General/Chapter 9 Study Guide

Section 9.1: Electrical Safety

• Most radio equipment gets power from AC grid

Basic safety:

• Master onn-off switch
• Switch shoudl be clearly labeled
• Don't work on live equipment
• Don't work alone on energized equipment
• Don't assume equipment is off (check with meter)
• Don't activate transmitter or amplifier while working on feed lines/antennas
• keep one hand in pocket while probing/testing energized equip
• wear insulated sole shoes, no jewelry
• Remove, insulate, or secure loose wires and cables
• Residual charges on capacitors can present hazardous voltages, use bleeder resistors
• Use grounding stick to remove capacitor charges
• Ensure exposed conductors are at ground potential

Electrical shocks:

• Result from electrical flow through body
• Not voltage, but current that causes shock
• Shocks of more than 5 mA can be painful
• Most dangerous currents are arm-to-arm or arm-to-foot
• Low frequency current (50-60 Hz like household AC) is most dangerous because it can disrupt the heart
• after shock, heart may resynchronize to usual rythm, enter uncoordinated state called fibrillation, or stop beating algtogether
• Voltages < 30 V can cause enough current flow to be dangerous
• Unconsciousness requires CPR

Wiring and safety grounding

• NEC describes how to handle AC wiring safely
• Building Codes also important to follow
• AC wiring: sue standard color conventions
  • Hot - carries voltage - red or black wire, brass terminal
  • Neutral = white wire, silver terminal
  • Ground = green wire, green or bare copper terminal
• Always connect ground wire to metal chassis to prevent hazardous voltages on equipment
• Don't use piping as ground unless you have verified it is an effective ground rod
• Use cable sufficiently rated for current load (ampacity)
• Most common (household): 12 gauge (20 Amps), 14 gauge (15 Amps)

Protective components:

• Prevent electrical damage on safety hazards
• Power control devices: fuses, circuit breakers

Fuses/circuit breakers:

• Fuses put thin metal between 2 points, metal melts under extreme loads
• Fuses rated too low can arc (don't use 12 V fuse in 120/240 V circuit)
• Circuit breakers act as trip, open circuits - these can also be reset
• Never put fuse/breaker on neutral or ground circuits
• Never replace fuse/circuit breakers with larger rated one - FIX THE PROBLEM
• Household: red and black each 120 V, for 240 V total, appliances use separate ground and neutral wires, 4 cables total.

Shock prevention:

• Safety interlock is shock prevention device
• Prevents dangerous voltages/intense RF from being present when cabinet/enclosure open
• One interlock type disconnects high voltage when activated
• Another type shorts/grounds high voltage when activated
• GFCI (ground fault circuit interruptor) used in AC circuits
• Trips if imbalance detected in currents carried by hot and neutral conductors (e.g., bathroom plug)

Generator safety:

• portable source of electricity, usually gas/diesel/propane
• Fueling/ventilation is critical for safety
• Install outdoors, avoid CO buildup
• When using generator, install CO detector alarms
• SHut off generators before refueleling to avoid fire
• Refuel in team of 2, one refuellingand one with fire extinguisher
• Store furel away from generator and hot exhaust
• Metal frame of generator not connected to ground, so use grounding rod
• If generator used at home, careful connecting to home circuit
• To connect home circuit to generator, disconnect from power grid
• Open "main" breakers at home to disconnect home from elec. grid
• Connecting generator to house without disconnecting from grid (back-feeding) can cause large, lethal voltages on utility lines (transformers step them up), dangerous for utility workers and neighbors
• If power is restored, can damage generator if connected to utility lines
• Use transfer switch to switch source

Lightning:

• Provide fire prevention and reduce electrical damage via lightning prevention
• Disconnect cables and unplug equip. power cords before storm
• AC power lines, telephone, network wires all conductors
• Metal entry panel to house where signal/control cables enter is good lightning ground location
• Panel should be grounded to nearby ground rod with short, heavy metal strap
• Connect ground rod to AC service entry ground rod with heavy bonded conductor
• ground wires/straps: as short and direct as possible
• Towers, masts, antennas mounts should all be grounded
• Lightning rods should be bonded to other safety grounds
• Don't use solder - will melt
• Mechanical clamps, brazing, or welding
• Determine if homeowners insurance covers lightning damage
• Coverage of external structures

Section 9.1 Summary

• With 4 conductor wire in 240 V AC equipment, connect to fuse/circuit breakers with the two hot wires
• minimum wire size to carry 20 A circuit is 12 gauge wire
• For circuit using 14 gauge wire, use a fuse/breaker rated to 15 Amps
• Don't place gas-powered generator in occupied area b/c of danger of CO poisoning
• GFCI will disconnect 120/240 V power if current flows from one or more hot wires to ground wire
• Metal enclosure of station equipment should be grounded to ensure hazardous voltages do not appear on chassis
• Soldered joints should not be used with system connecting to grounding rods because soldered joints will melt and be destroyed
• Lead-tin solder can contaminate if hands not washed after soldering
• Good practice for lightning protection grounds is to bond together with all other grounds
• Purpose of power supply interlock is to ensure dangerous voltages are removed if equipment is opened/worked on
• When powering home from emergency generator, disconnect incoming utility grid feed
• NEC covers electrical safety in the ham shack
• Emergency generator installation shoudl be in a well-ventilated area (risk of CO poisoning)

Section 9.2: RF Exposure

• Low levels of RF exposure not haazardous
• At high levels, can be hazardous
• There are several factors to consider: power level and density, avg exposure time, duty cycle of transmitter
• Primary factors affecting absorption of RF by body are power density and frequency

Power density

• Body tissue absorbs RF and heats up
• RF intensity is power density (mW/cm%2)
• Power density is highest near antennas and in direction of highest gain
• Rate of absorption of RF energy is specific absorption rate (SAR)
• SAR is best measure of RF exposure to ham operators
• Depends on frequency and body part
• Limbs and torso ahve highest resonance (most affected) by energy at 30-300 MHz
• Head most affected by RF at 300 MHz - 3 GHz
• Eyes most affected by RF at 1+ GHz
• Frequencies with highest SAR are 30-1500 MHz
• Safe levels of SAR established by FCC in form of MPE (maximum permissible exposure) limits
• Vary with frequency

Averaging/Duty Cycle

• Exposure to RF limited over time interval
• Time-averaging: total RF exposure over interval

Controlled vs uncontrolled:

• Controlled - aware of environment and exposure, take reasonable steps to limit exposure
• Transmitting facilities and near antennas
• Restricted access
• Uncontrolled - accessible by general public, unaware of exposure, less likely to receive continuous exposure (so density limits are higher)
• Averaging period:
  • Controlled: 6 minutes
  • Uncontrolled: 30 minutes

Duty cycle:

• Ratio of TX time on to total time during exposure
• Lower transmission duty cycles lead to lower average exposure
• lower transmission duty cycles permit greater short term exposure levels
• Operating duty cycle usually 50% or less
• Different modes have different duty cycles
• SSB (no speech proc.) has emissions duty cycle of 20%
• FM (constant power mode) has emissions duty cycle of 100%
• To get average power output:
  • (TX PEP) x (Emission Duty Cycle) x (Operating Duty Cycle)
• Example calculation:
  • SSB, no spech proc., TX/RX for equal times, PEP of 150 W:
  • (150 W) x (20 %) x (50 %) = 15 W
• Example calculation:
  • Station sending SSB AFSK, listening 1/4 of the time, at 100 W PEP:
  • (100 W) x (100% emissions duty cycle) x (75% operating duty cycle) = 75 W

Angenna gain:

• antenna gain can also affect average power output
• Significant feed line losses also affect avg power output
• Example calculation:
  • 6 dB gain antenna, with SSB, no speech processing, TX/RX for equal amt of time, PEP of 150 W:
  • (150 W) x (50%) x (20%) x (6 dB) = 60 W

Estimating exposure/station evaluation:

• All sations must evaluate capability to cause RF exposure, regardless of power amount
• LImits of RF exposure must be evaluated, depend on PEP and frequency
• If power above a certain threshold on a band, must evaluate exposure caused by it
• Measure RF field strength using calibrated field strenght meters and antennas
• Easiest to use ARRL tables or online calculator
• Requires knowing: power at antenna, duty cycel, feed line losses, antenna gain, antenna height, oeprating frequency
• 2 ways to determine exposure:
  • 1 - determine power density at a known distance
  • 2 - determine minimum distance from antenna at which MPE limit is satisfied
• If changes made, must re-evaluate unless power output reduced

Exposure safety measures

• If evaluation results exceed MPE limits, can take steps:
  • Move antenna away
  • Move peopel away
  • Point antenna away
  • Raise antenna
  • Reduce average power
• Use dummy load or dummy antenna when testing transmitter
• Reduce power and duty cycle

Section 9.2 Summary

• RF energy affects body tissue by heating it
• Estimating whether RF signal exceeds maximum permissible exposure (MPE) requires knowing ALL of these:
  • Duty cycle
  • Frequency
  • Power density
• Determine if station complies with FCC RF exposure regulations by doing ALL the following:
  • Calculation based on FCC OET Bulletin 6T
  • Calculations based on computational modeling
  • By measuring with field strength meter equipment
• Time averaging, for RF radiation exposure calculation, means total RF exposure averaged over a certain time
• If station evaluation shows RF energy exceeds MPE, take action to prevent human exposure to excessive RF fields
• When installing ground-mounted antenna, install in a way that prevents unauthorized access
• When evaluating RF exposure, transmitter duty cycle has effect of: lower transmitter duty permits greater short term exposure levels
• To comply with RF exposure safety regulations in FCC Part 97.13, amateurs must perform routine RF exposure evaluation
• To accurately measure RF field, use a calibrated field strength meter with a calibrated antenna
• If evaluation shows neighbor might receive more than an MPE of RF from main lobe of directional antenna, can take precautions to ensure antenna not pointed in their direction
• If installing indoor transmitting antenna, ensure MPE limits not exceeded in occupied areas

Section 9.3: Outdoor Safety

• Place all antennas and feed lines clear of power lines
• 10 feet or more in distance
• Spearate all parts of antenna and support from power lines at least 150% of total height of mast/height
• Keep clear and follow manufacturer's specifiations and directions

Antenna/tower maintenance:

• Before climbing or starting, run through safety checklist
• Inspect tower guying and support hardware
• fully rested/blocked (crank-up towers)
• double-check belts/lanyards
• inspect load-bearing equipment like pulleys
• Secure all electrical and RF equipment
  • TX off, disconnected from feed line
  • Unplug all AC equipment, lock circuits out and tag them
  • Check the weather

Be sure gear is secure

• Belts and harnesses within service life and adequately rated
• Carabiners closed
• Safety lanyard and redundant lanyards
• Follow manufacturer's directions

Section 9.3 Summary

• Whenever making adjustments or repairs to an antenna, turn off transmitter and disconnect feed line
• When climbing a tower using a safety belt, confirm belt is rated for weight and within its service life
• When climbing a tower supporting electrically powered devices, ensure circuits that supply power are locked out and tagged out.