Electrical and RF Safety Flashcards
Feed lines: characteristic impedance and attenuation; standing wave ratio (SWR) calculation, measurement, and effects; antenna feed point matching
What is one way that RF energy can affect human body tissue?
A. It heats body tissue
B. It causes radiation poisoning
C. It causes the blood count to reach a dangerously low level
D. It cools body tissue
A. It heats body tissue
RF energy can heat body tissue and cause burns. It is important to use proper precautions against excessive exposure to high amounts of RF energy. Just remember that Microwave ovens use RF energy to cook food!
If RF radiation is absorbed by the body in large enough amounts, it can produce heat. This can lead to burns and body tissue damage. Although RF radiation is not thought to cause cancer by damaging the DNA in cells the way ionizing radiation does, there has been concern that in some circumstances, some forms of non-ionizing radiation might still have other effects on cells that might somehow result in cancer.
Which of the following is used to determine RF exposure from a transmitted signal?
A. Its duty cycle
B. Its frequency
C. Its power density
D. All these choices are correct
D. All these choices are correct
Maximum Permissible Exposure (MPE) is the maximum level of radiation to which a person may be exposed without hazardous effects or biological changes in the eye or skin. The MPE is determined by the wavelength, the energy involved, and the duration of the exposure.
MPE is according to field strength and power density limits based on whole-body Specific Absorption Rate (SAR) averaging concepts that are used for evaluating the exposure conditions of fixed and mobile exposure conditions. SAR compliance is with respect to both whole-body and partial body SAR limits.
Think of defrosting chicken breasts in a microwave oven (which uses RF energy to cook food): below the MPE the chicken thaws, above the MPE the chicken COOKS!
How can you 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 these choices are correct
D. All these choices are correct
What does “time averaging” mean when evaluating RF radiation exposure?
A. The average amount of power developed by the transmitter over a specific 24-hour period
B. The average time it takes RF radiation to have any long-term effect on the body
C. The total time of the exposure
D. The total RF exposure averaged over a certain period
D. The total RF exposure averaged over a certain period
Because the amount of time that a person is subjected to sources of RF radiation is important in determining the safe amount of RF exposure (the greater the exposure time, the less RF you should be exposed to over that period), the FCC uses a “time averaging” method for calculating the limits for Maximum Permissible Exposure (MPE), or looking at the total RF exposure averaged over a certain time.
They look at the different factors that affect RF dose and set limits based on either a 6 or 30 minute time-averaged exposure period. As an example of how voltage affects RF dose over time, a microwave oven (which uses RF energy to cook food) can safely thaw a pound of chicken at 10% power for 5 minutes and not be completely thawed, however, those same 5 minutes at 100% power will burn your popcorn!
What must you do if an evaluation of your station shows that the RF energy radiated by your station exceeds permissible limits for possible human absorption?
A. Take action to prevent human exposure to the excessive RF fields
B. File an Environmental Impact Statement (EIS-97) with the FCC
C. Secure written permission from your neighbors to operate above the controlled MPE limits
D. All these choices are correct
A. Take action to prevent human exposure to the excessive RF fields
Reducing the total RF exposure may be accomplished by reducing any or a combination of factors:
Reduce power
Move your antenna
Move the RF source farther away from possible contact
Improve shielding or insulation.
What must you do if your station fails to meet the FCC RF exposure exemption criteria?
A. Perform an RF Exposure Evaluation in accordance with FCC OET Bulletin 65
B. Contact the FCC for permission to transmit
C. Perform an RF exposure evaluation in accordance with World Meteorological Organization guidelines
D. Use an FCC-approved band-pass filter
A. Perform an RF Exposure Evaluation in accordance with FCC OET Bulletin 65
The standard requirement is that it is mandatory for ALL transmitting stations, with timed exposure greater than 1 milliwatt, to conduct an RF Exposure Evaluation on their stations. The FCC does however give a few exemptions to this rule, if you meet that criteria the RF Exposure Evaluation is not required.
What is the effect of modulation duty cycle on RF exposure?
A. A lower duty cycle permits greater power levels to be transmitted
B. A higher duty cycle permits greater power levels to be transmitted
C. Low duty cycle transmitters are exempt from RF exposure evaluation requirements
D. High duty cycle transmitters are exempt from RF exposure requirements
A. A lower duty cycle permits greater power levels to be transmitted
The duty cycle refers to the amount of time you are actually transmitting. No transmitter is exempt from RF exposure requirements.
CW and phone operations only use intermittent power (you are sending part of the time, pausing to listen or receive), so these are lower duty cycle transmissions.
Data transmissions such as RTTY and packet use much closer to 100% power duty cycles.
Because the lower duty cycle transmissions use less RF energy over time (e.g. 20-60% instead of 100%), they permit a greater short-term exposure level.
Which of the following steps must an amateur operator take to ensure compliance with RF safety regulations?
A. Post a copy of FCC Part 97.13 in the station
B. Notify neighbors within a 100-foot radius of the antenna of the existence of the station and power levels
C. Perform a routine RF exposure evaluation and prevent access to any identified high exposure areas
D. All these choices are correct
C. Perform a routine RF exposure evaluation and prevent access to any identified high exposure areas
Routine RF exposure tests must be performed to make sure that our amateur station does not produce RF doses higher than the maximum permitted exposure (MPE). You need to know that you are not going to expose yourself or others who may be near your station.
What type of instrument can be used to accurately measure an RF field strength?
A. A receiver with digital signal processing (DSP) noise reduction
B. A calibrated field strength meter with a calibrated antenna
C. An SWR meter with a peak-reading function
D. An oscilloscope with a high-stability crystal marker generator
B. A calibrated field strength meter with a calibrated antenna
The most cost-effective and accurate way to measure an RF field around your equipment is to use a field-strength meter and antenna, both of which have been properly calibrated for the range of RF energy being measured.
Hint: The words “field strength” are in both the question and the answer.
What should be done if evaluation shows that a neighbor might experience more than the allowable limit of RF exposure from the main lobe of a directional antenna?
A. Change to a non-polarized antenna with higher gain
B. Use an antenna with a higher front-to-back ratio
C. Take precautions to ensure that the antenna cannot be pointed in their direction when they are present
D. All these choices are correct
Take precautions to ensure that the antenna cannot be pointed in their direction when they are present
You should always try to avoid pointing a directional antenna directly toward nearby people. Steps must be taken to make sure that the antenna cannot be directly pointed in their direction, such as marking the conflict points on your antenna rotator control, so that you do not use that directional heading.
What precaution should be taken if you install an indoor transmitting antenna?
A. Locate the antenna close to your operating position to minimize feed-line radiation
B. Position the antenna along the edge of a wall to reduce parasitic radiation
C. Make sure that MPE limits are not exceeded in occupied areas
D. Make sure the antenna is properly shielded
C. Make sure that MPE limits are not exceeded in occupied areas
You should use testing equipment (calibrated field-strength meter and calibrated antenna) to make sure that the maximum permissible exposure (MPE) limits are not exceeded.
ARRL: RF Radiation and Electromagnetic Field Safety
What stations are subject to the FCC rules on RF exposure?
A. All commercial stations; amateur radio stations are exempt
B. Only stations with antennas lower than one wavelength above the ground
C. Only stations transmitting more than 500 watts PEP
D. All stations with a time-averaged transmission of more than one milliwatt
D. All stations with a time-averaged transmission of more than one milliwatt
Determination of exemption.
(i) For single RF sources (i.e., any single fixed RF source, mobile device, or portable device, as defined in paragraph (b)(2) of this section): A single RF source is exempt if:
(A) The available maximum time-averaged power is no more than 1 mW, regardless of separation distance.
Which wire or wires in a four-conductor 240 VAC circuit should be attached to fuses or circuit breakers?
A. Only the hot wires
B. Only the neutral wire
C. Only the ground wire
D. All wires
A. Only the hot wires
Only fuse the HOT wires! Installing fuses on the other lines will not let excess or shorted voltage run to ground. Excess voltage would then remain in your equipment as a shock hazard.
Fusing HOT wires keeps the system COOL, fusing COOL wires keeps the system HOT! OUCH!
The answer of “Only the two wires carrying voltage” is written incorrectly. The answer choices should read:
A. Only the ungrounded conductors (the “hot” wires) B. Only the grounded conductor (the neutral wire) C. Only the grounding conductor D. All of the wires
According to the National Electrical Code, what is the minimum wire size that may be used safely for wiring with a 20-ampere circuit breaker?
A. AWG number 20
B. AWG number 16
C. AWG number 12
D. AWG number 8
C. AWG number 12
Use at least AWG number 12 wire as the minimum size for a 20-amp circuit, as this wire is rated to safely handle greater than 20 amperes of maximum current load.
Silly hint: The two most common shotgun sizes are “12” and “20” gauge.
Which size of fuse or circuit breaker would be appropriate to use with a circuit that uses AWG number 14 wiring?
A. 30 amperes
B. 25 amperes
C. 20 amperes
D. 15 amperes
D. 15 amperes
Using a 15A fuse or circuit breaker with a circuit using AWG #14 wiring will protect the wire from exceeding its maximum allowable current.
Silly hint: of the options, the number 15 is the closest to the number 14 in the question.
Where should the station’s lightning protection ground system be located?
A. As close to the station equipment as possible
B. Outside the building
C. Next to the closest power pole
D. Parallel to the water supply line
B. Outside the building
The terminations on the wires and cables for your ground system have a high resistance and therefore have the highest potential for overheating and/or exploding from an overcurrent situation such as a lightning strike. Running as many terminations as possible outside Will minimize the risk of the bodily or equipment damage inside the structure.
Which of the following conditions will cause a ground fault circuit interrupter (GFCI) to disconnect AC power?
A. Current flowing from one or more of the hot wires to the neutral wire
B. Current flowing from one or more of the hot wires directly to ground
C. Overvoltage on the hot wires
D. All these choices are correct
B. Current flowing from one or more of the hot wires directly to ground
A GFCI outlet measures the power coming in and then returning to your panel as it completes the circuit. It was designed to detect any difference of more than 4 or 5 milliamps and to react quickly (less than one-tenth of a second) to shut down any circuit out of this small range. Shorts will also cause this but the primary function is to measure what is going out versus what is coming back (hot/neutral).
The word “ground” is in both the question and the answer.
Which of the following is covered by the National Electrical Code?
A. Acceptable bandwidth limits
B. Acceptable modulation limits
C. Electrical safety of the station
D. RF exposure limits of the human body
C. Electrical safety of the station
The NEC or National Electrical Code sets the US standard for wiring and electrical equipment installations (including electrical safety inside the ham shack). The NEC publishes code books and references with guidelines for safe electrical installations and practices.
Which of these choices should be observed when climbing a tower using a safety harness?
A. Always hold on to the tower with one hand
B. Confirm that the harness is rated for the weight of the climber and that it is within its allowable service life
C. Ensure that all heavy tools are securely fastened to the harness
D. All these choices are correct
B. Confirm that the harness is rated for the weight of the climber and that it is within its allowable service life
Hint: while all these options are valid, the question specifically addresses belts.
Safety belts and harnesses are designed to stop the fall of the wearer, preventing serious injury. The gear must be capable of halting the weight of the person wearing it, along with the weight of any other gear that may be attached to the person or harness.
Any gear attached to the harness should be attached at the provided attachment points. All OSHA approved gear is rated to handle a maximum weight, to include the person wearing it and any tools or gear the person has, and exceeding this weight risks the failure of the fall protection gear.
The ratings and limitations of the gear are found on a label attached to the gear. This label should never be removed, and should be legible.
Safety gear is stringently designed and tested, and should be approved by OSHA. The materials used in a harness will degrade over time. The length of time will vary, depending on the quality of the materials. In order to be safe, do not use gear that has passed its expiration date.
Any fall protection device that has, in the past, arrested a fall, or that is frayed or damaged, or that is missing the label, or is past its expiration date, should be destroyed in a manner that it can no longer be used, and discarded appropriately.
What should be done before climbing a tower that supports electrically powered devices?
A. Notify the electric company that a person will be working on the tower
B. Make sure all circuits that supply power to the tower are locked out and tagged
C. Unground the base of the tower
D. All these choices are correct
B. Make sure all circuits that supply power to the tower are locked out and tagged
Tower climbing is always risky and even more so when power is involved.
Disconnecting the ground/ungrounding would add risk because it removes a path for energy to dissipate. That rules out “All” as well.
The electric company would only be involved if they had equipment on the tower, which is not likely if you’re the one climbing it.
In every case you must make absolutely sure that all circuits that supply power to the tower are totally DISCONNECTED, LOCKED OUT and TAGGED OUT (LOTO). This is so that someone below who does not know that you are on the tower will see the tagged circuits and know to NOT reconnect the power.
Which of the following is true of an emergency generator installation?
A. The generator should be operated in a well-ventilated area
B. The generator must be insulated from ground
C. Fuel should be stored near the generator for rapid refueling in case of an emergency
D. All these choices are correct
A. The generator should be operated in a well-ventilated area
Emergency generators, especially those fueled by gasoline or diesel, must always be located in well ventilated areas. Inside a garage, basement or other occupied location are NOT good places for such equipment as harmful exhausts, especially carbon monoxide can poison or KILL you.
Which of the following is a danger from lead-tin solder?
A. Lead can contaminate food if hands are not washed carefully after handling the solder
B. High voltages can cause lead-tin solder to disintegrate suddenly
C. Tin in the solder can “cold flow,” causing shorts in the circuit
D. RF energy can convert the lead into a poisonous gas
A. Lead can contaminate food if hands are not washed carefully after handling the solder
Lead is known to be a toxic substance, and so contact with skin and especially foods should be avoided. Wear gloves when possible and make sure to wash your hands thoroughly after using solder, especially before eating.
It is also good practice to never eat at your work bench – you don’t know what chemicals or hazards you might pick up with your sandwich!
Tin whiskers only in lead free solder
Which of the following is required for lightning protection ground rods?
A. They must be bonded to all buried water and gas lines
B. Bends in ground wires must be made as close as possible to a right angle
C. Lightning grounds must be connected to all ungrounded wiring
D. They must be bonded together with all other grounds
D. They must be bonded together with all other grounds
It’s important to bond all of your grounds lines together (shack and house, etc.) so that in case of a lightning strike current cannot jump from one ground to another causing destructive surges.
What is the purpose of a power supply interlock?
A. To prevent unauthorized changes to the circuit that would void the manufacturer’s warranty
B. To shut down the unit if it becomes too hot
C. To ensure that dangerous voltages are removed if the cabinet is opened
D. To shut off the power supply if too much voltage is produced
C. To ensure that dangerous voltages are removed if the cabinet is opened
Interlock circuits are intended to provide safety by preventing you from coming into contact with components of your equipment which may have dangerous voltages.
The interlocks ensure that dangerous voltages are removed if you open the equipment cabinet.
Where should lightning arrestors be located?
A. Where the feed lines enter the building
B. On the antenna, opposite the feed point
C. In series with each ground lead
D. At the closest power pole ground electrode
A. Where the feed lines enter the building
In the event of a lightning strike nearby, the lightning arrestor conductors can carry very high fault current levels. These high currents do have a risk of causing fire or shock, depending on installation effectiveness, as such, arrestors are located outside of the dwelling. We do want all outside conductors protected from lightning induction voltages as well, therefore we want arrestors as close to dwelling as possible.
