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T0A

SAFETY

Power circuits and hazards: hazardous voltages, fuses and circuit breakers, grounding, electrical code compliance; Lightning protection; Battery safety

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T0A011 of 12

Which of the following is a safety hazard of a 12-volt storage battery that lacks internal protection circuitry?

Why A lead-acid storage battery has very low internal resistance, so a short across its terminals can push hundreds or even thousands of amps through whatever bridges them. That current heats wires, tools or jewelry red hot in seconds, and the sparks can ignite the hydrogen gas that lead-acid batteries vent while charging, which is why a shorted battery can literally explode. The danger is the available current, not the voltage.
Watch out Twelve volts cannot push meaningful current through dry skin, so hand-to-hand shock is not the hazard; it generally takes on the order of 30 volts or more before shock becomes a real concern. RF from a nearby transmitter does not make battery electrolyte give off poison gas.
12 V won't shock you, but it will weld your wrench. Low voltage, huge current.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A022 of 12

What health hazard is posed by electrical current flowing through the body?

Why Current through tissue does damage in three distinct ways, so all of the listed effects are real. Resistive heating burns tissue along the current path (often worst at entry and exit points), the current interferes with the small bioelectric signals cells use, including the heart's rhythm, and it drives involuntary muscle contraction that can prevent you from letting go of the conductor. Even modest currents, on the order of 100 mA through the chest, can be fatal by causing ventricular fibrillation.
Watch out Each single-effect choice is true by itself but incomplete, so picking one of them misses the fact that the pool lists all three as valid hazards.
Current burns, confuses and clamps: heat, cell signals, muscle lock. Three hazards, so the all-of-these answer.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A033 of 12

In the United States, what circuit does black wire insulation indicate in a three-wire 120 V AC cable?

Why In US residential AC wiring the color code for a three-wire 120 V cable is fixed: black (or red for a second hot) is the ungrounded "hot" conductor carrying the live 120 V, white is the grounded neutral that returns current, and green or bare copper is the equipment safety ground. So black insulation marks the conductor you must treat as energized.
Watch out Neutral is the tempting pick because it also carries current, but neutral is white; it is bonded to ground at the service panel and normally sits near zero volts. "Negative" does not apply at all, since 120 V house wiring is AC, not DC.
Black = Back-feed danger (hot), White = neutral, Green = ground. Only the black one bites.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A044 of 12

What is the purpose of a fuse in an electrical circuit?

Why A fuse contains a thin metal element that melts and opens the circuit when current exceeds its rating, cutting power before wiring or components overheat and start a fire. It is an overcurrent protection device, rated in amperes, and it is sized to protect the wiring and equipment behind it. Circuit breakers do the same job but can be reset instead of replaced.
Watch out The idea that a fuse limits current to prevent shocks is tempting, but a fuse does not limit current, it interrupts it, and it blows at levels far above the few milliamps that are dangerous to people. Ripple is handled by filter capacitors in the power supply, not by a fuse.
A fuse does not limit current, it quits. Too much current, it opens and the power goes away.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A055 of 12

Why should a 5-ampere fuse never be replaced with a 20-ampere fuse?

Why A fuse is chosen to protect the wiring and the device, not the fuse itself. It should open before the current exceeds what the conductors and components can safely carry. Putting in a 20 A fuse where a 5 A fuse belongs lets up to four times the rated current flow through wiring sized for 5 A, so the wire or component can overheat and start a fire before the fuse ever opens.
Watch out The idea that the bigger fuse blows more easily reverses the rating: a higher amperage rating means it takes more current to melt it, so it blows less readily, which is exactly the problem.
The fuse protects the wire, not the fuse. Never fit a fuse bigger than the circuit is rated for.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A066 of 12

What is a good way to guard against electrical shock at your station?

Why Shock protection at a station is layered, and all three practices listed do real work. Three-wire grounded cords and plugs give equipment chassis a low-resistance path back to the service ground so a fault trips the breaker instead of energizing the case. Bonding all gear to a single common safety ground prevents dangerous voltage differences between chassis that you might bridge with your body. And high-voltage filter capacitors can hold a lethal charge long after power is removed, so they must be bled down and verified discharged before you reach inside.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A077 of 12

Where should a lightning arrester be installed in a coaxial feed line?

Why Lightning protection works by giving surge energy a short, low impedance path to earth before it can get inside the house. That means the arrester belongs on a grounded entry panel right where the feed lines come through the wall, bonded to the station's single-point ground, which is in turn bonded to the AC service ground. Surge current then dumps into earth outside rather than traveling along the coax to your equipment.
Watch out Putting it at the antenna feed point or at the transceiver still lets surge energy travel inside the building or through your gear; the AC service panel protects power wiring, not the coax.
Stop the surge at the door: ground panel where feed lines enter, bonded to a single-point ground.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A088 of 12

Where should a fuse or circuit breaker be installed in a 120V AC power circuit?

Why Overcurrent protection must interrupt the current, so the fuse or breaker goes in series with the conductor carrying the supply voltage, which is the hot (ungrounded) side. Opening only the hot conductor guarantees that when the device trips, the equipment is disconnected from the 120V source and nothing downstream stays energized. The National Electrical Code prohibits putting an overcurrent device in the grounded neutral, because a blown neutral fuse would leave the gear still connected to the hot line while appearing dead.
Watch out Fusing both hot and neutral sounds like extra safety but is actually a code violation and a hazard: if only the neutral fuse opens, the chassis can remain at line potential even though the equipment is off.
Fuses go in series (they must break the current) and only in the hot wire. Never fuse the neutral.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A099 of 12

What should be done to all external ground rods or earth connections?

Why The National Electrical Code requires all grounding electrodes at a building, the AC service ground rod, the telephone and cable entry grounds, and any tower or antenna ground rods, to be bonded together into one grounding electrode system. If they are left separate, a lightning strike or fault can raise one rod to thousands of volts above another, and the difference appears across your equipment and its cables. Bonding with heavy wire or wide copper strap keeps everything at the same potential so surge current flows to earth instead of through your radios or through you.
Watch out Keeping the rods far apart is exactly backwards: separated, unbonded electrodes create the voltage difference that damages gear. Ground rods are not tuned or resonant, and sealing them from soil moisture would only degrade the earth connection.
One station, one ground: bond every rod together. Separate grounds means dangerous voltage differences.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A1010 of 12

What hazard exists when rapidly charging or discharging an unprotected battery?

Why Fast charge or discharge pushes large currents through the cell's internal resistance, and that I squared R loss turns into heat inside the battery. Heat drives chemical breakdown: lead-acid and NiCd cells vent hydrogen and oxygen gas, and lithium cells can go into thermal runaway and burn or rupture. That is why batteries are used with protection circuits, fuses, and chargers matched to the chemistry and capacity, and why charging areas need ventilation.
Watch out Electric shock is a real concern with high-voltage battery banks, but the specific hazard tied to a high charge or discharge rate on an unprotected cell is heat and vented gas, not shock. Ripple and overvoltage describe power supply output quality, not what happens inside a stressed battery.
Fast current equals heat equals gas. Hydrogen plus a spark is the battery room explosion story.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A1111 of 12

What hazard exists in a power supply immediately after turning it off?

Why A power supply's filter capacitors store energy to smooth the rectified DC, and that charge does not vanish when you flip the switch. Large electrolytics in a high-voltage supply can hold hundreds of volts for minutes or longer, enough to be lethal. That is why supplies include bleeder resistors, and why you verify with a meter and discharge the caps before reaching inside.
Watch out Circulating currents in the filter and leakage flux in the transformer both require power to be flowing, so they stop when the supply is off; kickback diode transients belong to switching inductive loads, not a shut-down supply.
Power off is not safe: capacitors keep the charge. Meter it, bleed it, then touch it.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T0A1212 of 12

Which of the following precautions should be taken when measuring high voltages with a voltmeter?

Why Voltmeters and their test leads carry voltage and category ratings that reflect how much insulation and creepage distance they have. Exceeding that rating can cause arcing across the probe insulation or inside the meter, which can injure you or destroy the instrument. Before touching a high voltage circuit, confirm both the meter and the leads are rated at or above the voltage you expect to see.
Watch out Low impedance is the opposite of what you want in a voltmeter: a good voltmeter has very high input impedance so it draws almost no current and does not load the circuit being measured. A voltmeter is also never meant to be the ground path for a circuit, and meters are not 'set to a frequency'.
The leads count too. Meter AND leads must be rated for the voltage you are poking at.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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