16CH12v4 4/13/04 10:05 AM Page 453 OBJECTIVES This chapter helps you to prepare for the Core Hardware module of the A+ Certification examination by covering the following objectives within the “Domain 3.0: Preventive Maintenance” section.2 Identify various safety measures and procedures and when and how to use them. Content may include the following: • ESD (electrostatic discharge) precautions and procedures • What ESD can do, how it may be apparent or hid- den • Common ESD protection devices • Situations that could present a danger or hazard • Potential hazards and proper safety procedures relating to • High-voltage equipment • Power supplies • CRTs • Heavy objects • Clothing 3.3 Identify environmental protection measures and procedures and when and how to use them. 12 Content may include the following: • Special disposal procedures that comply with environmental guidelines C H A P T E R • Batteries • CRTs • Chemical solvents and cans • MSDS (Material Safety Data Sheets) System Protection 16CH12v4 4/13/04 10:05 AM Page 454 OBJECTIVES OUTLINE Computer technicians should be aware of potential Introduction 456 environmental hazards and know how to prevent them from becoming a problem. Safety is an issue in every profession.
Technicians should be aware of the potential Electrostatic Discharge 456 hazards associated with certain areas of the computer and with certain types of peripheral equipment. MOS Handling Techniques 458 Understanding Grounds 460 Concerns for the world environment are at their highest. Many of the materials used in the construction of com- puter-related equipment can be harmful. Also, many of Protection During Storage 462 the products used to service computer equipment can have an adverse effect on the environment.
Therefore, technicians should be aware of requirements associated Disposal Procedures 463 with the disposal of this equipment and these materials. PC repair personnel should be aware of the causes and Avoiding High-Voltage Hazards 464 damaging effects of ESD so that they can prevent its occurrence. A good place to start checking for environ- mental hazards is from the incoming power source. The Avoiding Laser and Burn Hazards 465 following sections deal with power-line issues and solu- tions.
Chapter Summary 466 Key Terms 466 Apply Your Knowledge 467 Review Questions 467 Answers and Explanations 468 Challenge Solutions 470 Suggested Readings and Resources 470 16CH12v4 4/13/04 10:05 AM Page 455 STUDY STRATEGIES To prepare for the Preventive Maintenance objective of the • Remember that ESD is destructive and EMI is not. Core Hardware exam: • Know that the best device for transporting computer ➤ Use all the traditional study tools we’ve placed in the equipment is the original manufacturer’s packaging, chapter—Pay attention to the Objectives, Challenges and including the antistatic foam and bags used to pack end-of-chapter questions and use them to learn the mate- it. • Remember that toner cartridges from a laser printer ➤ Use the pedagogy in this chapter to focus on the should be recycled. Also, be aware that the proper exam-specific material—We’ve included lots of features disposal method for batteries is to recycle them.
geared expressly to the A+ exam. The Exam Tips scat- • Be aware of the voltage levels that are present tered throughout the chapter are placed there to point to inside a CRT cabinet. Also, know that a long, flat- known exam-related materials. The same is true of the blade screwdriver is the proper tool to use for dis- embedded Challenge items.
charging the high-voltage anode of a cathode-ray ➤ Key in on Exam Tips in the chapter—While reading tube. through the chapter, make sure to concentrate on the fol- • Know the best way to protect computer equipment in lowing test-related items: an electrical storm. • Remember what the abbreviation ESD stands for. • Know the areas of the computer system that are Also, memorize the conditions that make ESD more dangerous for personnel and how to prevent injury likely to occur.
Know when not to wear an antistatic from these areas. • Remember the type of fire extinguisher that must be • Know that high-voltage ratings do not make a partic- used with electrical systems, such as a PC. ular contact point more dangerous than one with a lower voltage; they have a higher current potential. • Be aware of the effects that temperature cycling can have on socket-mounted devices.
16CH12v4 4/13/04 10:05 AM Page 456 456 C O R E HARDWARE SERVICE TECH N I C I A N INTRODUCTION This chapter deals with environmental hazard conditions that can damage computer equipment or injure the user or technician. These conditions include power-supply variations, electrostatic discharge conditions, and potentially hazardous areas of the system. The chapter also describes procedures for properly disposing of com- puter equipment when it fails or reaches the end of its useful life cycle. After completing the chapter, you should be able to describe different types of typical power-supply variations and describe equipment that can be employed to minimize or remove these variations from the system.
Likewise, you should be able to identify sources of ESD and specify precautions that can be taken to prevent static discharge. Finally, you should be able to identify potentially hazardous areas of the comput- er and its peripherals. ELECTROSTATIC DISCHARGE An electrostatic discharge (ESD) is the most severe form of electro- EXAM TIP Remember what the abbreviation ESD stands for. The human body can build up static charges that range up to 25,000 volts.
These build-ups can discharge very rapidly into an electrically grounded body or device. Placing a 25,000-volt surge through any electronic device is potentially damag- ing to it. At this point you may be wondering why the 25,000 volts associated with video monitors are deadly, whereas the 10,000 to 25,000 volts associated with ESD are not harmful to humans. The reason is the difference in current-delivering capabilities created by the voltage.
Electronics instructors reiterate that it isn’t the voltage that will kill you; it’s the current (amperage). The capability of the voltage associated with a video monitor to push current through your body is significant (several amps), whereas the same capability associated with static is very low (micro-amps, or 16CH12v4 4/13/04 10:05 AM Page 457 C h a p te r 1 2 SYSTEM PROTECTION 457 thousandths of an amp). Therefore, it is possible for a lower voltage EXAM TIP device with a higher current rating (such as a 110 Vac power supply) Know that high-voltage ratings do not make to be much more dangerous than a higher voltage source that has a a particular contact point more dangerous than one with a lower voltage; they have a lower current-producing capability (such as static). higher current potential.
Static can easily discharge through digital computer equipment. The electronic devices that are used to construct digital equipment are Memorize the conditions that make ESD particularly susceptible to damage from ESD. As a matter of fact, more likely to occur. ESD is the most damaging form of electrical interference associated with digital equipment.
Be aware that compressed air can be used The first step in avoiding ESD is being able to identify when and to blow dust out of components and that it why it occurs. The most common causes of ESD are does not create ESD. ➤ Moving people ➤ Improper grounding ➤ Unshielded cables ➤ Poor connections ➤ Moving machines ➤ Low humidity (hot and dry conditions) When people move, their clothes rub together and can produce large amounts of electrostatic charge on their bodies. Walking across car- peting can create charges in excess of 1,000 volts.
Motors in electri- cal devices, such as vacuum cleaners and refrigerators, also generate high levels of ESD. Some repair shops do not permit technicians to use compressed air to blow dust out of keyboards and other computer equipment because it has erroneously been linked to creating ESD. ESD is most likely to occur during periods of low humidity. If the relative humidity is below 50%, static charges can accumulate easily.
ESD generally does not occur when the humidity is above 50%. Normal air-conditioning works by removing moisture from the atmosphere. Therefore, its presence can increase the potential for ESD by lowering the humidity even further. Anytime the static charge reaches around 10,000 volts, it is likely to discharge to grounded metal parts.
In many high-ESD situations, it is useful to install a humidifier to raise the level of humidity in the work area. 16CH12v4 4/13/04 10:05 AM Page 458 458 C O R E HARDWARE SERVICE TECH N I C I A N CHALLE NGE #1 You have been asked to consult on the design of your company’s new repair facility near Phoenix, Arizona. In particular, management wants to know how to equip the work areas of the new facility. You have not been to the site, but you know that it is in a hot desert environment.
Also, the building will be air-condi- tioned. How should you advise the management team about precautions that should be taken with the work area? Refer to the “Challenge Solutions” section at the end of this chapter for the reso- lution to the challenge. MOS Handling Techniques Metal oxide semiconductor (MOS) devices are sensitive to voltage spikes and static electricity discharges. For example, the level of stat- ic electricity present on your body is high enough to destroy the inputs of a CMOS device if you touch its pins with your fingers.
Professional service technicians employ a number of precautionary steps when they are working on systems that may contain MOS devices. These technicians normally use a grounding strap, like the one depicted in Figure 12. This antistatic device may be placed around the wrist or ankle to ground the technician to the system being worked on. These straps release any static present on the tech- nician’s body and pass it harmlessly to ground potential.
Antistatic wrist or ankle straps should never be worn while working on higher voltage components, such as monitors and power-supply units. Some technicians wrap a copper wire around their wrist or ankle and connect it to the ground side of an outlet. This practice is not safe because the resistive feature of a true wrist strap is missing. As an alternative, most technicians’ work areas include antistatic mats made out of rubber or other antistatic materials that they stand EXAM TIP Know when not to wear an antistatic wrist strap.
on while working on the equipment. These mats are particularly help- ful in carpeted work areas because carpeting can be a major source of ESD buildup. Some antistatic mats have ground connections that should be connected to the safety ground of an AC power outlet. 16CH12v4 4/13/04 10:05 AM Page 459 C h a p te r 1 2 SYSTEM PROTECTION 459 FIGURE 12.1 Typical antistatic devices.
To avoid damaging static-sensitive devices, follow these procedures to help minimize the chances of destructive static discharges: ➤ Before touching any components inside the system, touch an exposed part of the chassis or the power-supply housing with your finger, as illustrated in Figure 12. Grounding yourself in this manner ensures that any static charge present on your body is removed. This technique should be used before handling a circuit board or component. Of course, you should be aware that this technique works safely only when the power cord is attached to a grounded power outlet.
The ground plug on a standard power cable is the best tool for overcoming ESD problems. ➤ Be aware that normal operating vibrations and temperature cycling can degrade the electrical connections between ICs and sockets over time. This gradual deterioration of electrical con- tact between chips and sockets is referred to as chip creep. 16CH12v4 4/13/04 10:05 AM Page 460 460 C O R E HARDWARE SERVICE TECH N I C I A N FIGURE 12.2 Discharging through the power-supply unit.
➤ Use antistatic sprays or solutions on floors, carpets, desks, and EXAM TIP Be aware of the effects that temperature computer equipment.