Power circuits and hazards, hazardous voltages, fuses and circuit breakers, grounding, and battery safety
BE SAFE!
When operating or working on amateur radio equipment, it’s possible to come into contact with dangerous voltages and currents. People have died working on high-voltage circuits. Because it would be a shame to lose a single person, it’s important to know how to be safe when working with electricity.
30 volts is the commonly accepted value for the lowest voltage that can cause a dangerous electric shock, and only 100 mA flowing through the body can cause death. These are not very large values.
QUESTION: What health hazard is presented by electrical current flowing through the body? (T0A02)
ANSWER: All of these choices are correct
• It may cause injury by heating tissue
• It may disrupt the electrical functions of cells
• It may cause involuntary muscle contractions
Start by ensuring that your amateur radio station has three-wire electrical outlets that are properly grounded. When properly wired, three-wire electrical outlets and plugs are safer than two-wire outlets and plugs, and you should use three-wire plugs for all of your amateur radio equipment. The third wire provides an independent, or safety ground.
QUESTION: In the United States, what is connected to the green wire in a three-wire electrical AC plug? (T0A03)
ANSWER: Equipment ground
QUESTION: What is a good way to guard against electrical shock at your station? (T0A06)
ANSWER: All of these choices are correct
• 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
Installing fuses in an electrical circuit is the main way to protect a circuit from excessive current flow. Fuses are designed to “blow” or disconnect power if the current flowing through them exceeds a specified value.
QUESTION: What is the purpose of a fuse in an electrical circuit? (T0A04)
ANSWER: To interrupt power in case of overload
When replacing a fuse, always replace the blown fuse with a fuse of the same type and value.
QUESTION: Why is it unwise to install a 20-ampere fuse in the place of a 5-ampere fuse? (T0A05)
ANSWER: Excessive current could cause a fire
If you plan to build your own equipment, be sure to include fuses in your designs.
QUESTION: What safety equipment should always be included in home-built equipment that is powered from 120V AC power circuits? (T0A08)
ANSWER: A fuse or circuit breaker in series with the AC hot conductor
Whenever you’re working on equipment, be sure to disconnect it from the power lines, and even then be careful working around a power supply’s capacitors.
QUESTION: What kind of hazard might exist in a power supply when it is turned off and disconnected? (T0A11)
ANSWER: You might receive an electric shock from the charge stored in large capacitors
You also need to take precautions when using batteries to power your amateur radio station. Conventional 12-volt storage batteries present several safety hazards.
QUESTION: Which of the following is a safety hazard of a 12-volt storage battery? (T0A01)
ANSWER: Shorting the terminals can cause burns, fire, or an explosion
QUESTION: What can happen if a lead-acid storage battery is charged or discharged too quickly? (T0A10)
ANSWER: The battery could overheat, give off flammable gas, or explode
Antenna and tower safety
Antenna safety is also of primary concern. There are two aspects of antenna safety—being safe when installing an antenna and safely operating an antenna. When installing an antenna, make sure you note where the power lines are and stay clear of them.
QUESTION: Which of the following is an important safety precaution to observe when putting up an antenna tower? (T0B04)
ANSWER: Look for and stay clear of any overhead electrical wires
QUESTION: What is the minimum safe distance from a power line to allow when installing an antenna? (T0B06)
ANSWER: Enough so that if the antenna falls unexpectedly, no part of it can come closer than 10 feet to the power wires
QUESTION: Why should you avoid attaching an antenna to a utility pole? (T0B09)
ANSWER: The antenna could contact high-voltage power lines
You also should position the antenna so that no one can touch it while you are transmitting
QUESTION: What could happen if a person accidentally touched your antenna while you were transmitting? (T0C07)
ANSWER: They might receive a painful RF burn
There are a number of things you should keep in mind when putting up a tower and installing antennas on them.
QUESTION: When should members of a tower work team wear a hard hat and safety glasses? (T0B01)
ANSWER: At all times when any work is being done on the tower
QUESTION: What is a good precaution to observe before climbing an antenna tower? (T0B02)
ANSWER: Put on a carefully inspected climbing harness (fall arrester) and safety glasses
QUESTION: Under what circumstances is it safe to climb a tower without a helper or observer? (T0B03)
ANSWER: Never
QUESTION: Which of the following is an important safety rule to remember when using a crank-up tower? (T0B07)
ANSWER: This type of tower must not be climbed unless retracted or mechanical safety locking devices have been installed
QUESTION: What is the purpose of a safety wire through a turnbuckle used to tension guy lines? (T0B13)
ANSWER: Prevent loosening of the guy line from vibration
And, you should use a gin pole designed for use with the tower that you’re installing.
QUESTION: What is the purpose of a gin pole? (T0B05)
ANSWER: To lift tower sections or antennas
Grounding is very important when installing a tower. The tower is, after all, basically a big lightning rod.
QUESTION: Which of the following establishes grounding requirements for an amateur radio tower or antenna? (T0B11)
ANSWER: Local electrical codes
QUESTION: What is considered to be a proper grounding method for a tower? (T0B08)
ANSWER: Separate eight-foot long ground rods for each tower leg, bonded to the tower and each other
QUESTION: Which of the following is true when installing grounding conductors used for lightning protection? (T0B10)
ANSWER: Sharp bends must be avoided
QUESTION: Which of the following is good practice when installing ground wires on a tower for lightning protection? (T0B12)
ANSWER: Ensure that connections are short and direct
QUESTION: What should be done to all external ground rods or earth connections? (T0A09)
ANSWER: Bond them together with heavy wire or conductive strap
Lightning can also be conducted down a feed line and into your station. To prevent this, several manufacturers make devices designed to conduct this current to ground before it gets into the station.
QUESTION: Which of these precautions should be taken when installing devices for lightning protection in a coaxial cable feed line? (T0A07)
ANSWER: Mount all of the protectors on a metal plate that is in turn connected to an external ground rod
RF hazards and radiation exposure
Over-exposure to radio waves can also be a safety hazard. When using as little as 50 watts, you may be required to perform an RF exposure evaluation for your amateur radio station, even though VHF and UHF radio signals are non-ionizing radiation.
QUESTION: What type of radiation are VHF and UHF radio signals? (T0C01)
ANSWER: Non-ionizing radiation
QUESTION: How does RF radiation differ from ionizing radiation (radioactivity)? (T0C12)
ANSWER: RF radiation does not have sufficient energy to cause genetic damage
Even so, small levels of RF energy can be hazardous.
QUESTION: What is the maximum power level that an amateur radio station may use at VHF frequencies before an RF exposure evaluation is required? (T0C03)
ANSWER: 50 watts PEP at the antenna
How do you perform an RF exposure evaluation?
QUESTION: Which of the following is an acceptable method to determine that your station complies with FCC RF exposure regulations? (T0C06)
ANSWER: All of these choices are correct
• By calculation based on FCC OET Bulletin 65
• By calculation based on computer modeling
• By measurement of field strength using calibrated equipment
One of the factors to consider when performing an RF exposure evaluation is the duty cycle of your transmissions. A transmission with a lower duty cycle would be less hazardous than a high duty cycle transmission.
QUESTION: What is the definition of duty cycle during the averaging time for RF exposure? (T0C11)
ANSWER: The percentage of time that a transmitter is transmitting
QUESTION: Why is duty cycle one of the factors used to determine safe RF radiation exposure levels? (T0C10)
ANSWER: It affects the average exposure of people to radiation
Consider this example.
QUESTION: If the averaging time for exposure is 6 minutes, how much power density is permitted if the signal is present for 3 minutes and absent for 3 minutes rather than being present for the entire 6 minutes? (T0C13)
ANSWER: 2 times as much
Because of the way radio waves interact with the body, the exposure limits are different for each amateur radio band.
QUESTION: Why do exposure limits vary with frequency? (T0C05)
ANSWER: The human body absorbs more RF energy at some frequencies than at others
QUESTION: Which of the following frequencies has the lowest value for Maximum Permissible Exposure limit? (T0C02)
ANSWER: 50 MHz
QUESTION: What factors affect the RF exposure of people near an amateur station antenna? (T0C04)
ANSWER: All of these choices are correct
• Frequency and power level of the RF field
• Distance from the antenna to a person
• Radiation pattern of the antenna
So, what should you do if your RF exposure evaluation shows that people are being exposed to excessive RF?
QUESTION: Which of the following actions might amateur operators take to prevent exposure to RF radiation in excess of FCC-supplied limits? (T0C08)
ANSWER: Relocate antennas
You could also lower the power or simply transmit less.
You should regularly evaluate your station for RF safety.
QUESTION: How can you make sure your station stays in compliance with RF safety regulations? (T0C09)
ANSWER: By re-evaluating the station whenever an item of equipment is changed
Dave New, N8SBE says
“When replacing a fuse, always replace the blown fuse with a fuse of the same type and value?” — replace question mark with a period.
Dan KB6NU says
Fixed….thanks.