HANDOUT Electrical Safety How do we make sure that humans and computer equipment are safe from electrical hazards?

Transcription

1 How do we make sure that humans and computer equipment are safe from electrical hazards? ESD Although not a potential hazard that endangers the PC technician, ESD can easily destroy sensitive computer components, at great financial cost. ESD stands for electrostatic discharge. It happens when you have 2 items with a different voltage built up on them. ESD can be caused by many things, like the shuffling of one s feet across carpet, for one. Static electricity thrives in cold, dry conditions. Static discharges could theoretically be as large as 12,000 volts, but most integrated circuits can be affected by as little as 10 volts. The human body can only detect discharges of about 2,500 volts. There are 3 different types of ESD damage: Catastrophic: This type causes permanent damage and is noticeable right away. When you touch a component, and it is fried right now, that is catastrophic ESD. Devices with catastrophic failure can be replaced during the building and testing stage before a customer would get the equipment. Latent: This type of ESD is not noticeable right away and may allow a component to limp along for some time, but the component s performance gets worse and worse over time. Many technicians not wearing anti-static straps will cause Latent ESD to occur. They don t realize they are causing failures down the road. Devices damaged by Latent ESD will probably pass initial testing, and those computers will get shipped home to the customer who will have the component die on them later on. Upset: This type of ESD is similar to latent in that the device is damaged, and it will still pass initial testing. Such a device will have chips that develop gate leakage which may cause software and data loss and other problems in the future. These devices may fail completely once they get into the customers hands. 1 P age

2 ESD Protection In order to protect against ESD, there are several measures one can implement. Use of the antistatic wrist strap essential to all PC technician toolkits is your first line of defense against ESD. Anti-static work mats are also very helpful preventative measures against ESD. If you have a way of raising the humidity in your area, that would also be helpful. Some resources state that if you put fabric softener in a water mixture and spray carpets and curtains in the area with it, it will help reduce static electricity. There are also anti-static sprays that you can spray surfaces with. And lastly, if you have the option of getting rid of the carpet in the area where you have electronic equipment, it would be a great idea to do that. ESD straps are usually worn on the non-dominant hand (the left wrist for a right-handed person). The strap should be connected to a ground through a coiled retractable cable and a resistor. This resistor allows high-voltage charges to leak through but prevents a shock hazard when working with low-voltage parts. Because high-voltage charges can leak through, be sure to remove the ESD strap when working in any situation where high voltage may be a danger. One way to prevent ESD is to equalize the electric potential of your body and the components on which you're working. When working on the inside of the computer case, installing or cleaning components, make certain that the personal static charge has been discharged by touching the inside of the metal case housing before handling components. Touching the chassis of the computer while working on components helps to equalize the charge between you and the chassis. An ESD strap will also equalize the charge, but if no ESD strap is available, then touching the chassis is a good alternative. In general, ESD damage can be avoided by properly grounding yourself before working on a PC. Finally, to protect your individual components before installing them into a PC, remember to always store static sensitive components (especially RAM and CPUs) in the anti-static bag or packaging that they came in. These bags prevent a static discharge from building on the components themselves which could lead to a discharge that could destroy the component before installation. 2 P age

3 EMI/RFI Another potential hazard that is more likely to put PC components at risk than PC Technicians is EMI. Electromagnetic Interference (EMI) may be generated by the PCs themselves or by other electronic devices. However, a properly grounded PC prevents the transmission of EMI and protects itself from EMI from other sources. RFI (Radio Frequency Interference) is grouped in with EMI many times because it causes electrical noise to happen. Interference caused by cell phones and microwaves fall into the RFI category. Electromagnetic interference typically happens when one electronic device interferes with another electronic device. You might have seen EMI manifest itself on your TV when a vacuum cleaner is run nearby. You would have seen lines going across the screen in waves. When the source of interference stops, the waves go away. Magnetic fields from magnets, power lines, and fluorescent lights, to name a few, can also cause EMI. Prevention: When installing network cabling, it is very important to not run the cabling near fluorescent lights or around power lines because of their magnetic fields. Also, while working on and around PCs and magnetic drives, you should avoid the use of extension magnets. Don t use devices like electric pencil sharpeners around a box of floppy disks, and avoid sitting those same disks on speakers with large magnets in them. Electrical safety Many electrical devices can cause harm to humans. Technicians should follow good safety practices when working around these devices. CRT: There is a high voltage power source in a CRT monitor. This high voltage source contains anywhere from 20,000 to 50,000 volts. This is one of the reasons that CRT monitors are not serviced in CET courses. It would be extremely dangerous for a technician to wear an ESD wrist strap while inside a CRT monitor because the ESD wrist strap is made to remove small voltages, not like the ones contained in a CRT. 3 P age

4 Power supply: Power supplies contain capacitors that hold a charge even when power has been removed from the power supply itself. There is a warning message written on the top of power supplies that says, in many languages, that it is very dangerous to open a power supply. We do not service power supplies in our CET courses for this reason. Besides, if you look at the cost of new power supplies, it does not seem economical to spend a technician s time to service a part that can be replaced fairly inexpensively. Traditional computer power supplies range from 250W to 650W output capacity. Some computers may need power supplies with 850W or higher. When building a computer, select a power supply with sufficient wattage to power all of your components. Each component inside the computer uses a certain amount of power. You can obtain the wattage information for each of these components from the manufacturer's documentation. When deciding on a power supply, make sure to choose a power supply that has more than enough power for the current components. A power supply with a higher wattage rating has more capacity; therefore, it can handle more devices. On the back of the power supply is a small switch called the voltage selector switch. This switch sets the input voltage to the power supply to 110V / 115V or 220V / 230V. The correct voltage setting is determined by the country where the power supply will be used. Setting the voltage switch to the incorrect input voltage could damage the power supply and other parts of your computer. Some power supplies may not have the voltage selector switch. If you have a power supply without a switch, check the PSU documentation to see if it will automatically detect and set the correct voltage. Power Inverter: A power inverter allows you to convert DC to AC voltage. Some electronic devices need AC because their internal power supply or power adapter already does the AC to DC conversion needed for that device. Many mobile users have power inverters that they can plug into the cigarette lighter of their car or camper to power their laptops. These power inverters can be dangerous (burns or fires) if hooked up directly to batteries, and you should follow the manufacturer s recommendations on their use. Laser Printer: Laser printers contain several components that can cause the user harm. First, since laser is in the name of a laser printer, let us remember that images are written to the drum of a laser printer using a laser. While it would be extremely difficult to make a laser printer operate with the machine open, it is necessary to say 4 P age

5 that lasers can cause damage to the eyes. So be careful to never look into a laser while it is in operation. Another danger in a laser printer is the fuser assembly. The fuser gets extremely hot, and, as such, it has warning labels stating to not touch because it is extremely hot and will cause burns. Matching power requirements of equipment with power distribution and UPSs Computers and other electronic equipment need a continuous supply of power that does not fluctuate in order to operate properly. For this reason, surge protection and battery backup are important to not on ly the life of the computer s power supply but also to the computer itself. Devices to attach PCs to a power source are as follows: Power strips: These devices typically do not protect against power failure or surges. They are used mainly to connect multiple devices to a power source. Surge protectors: Surge protectors can work in a couple of different ways. One type is a shunt type. This kind works to absorb a surge and is typically rated in joules. A second type of surge protector works off of a clamping voltage. This type detects the voltage coming through and blocks it when the voltage goes higher than what the clamping voltage is set to. Many surge protectors will have a light that shows whether their surge protection is still working because when lightning hits they may become damaged, and the user needs some indication of that. Line conditioners: Line conditioners regulate or provide a continuous voltage during low voltage (brownouts or sags) or 5 P age

6 high voltage (swells or spikes) conditions. These devices are typically small desktop units. UPS: Uninterruptible Power Supplies are devices that provide battery back-up power in case of complete power failure. These devices contain a battery that will supply power for a short period of time so that a proper shutdown can be done. These devices many times also provide line conditioning to prevent power surges or brownouts from causing harm. The UPS is a very heavy component that plugs into an AC outlet and allows you to plug electronic devices into it. One way to test a UPS is to unplug the UPS device from the wall outlet and see if it will still power the devices attached to it. If it powers the attached devices, then the battery backup unit is working properly. 6 P age