FCC Technician Class Exam Study Guide - 2014-2018/Subelement T0 Group A

Power circuits and hazards: hazardous voltages; fuses and circuit breakers; grounding; lightning protection; battery safety; electrical code compliance

Amateur Radio Operators occasionally need to handle items that are energized with high voltages or have enormous amounts of energy stored in them. Such powerful sources must always be handled with care to prevent serious accidents from occurring.

Human Health. There is a misconception that "voltage kills", but this is actually untrue considering that we may become charged with tens of thousands of volts simply by handling a latex balloon while wearing a woolen sweater. We must rely on the truth that "current kills". It only takes between 10-100ma to trigger muscle spasms, disrupt cellular activity or cause the burning of flesh do to Ohmic heating. Always assume that we are at earth potential of zero volts and that all higher voltages will induce a current through us.

AC Power Sources. North American electrical service in a modern structure has three wires delivered to your receptacle. Inside this receptacle, these three conductors are a green wire for Ground, a white wire for Neutral and a wire of a different color for the Hot. Traditionally Hot is a black wire, but that isn't a requirement. The Hot should never be a white or green wire as those are reserved exclusively for Neutral or Ground. It is always worthwhile to test that your house is wired and grounded correctly before setting up your ham shack or lab bench.

AC sources provide a maximum current that is determined by the cross-sectional dimension of the wire. Because the conductors have a resistance, current flow would cause heat to be released. Typically the heat is released in junctions where wires are fastened together as those regions have the highest resistance. The NFPA (National Fire Protection Association) has done extensive research to create an elegant set of rules for correct dimensioning and insulation of conductors. Overcurrent conditions that can cause heating of the conductor are mitigated with a circuit breaker (or fuses for pre-1960 installations).

The circuit breakers, or fuses, primarily serve the goal of preventing fires, but our additional green ground wire provides a human safety benefit. Lets say the Hot wire comes loose inside your appliance and the metal frame becomes energized, it is a shock hazard. A grounded appliance will bleed this voltage to ground potential of zero volts. This typically results in an over current where a circuit breaker trips or a fuse blows.

When a circuit breaker trips or a fuse burns out, interpret that as a warning. You have just prevented a fire or electrocution by a safety feature being activated. When you defeat these systems by installing an over-dimensioned breaker or fuse, you are defeating safety features that solely exist to protect life and limb. Always determine the cause of an incident. If you don't have an exact replacement, it is acceptable to use a smaller dimensioned protection at the risk of a nuisance tripping that rapidly reduces the life of a breaker.

Current flow in an AC system is between two conductors, the Hot and Neutral. The Hot alternates between a high positive potential and a high negative potential and the Neutral is the return path for the electron flow. There are a number of high quality technologies that provide greater protection for the user. Arc-Fault breakers contain an embedded system that measures current flow and disables the current flow if it detects what appears to be arcing in the load. GFCI breakers, are Ground Fault Current Interrupting where if more that 20ma is unaccounted for in the flow between Hot and Neutral, the current flow is shut off. Some companies, such as E-T-A, even produce circuit breakers that are drop in replacements for fuses and will fit in the existing holders.

Battery Sources. These are particularly dangerous as they can deliver much higher currents that AC electrical service. A lead-acid battery from a car can easily deliver hundreds of amperes of current. This enormous current capability means that an incredible amount of heating can occur during a short circuit. There are instances of screwdrivers melting after being sec upon the terminals of the batter. The same current that passes through the short circuit also passes through the battery and ohmic heating can rapidly overheat the battery. Treat the battery with the same amount of care as you would a fuel canister. The same energy that can power your transmitter over the course of an afternoon can also start a fire within seconds.

I am guilty of short circuiting a car battery on one occasion. In this case a solar cell was charging the battery. While unplugging the solar cell, the terminals landed on each other. Immediately, the entire wire turned to a very beautiful smoke cloud that rose straight up (and very uniformly) across the entire length of wire. Each loop the wires made across the floor made perfect smoke rings as well.

Some components such as inductors, transformers and certain electric motors must never be attached to batteries as they will quickly become short circuited once the magnetic fields are created.

When batteries overheat, either through overcharging or over current, the water in the battery will separate into molecular oxygen and hydrogen. These gases are violently explosive in their mutual presence, not to mention, already heated due to current flow. An outgassing battery is an explosion hazard, especially if it's outgassing faster than pressure can be released. Disconnect a noisy battery and ventilate the room to dissipate the gases.

The Question Pool

Question 1

Which of the following is a safety hazard of a 12-volt storage battery?

A. Touching both terminals with the hands can cause electrical shock

B. Shorting the terminals can cause burns, fire, or an explosion

C. RF emissions from the battery

D. All of these choices are correct

The voltage is harmless, but the short circuit current can be enormous. Ohm's Law tells us that heat released is proportional to the current squared. Each time the current doubles, the power output multiplies by four. The correct answer is B.

Question 2

How does current flowing through the body cause a health hazard?

A. By heating tissue

B. It disrupts the electrical functions of cells

C. It causes involuntary muscle contractions

D. All of these choices are correct

All these things can happen. Ohmic heating of tissue and involuntary muscular contractions are often associated with electrical injuries. Cellular damage can also occur due to a potential difference across the cell. The correct answer is D.

Question 3

What is connected to the green wire in a three-wire electrical AC plug?

A. Neutral

B. Hot

C. Safety ground

D. The white wire

Green is a reserved color for ground. Don't ever use it for any other purpose. The ground is a return path for current in case a wiring fault makes an appliance hot, so that the breaker may trip if it draws enough current. The correct answer is C.

Question 4

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

A. To prevent power supply ripple from damaging a circuit

B. To interrupt power in case of overload

C. To limit current to prevent shocks

D. All of these choices are correct

The fuse is equivalent in application as a circuit breakers as the protect from over-current situations. They prevent fires, not shocks. The correct answer is B.

Question 5

Why is it unwise to install a 20-ampere fuse in the place of a 5-ampere fuse?

A. The larger fuse would be likely to blow because it is rated for higher current

B. The power supply ripple would greatly increase

C. Excessive current could cause a fire

D. All of these choices are correct

The components and wiring and specified for a 5 ampere fuse. If you exceed that current, you will have excessive heating which would be a fire hazard. In this case, the current is allowed to flow four times the specified max, which means ohmic losses will be squared, sixteen times as much heat will be dissipated. The correct answer is C.

Question 6

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

A. Use three-wire cords and plugs for all AC powered equipment

B. Connect all AC powered station equipment to a common safety ground

C. Use a circuit protected by a ground-fault interrupter

D. All of these choices are correct

Ground Fault Breakers save lives. Rumor has it you still feel the shock, but they interrupt before harm is done. Properly grounded equipment ensures the potential of the housing is at earth potential of zero volts. All grounds should be connected to each other to prevent potential difference between grounds. The correct answer is D.

Question 7

Which of these precautions should be taken when installing devices for lightning protection in a coaxial cable feed line?

A. Include a parallel bypass switch for each protector so that it can be switched out of the circuit when running high power

B. Include a series switch in the ground line of each protector to prevent RF overload from inadvertently damaging the protector

C. Keep the ground wires from each protector separate and connected to station ground

D. Ground all of the protectors to a common plate which is in turn connected to an external ground

Keep all your ground conductors connected. Lightning strikes have enormous voltages and can deliver enormous currents. In most cases, the grounds may have a small voltage across them, but during a lightning strike, these voltages can reach into the thousands of volts and that is a fire hazard. Even if the strike isn't direct, it's possible for the ground wires to act as antennas and become highly charged. Keep your grounds connected. The correct answer is D.

Question 8

What safety equipment should always be included in home-built equipment that is powered from 120V AC power circuits?

A. A fuse or circuit breaker in series with the AC hot conductor

B. An AC voltmeter across the incoming power source

C. An inductor in series with the AC power source

D. A capacitor across the AC power source

Circuit breakers will restrict over-currents to the smaller components that could otherwise combust during a runaway failure. The correct answer is A.

Question 9

What kind of hazard is presented by a conventional 12-volt storage battery?

A. It emits ozone which can be harmful to the atmosphere

B. Shock hazard due to high voltage

C. Explosive gas can collect if not properly vented

D. All of these choices are correct

Hydrogen and Oxygen are both released from the water in a battery. Two dangerous gases in a heated, sealed container. The correct answer is C.

Question 10

What can happen if a lead-acid storage battery is charged or discharged too quickly?

A. The battery could overheat and give off flammable gas or explode

B. The voltage can become reversed

C. The memory effect will reduce the capacity of the battery

D. All of these choices are correct

The excessive current will heat the battery rapidly. The same current that is being delivered is also flowing through the battery. The correct answer is A.

Question 11

What kind of hazard might exist in a power supply when it is turned off and disconnected?

A. Static electricity could damage the grounding system

B. Circulating currents inside the transformer might cause damage

C. The fuse might blow if you remove the cover

D. You might receive an electric shock from the charged stored in large capacitors

Some of the components can store energy long after the appliance is disconnected from power. Observe the voltage rating on the capacitors to get a hint on if the voltage is one you feel is dangerous. Remember, some metal capacitors may have energized cans around them. Typically these have plastic sleeves, but they also fall off or disappear. Glass or foil capacitors may have a very long self discharge time. Be careful discharging these because they can provide enormous currents; always use a resistor to discharge. The correct answer is D.