's no particular reason to place them far apart. There's certainly no danger in placing them close together.

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What hazard is caused by charging or discharging a battery too quickly?
  • Correct Answer
    Overheating or out-gassing
  • Excess output ripple
  • Half-wave rectification
  • Inverse memory effect

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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What hazard exists in a power supply immediately after turning it off?
  • Circulating currents in the dc filter
  • Leakage flux in the power transformer
  • Voltage transients from kickback diodes
  • Correct Answer
    Charge stored in filter 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 22

Which of the following precautions should be taken when measuring high voltages with a voltmeter?
  • Ensure that the voltmeter has very low impedance
  • Correct Answer
    Ensure that the voltmeter and leads are rated for use at the voltages to be measured
  • Ensure that the circuit is grounded through the voltmeter
  • Ensure that the voltmeter is set to the correct frequency

What they're getting at here, is YOUR SAFETY!

If you're measuring high voltage, you need to make sure the meter probes, leads and insides of the meter can take that high voltage without conducting that voltage to you or burning up. Both of those are undesirable features for a high voltage voltmeter.

Voltmeters have a very high impedance - indeed, the standard for voltmeters used to be 20,000 Ohms/Volt. You want to make sure the voltmeter doesn't affect the circuit that you're measuring, so it has to have a very high impedance. So the choice of low impedance is just wrong.

You always hear that things have to be grounded well. But you don't want to be grounding the high voltage through your meter! You might want to measure the difference in voltage between ground and some circuit, but the intention is not to ground the circuit. If you did that, the difference would be zero, and that measure would only obtain for a short while, the length of time it took to blow up your circuit or the meter.

Voltmeters might be measuring RF energy, but you usually want a voltmeter that will measure a wide range of RF frequencies, not a particular frequency. So setting it to the "correct frequency" is a bit of nonsense.

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