Module 20 - ELECTRICAL SAFETY
- READING ASSIGNMENT
- ELECTRICAL INJURIES
- ELECTRICAL SAFETY
- ELECTRICAL INJURY RESPONSE
- ELECTRICAL GROUNDING &
- LIGHTNING SAFETY
- QUIZ 20
- RECOMMENDED READING
Read Chapter 9, Pages 9-1 to 9-5 in your text before you continue.
Shocks. Burns. Even small currents can cause problems
- 1 milliamp
- - Just Perceptible
- 5 milliamps
- - Maximum harmless current
- 10-20 milliamps
- - Lower limit for sustained muscular contractions
- 30-50 milliamps
- - Pain
- 50 milliamps
- Pain, possible fainting.
- 100-300 milliamps
- - Normal heart rhythm disturbed. Electrocution if sustained current.
- 6 amps
- - Sustained heart contractions. Burns if current density is high.
A home's safety ground is adequate to control shock hazards for 60Hz ac power systems. But with radio frequencies, the safety ground wiring usually acts more like an antenna than a ground. Poor RF grounding could lead to RF burns.
RF voltage can be present on the outside of a connector, cable or equipment enclosure. While they can be painful, they generally don't do much damage. Using a common ground keeps all of the radio equipment at the same RF voltage (even if it's not exactly at ground).
30 volts is a commonly accepted value for the lowest voltage that can cause a dangerous electric shock. Remember more than thirty can put you in the dirty!
Current flowing through the body cause a health hazard. By heating tissue, disrupting s the electrical functions of cells & causing involuntary muscle contractions.
An electric shock is received when current passes through the body. The severity of the shock depends on the Path of current through the body, the amount of current flowing through the body & how long the body is in the circuit.
This can cause death in a few minutes unless a defibrillator is used to shock the heart back into normal rhythm. It is ironic that more current will freeze the heart and not cause fibrillation, but the heart may have to be restarted
If you are in contact with an energized wire or any energized electrical component, and also with any grounded object, YOU WILL RECEIVE A SHOCK. You can even receive a shock when you are not in contact with a grounded object
Most common shock-related, nonfatal injury is a burn. Burns occur when you touch electrical wiring or equipment that is improperly used or perhaps not properly maintained and the current flows through only a small path in the body. Burns typically occurs on the hands or arms. Electrical burns are very serious injuries that need immediate attention because they can easily be third degree burns.
Another burn associated with electrical circuits is a flash burn. These burns usually occur when an electrical circuit is short circuited and an electrical flash results or a high current circuit is switched on or off when you are close to the arc. These incidents can cause molten metal to be discharged which can solidify in the eyes or on the skin causing burns. Flash burns to the eyes can be very serious
Avoiding contact is the most effective way of practicing electrical safety. Most modern radio equipment uses currents that are not as dangerous as older equipment but precautions still must be taken
Always turn off power when working inside equipment. Make certain equipment is properly grounded and circuit protected. Some recommend keeping one hand in your pocket when working around high voltage circuits
Power supplies contain large value capacitors to ensure good filtering. Usually there is a bleeder resistor across the terminals of the capacitor to 'bleed' the charge off after the equipment is turned off. But it is best to be cautious. Make doubly sure the capacitor is discharged. Check it with a DC voltmeter and confirm there is no voltage present. Then, and only then short across the terminals or short the high voltage to ground. Otherwise you might receive an electric shock from the stored charge in large capacitors.
If power is required, remove jewelry & avoid unintentional touching of circuitry. Never bypass safety interlocks or fuses.
Storage batteries are dangerous when shorted as they can supply large amounts of current is a very short time.
A fuse or circuit breaker in series with the AC "hot" conductor should always be included in home-built equipment that is powered by 120 V AC power circuits. Use cable and wire sufficiently rated for the expected current load.
Ground fault circuit breakers are designed to protect people from electrocution. These devices are crucial to electrical safety and in many instances are a regulatory requirement. Radio Amateur stations should be fitted with GFI'S
Ground-Fault Interrupter devices are based on the principle that the current entering a device should be the same as current exiting the device. Any discrepancy is due to current flowing somewhere it should not be flowing. The device works by comparing the active and neutral current that should be equal and opposite. When the currents in the active and neutral differ the device trips.
ELECTRICAL INJURY RESPONSE
REMOVE POWER before attempting any rescue or first aid. Have ON/OFF switches and circuit breakers clearly marked so they can be easily turned off if necessary. Call for help. Learn CPR and first aid.
Immediate effects of shock are often confusion and memory loss. Sometimes, a person who has received a shock may not be able to judge its seriousness and can injure themselves further. Vision and motor coordination between hands and eyes may also be affected.
Sometimes injury and damage will not immediately manifest itself to the victim. In some cases heart irregularities and breathing stoppage can start some time later. Additional serious long term and delayed affects of shock, including ventricular fibrillation (which can lead to heart attack), can occur up to 72 hours after the shock.
ELECTRICAL GROUNDING & CIRCUIT PROTECTION
Make sure your home is "up to code." Most ham equipment does not require special wiring or circuits. Always use 3-wire power cords. Use circuit breakers, circuit breaker outlets, or Ground Fault Circuit Interrupter (GFI) circuit breakers or outlets.
Use proper fuse or circuit breaker size. Don't overload single outlets.
The green wire in a three-wire electrical AC plug goes to ground. This is not the neutral current carrying leg but the safety ground.
Some of the most frequently violated electrical wiring standards are: Metal parts of an electrical wiring system that we touch (switch plates, ceiling light fixtures, conduit, etc.) These should be at zero volts relative to ground.
Housings of motors, appliances or tools that are plugged into improperly grounded circuits may become energized and present a shock hazard.
Antennas are not struck any more frequently than trees or tall structures. Ground all antennas. Use lightning arrestors.
Essentially, starting at your antennas, all towers, masts, and antenna mounts should be grounded according to local building codes.
This is done at the base or through a wire to a ground rod. Ground connections should be as short and direct as possible and avoid sharp bends.
Where cables and feed lines enter the house, use lightning arrestors grounded to a common plate that is connected to a nearby external ground such as a ground rod. if you know that a lightning storm's coming, disconnect all cables outside the house and unplug equipment power cords inside the house. This interrupts the lightning's path to get to ground through your equipment.
Regardless of how much protection you install, operating during a thunderstorm is a bad idea. Even a nearby strike can create a voltage surge of thousands of volts.
Your quiz will tell you what information you need to know. Pass the quizzes - pass the exam!
Tips on how to remember the correct answer are included.
You can take the quiz as often as you wish.
No one but you will see the quiz results.
As I have done several times previous in this course, extra information has been added to help you better understand electronics and in the case of this module help promote safer conditions. We too often get sloppy around electricity especially the common 120 VAC household current. Bad idea. Don't get sloppy. Don't hurry and cut corners. Unplug equipment before working on it. Make sure there is not any stored energy. Be safe!
Review the Term Glossary in Chapter 10 of your Ham Radio License Manual
Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.