General/Chapter 9 Study Guide

Section 9.1: Electrical Safety

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:

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

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)

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

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: