LESSON 7

SUBELEMENT T0 – AC power circuits, antenna installation, RF hazards – [3 Exam Questions - 3 Groups]

Lesson 7a

T0A – AC power circuits; hazardous voltages, fuses and circuit breakers, grounding, lightning protection, battery safety, electrical code compliance

Unfortunately every year people are injured or killed from electrical shock. Most of these incidents could have been avoided with a little safety percaution. 30 volts is the commonly accepted value for the lowest voltage that can cause a dangerous electrical shock.

Electric current flowing through the body causes a health hazard by heating the tissue, disrupting the electrical functions of cells, and causing involuntary muscle contractions.

Modern electrical outlets have three wires attached to them. The black or red wire is hot, the white wire is neutral, and the green wire is the safety ground. This should not be used for an RF ground but radio equipment using three wire plugs should never have the ground wire disabled or removed.

A fuse is designed to open up if the circuit draws to much current. Thus the fuse is designed to interrupt the power in case of an overload. Fuses should be replaced only with the proper size fuse designed for that circuit. Installing a 20 ampere fuse in a circuit designed for a 5 ampere fuse has the potential of excessive current and cause a fire.

One piece of safety equipment that should always be included in home build equipment that plugs into a 120 Volt power circuit is a fuse or circuit breaker in series with the AC hot conductor.

To protect yourself against electrical shock at your station you should use three-wire cords and plugs for all AC powered equipment, connect all AC power station equipment to a common safety ground, and use a circuit protected by a ground-fault interrupter.

You still might receive an electrical shock from a stored charge in large capacitors of power supplies even when they are turned off and unplugged.

12 volt lead-acid batteries have their own set of safety rules. Never add acid to a battery. If a battery needs to be recharged and commercial power is not available the battery can be connected to a car’s battery and run the engine. Conventional 12-volt storage batteries can have explosive gas collect if they are not properly vented. Lead-acid storage batteries should not be charged or discharged too quickly because they could overheat and give off flammable gas or explode.

All antennas should have lightning protection. If you install devices for lightning protection in a coaxial cable feedline precaution should be taken to ground all of the protectors to a common plate which is in turn connected to an external ground. Do not use the safety ground for lightning protection ground.

It is good practice when installing ground wires on a tower for lightning protection to ensure that connections are short and direct.

*T0A01
Which is a commonly accepted value for the lowest voltage that can cause a dangerous electric shock?
A. 12 volts
B. 30 volts
C. 120 volts
D. 300 volts
~~

*T0A02
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
~~

*T0A03
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
~~

*T0A04
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
~~

*T0A05
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
~~

*T0A06
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
~~

*T0A07
Which of these precautions should be taken when installing devices for lightning protection in a coaxial cable feedline?
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
~~

*T0A08
What is one way to recharge a 12-volt lead-acid station battery if the commercial power is out?
A. Cool the battery in ice for several hours
B. Add acid to the battery
C. Connect the battery to a car's battery and run the engine
D. All of these choices are correct
~~

*T0A09
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
~~

*T0A10
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
~~

*T0A11
Which of the following is good practice when installing ground wires on a tower for lightning protection?
A. Put a loop in the ground connection to prevent water damage to the ground system
B. Make sure that all bends in the ground wires are clean, right angle bends
C. Ensure that connections are short and direct
D. All of these choices are correct
~~

*T0A12
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 stored charge in large capacitors
~~

*T0A13
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
~~

Answers:
B, D, C, B, C, D, D, C, C, A, C, D, A

Lesson 7b

T0B – Antenna installation; tower safety, overhead power lines

It is good practice for all members of a tower work team to wear a hard hat and safety glasses at all times when any work is being done on the tower. It is also a good precautions to put on a climbing harness and safety glasses before climbing an antenna tower. It is never safe to climb a tower without a helper or observer.

On 10/12/2009 at 2040 Eastern Time a 55 year old man, his 49 year old wife, and their 15 year old son were electrocuted while attempting to raise an antenna… (see more: http://hamslife.com/?p=121). It is an important safety precaution to look for and stay clear of any overhead electrical wires when putting an antenna tower. The minimum safe distance from a power line to allow when installing an antenna is so that if the antenna falls unexpectedly, no part of comes closer then 10 feet to the power line.

Besides the danger of receiving a letter from the utility company telling you to remove it you should avoid attaching an antenna to a utility pole because the antenna could contact high-voltage power wires.

The local electrical codes establish grounding requirements for an amateur radio tower or antenna.

Separate eight-foot long ground rods for each tower leg, bonded to the tower and each other is considered to be a proper grounding method for a tower.

When grounding conductors used for lightning protection you must avoid sharp bends.

Crank-up towers are designed to be lowered when work is being done on the antenna system. This type of tower must never be climbed unless it is in the fully retracted position. If this safety rule is ignored there is a potential for the added weight of a person climbing the tower to cause the safety catch to give way and for the antenna to collapse and cut off finger and toes.

A gin pole is the tool used to lift tower sections or antennas and place them on the tower.

*T0B01
When should members of a tower work team wear a hard hat and safety glasses?
A. At all times except when climbing the tower
B. At all times except when belted firmly to the tower
C. At all times when any work is being done on the tower
D. Only when the tower exceeds 30 feet in height
~~

*T0B02
What is a good precaution to observe before climbing an antenna tower?
A. Make sure that you wear a grounded wrist strap
B. Remove all tower grounding connections
C. Put on a climbing harness and safety glasses
D. All of the these choices are correct
~~

*T0B03
Under what circumstances is it safe to climb a tower without a helper or observer?
A. When no electrical work is being performed
B. When no mechanical work is being performed
C. When the work being done is not more than 20 feet above the ground
D. Never
~~

*T0B04
Which of the following is an important safety precaution to observe when putting up an antenna tower?
A. Wear a ground strap connected to your wrist at all times
B. Insulate the base of the tower to avoid lightning strikes
C. Look for and stay clear of any overhead electrical wires
D. All of these choices are correct
~~

*T0B05
What is the purpose of a gin pole?
A. To temporarily replace guy wires
B. To be used in place of a safety harness
C. To lift tower sections or antennas
D. To provide a temporary ground
~~

*T0B06
What is the minimum safe distance from a power line to allow when installing an antenna?
A. Half the width of your property
B. The height of the power line above ground
C. 1/2 wavelength at the operating frequency
D. So that if the antenna falls unexpectedly, no part of it can come closer than 10 feet to the power wires
~~

*T0B07
Which of the following is an important safety rule to remember when using a crank-up tower?
A. This type of tower must never be painted
B. This type of tower must never be grounded
C. This type of tower must never be climbed unless it is in the fully retracted position
D. All of these choices are correct
~~

*T0B08
What is considered to be a proper grounding method for a tower?
A. A single four-foot ground rod, driven into the ground no more than 12 inches from the base
B. A ferrite-core RF choke connected between the tower and ground
C. Separate eight-foot long ground rods for each tower leg, bonded to the tower and each other
D. A connection between the tower base and a cold water pipe
~~

*T0B09
Why should you avoid attaching an antenna to a utility pole?
A. The antenna will not work properly because of induced voltages
B. The utility company will charge you an extra monthly fee
C. The antenna could contact high-voltage power wires
D. All of these choices are correct
~~

*T0B10
Which of the following is true concerning grounding conductors used for lightning protection?
A. Only non-insulated wire must be used
B. Wires must be carefully routed with precise right-angle bends
C. Sharp bends must be avoided
D. Common grounds must be avoided
~~

*T0B11
Which of the following establishes grounding requirements for an amateur radio tower or antenna?
A. FCC Part 97 Rules
B. Local electrical codes
C. FAA tower lighting regulations
D. Underwriters Laboratories' recommended practices
~~

Answers:
C, C, D, C, C, D, C, C, C, C, B

Lesson 7c
T0C - RF hazards; radiation exposure, proximity to antennas, recognized safe power levels, exposure to others

Debates have raged for decades whether or not RF radiation, in levels not high enough to start cooking the flesh, poses a serious health threat. The experts on the subject have no definitive answer and research seems to not produce any real proof of what level of RF radiation on a long term bases is safe. Even though there is no proof or agreement on the subject still there are standards set and questions on the ham radio test about those standards.

VHF and UHF radio signals are Non-ionizing radiation. So at least you do not have to worry about glowing in the dark after talking on your VHF or UHF radio.

50 watts Peak Envelope Power (PEP) at the antenna is the maximum power level that an amateur radio station may use at VHF frequencies before an RF exposure evaluation is required.

The frequency and power level of the RF field, the distance from the antenna to a person, and the radiation pattern of the antenna are all factors that affect the RF exposure of people near an amateur station antenna. The human body absorbs more RF energy at some frequencies than at others thus exposure limits vary with frequency.

If you fear you may be exposed to excessive levels of RF radiation you might relocate antennas to prevent exposure to RF radiation in excess of FCC-supplied limits.

Acceptable methods that can be used to determine that your station complies with FCC RF exposure regulations are by calculation based on FCC OET Bulletin 65, by calculation based on computer modeling, and by measurement of field strength using calibrated equipment. If you are asked this question on your test the answer is D all of the choices are correct.

You can make sure your station stays in compliance with RF safety regulations is by re-evaluating the station whenever an item of equipment is changed.


Of the following frequencies 3.5 MHz, 50 MHz, 440 MHz, and 1296 MHz 50 MHz has the lowest Maximum Permissible exposure limit.

The “duty cycle” when referring to RF exposure is the ratio of on-air time to total operating time of a transmitted signal. A pulse transmission may have a peak power of 1 KW but it is on only one tenth of a second then it is off for nine tenth of a second. When calculating the exposure power the transmitted power would be considered to be 100 watts. The “duty cycle” affects the average exposure of people to radiation thus it is one of the factors used to determine safe RF radiation exposure levels.

There is one bit of safety information that does have proof and has no question. Antennas should be kept where they are not apt to be touched by a person or pet. If a person accidentally touched your antenna while you were transmitting they might receive a painful RF burn. These RF burns can go deep and quick even when relatively low power is being used.

*T0C01
What type of radiation are VHF and UHF radio signals?
A. Gamma radiation
B. Ionizing radiation
C. Alpha radiation
D. Non-ionizing radiation
~~

*T0C02
Which of the following frequencies has the lowest Maximum Permissible Exposure limit?
A. 3.5 MHz
B. 50 MHz
C. 440 MHz
D. 1296 MHz
~~

*T0C03
What is the maximum power level that an amateur radio station may use at VHF frequencies before an RF exposure evaluation is required?
A. 1500 watts PEP transmitter output
B. 1 watt forward power
C. 50 watts PEP at the antenna
D. 50 watts PEP reflected power
~~

*T0C04
What factors affect the RF exposure of people near an amateur station antenna?
A. Frequency and power level of the RF field
B. Distance from the antenna to a person
C. Radiation pattern of the antenna
D. All of these choices are correct
~~

*T0C05
Why do exposure limits vary with frequency?
A. Lower frequency RF fields have more energy than higher frequency fields
B. Lower frequency RF fields do not penetrate the human body
C. Higher frequency RF fields are transient in nature
D. The human body absorbs more RF energy at some frequencies than at others
~~

*T0C06
Which of the following is an acceptable method to determine that your station complies with FCC RF exposure regulations?
A. By calculation based on FCC OET Bulletin 65
B. By calculation based on computer modeling
C. By measurement of field strength using calibrated equipment
D. All of these choices are correct
~~

*T0C07
What could happen if a person accidentally touched your antenna while you were transmitting?
A. Touching the antenna could cause television interference
B. They might receive a painful RF burn
C. They might develop radiation poisoning
D. All of these choices are correct
~~

*T0C08
Which of the following actions might amateur operators take to prevent exposure to RF radiation in excess of FCC-supplied limits?
A. Relocate antennas
B. Relocate the transmitter
C. Increase the duty cycle
D. All of these choices are correct
~~

*T0C09
How can you make sure your station stays in compliance with RF safety regulations?
A. By informing the FCC of any changes made in your station
B. By re-evaluating the station whenever an item of equipment is changed
C. By making sure your antennas have low SWR
D. All of these choices are correct
~~

*T0C10
Why is duty cycle one of the factors used to determine safe RF radiation exposure levels?
A. It affects the average exposure of people to radiation
B. It affects the peak exposure of people to radiation
C. It takes into account the antenna feedline loss
D. It takes into account the thermal effects of the final amplifier
~~

*T0C11
What is meant by "duty cycle" when referring to RF exposure?
A. The difference between lowest usable output and maximum rated output power of a transmitter
B. The difference between PEP and average power of an SSB signal
C. The ratio of on-air time to total operating time of a transmitted signal
D. The amount of time the operator spends transmitting
~~

Answers:
D, B, C, D, D, D, B, A, B, A, C