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Safety First. It is common practice, as taught in any electronics course, to ensure safety first when doing any maintenance. So that's what we are doing here, starting with this reminder, anything that plugs into the wall can kill!. Fuses and circuit breakers are used to stop excessive current events in overload situations. They react to, but do not prevent such events. They are fast, but not instantaneous, and they do not make any device 100% safe. When doing any kind of maintenance (routine, preventative, or unscheduled) with potentially lethal voltages present, safety dictates that it is essential to unplug the power cord and disconnect from that power source before digging around inside. This is essential because we must never assume a power supply unit (PSU), or the cord are safe.
There are three possible exceptions:
- IF the PSU has a master power switch on the back of the PSU (many do not) - these are great when testing fans, but do not assume that $.50 switch is not defective, or damaged
- When necessary to have power applied for troubleshooting
- When inspecting fans to ensure they are spinning
Of course these exceptions assume the power supply is functioning properly, has no manufacturing defects, and has not been physically damaged by some external force (dropped in shipping, a prime example). You can not assume with something that can kill. I have no doubt millions of off-brand, generic PSUs in use today were made under horrible working conditions in filthy unsafe factories under the watchful eye of unscrupulous factory owners and corrupt government officials. Some by children in servitude. 
You cannot trust the quality of the materials and parts used in construction, the training or skills of the assemblers, or the integrity of their quality control. Even name brand, quality PSUs can fail, or be damaged.
Unplug the power cord before doing scheduled maintenance even with a perfectly functioning power supply, or risk destroying the motherboard, RAM or other sensitive devices. The ATX Form Factor Standard, as indicated on page 21, paragraph 4.1.3.2, requires +5VDC @ 2 amperes "standby voltage" (normally designated as "+5Vsb") be applied across several motherboard points whenever the computer is in "Standby Mode". Standby Mode allows for such features as "soft power control" enabling the case's front panel power switch to power up the computer. It also allows other features, such as Wake on LAN, Wake on Modem, Wake on Keyboard, and Wake on Mouse. Standby Mode is enabled whenever the power supply is plugged into an AC power source (the wall - or UPS), and if equipped, the rear master power switch is turned on. Supplies with no master power switch are in Standby Mode whenever the power cord is simply connected.
The output voltages of a properly working PSU do not present a hazard to humans. However, if the +5Vsb comes across the wrong contact on a critical component, it can easily destroy the device. It is important to remember that RAM modules, CPUs, cards, etc., have many 100s of exposed electrical contacts/pins in very close proximity to each other. The likelihood of electricity jumping (arcing) from one conductor to another is dependent on 2 simple factors:
- The greater the voltage ("potential") the greater the chance of arcing
- The smaller the gap the greater the chance of arcing (because it takes less potential to arc across a shorter gap)
So even though the highest voltage we are talking about on the motherboard is 12VDC, the close proximity of the conductors (contacts/pins) on vulnerable devices greatly increases the chance electricity will jump the gap (arc) to an adjacent contact (and circuit), and destroy the component or a device in the related circuitry. Even if there is no arc, the mere proximity of the closely grouped contacts and pins DEMANDS perfect (straight in/straight out) steady-hand alignment by the user when inserting or removing such devices so that adjacent pins/contacts do not physically touch the wrong insertion point. If no voltage is present, damage from accidental contact is avoided.
Therefore power MUST be completely removed before doing maintenance to any computer, not just for personal safety, but to ensure a simple distraction or less than rock-steady hands, do not cause that +5VSB to contact the wrong point.
I know what safety and electronic technical/service manuals say, but in researching for this article, I wanted to see what the motherboard and video card makers say. I could find no documentation from any maker to support leaving the power cord attached when doing maintenance. I did find, over and over again, motherboard, RAM, and video card makers who feel unplugging the PSU is necessary. I note but a few examples:
Abit AT8 32X Motherboard Manual, Page 2-1:
Before the Installation: Turn off the power supply switch (fully turn off the +5V standby power), or disconnect the power cord before installing or unplugging any connectors or add-on cards. Failing to do so may cause the motherboard components or add-on cards to malfunction or damaged.
ASUS P5LD2 Deluxe Motherboard Manual, Page 2-1, Before you proceed (their bold emphasis):
Before you install or remove any component, ensure that the ATX power supply is switched off or the power cord is detached from the power supply. Failure to do so may cause severe damage to the motherboard, peripherals, and/or components.
ATI Radeon 1600 Graphics Card Manual, Page 8:
Installing the Radeon® graphics card
1 Turn off the computer, monitor, and other peripheral devices.
2 Unplug the computer’s power cord and disconnect all cables from the back of your computer.
Gigabyte GA-945P-DS3 Motherboard Manual, Page 9:
Preparing Your Computer
The motherboard contains numerous delicate electronic circuits and components which can become damaged as a result of electrostatic discharge (ESD). Thus, prior to installation, please follow the instructions below:
1. Please turn off the computer and unplug its power cord.
Intel D945GCCR Motherboard Manual, Page 30:
Before installing or removing the processor, make sure the AC power has been removed by unplugging the power cord from the computer; the standby power LED should not be lit (see Figure 3 on page 22). Failure to do so could damage the processor and the board.
If the AC power has been switched off and the standby power indicator is still lit, disconnect the power cord before installing or removing any devices connected to the board. Failure to do so could damage the board and any attached devices.
The Problem with ESD.
Electro-static charges (static electricity) easily builds up in the human body from simple acts of moving in our clothing, or walking across a carpeted floor. The "potential" (voltage) of the static electricity in humans can easily exceed 30,000 volts!! The threshold for human awareness is only around 3,500 volts, depending on many factors, including dryness of the skin, sensitivity, etc. Electro-static discharge (ESD) sensitive devices used in sophisticated electronics, such as computer processors (including CPUs and GPUs) and memory (RAM) integrated circuits (ICs or "chips"), can easily be destroyed by static with potentials as little as 30 volts! This means that a static discharge from your fingertips can totally destroy a sensitive device without you even being aware a discharge occurred!
So how do we prevent such damage? It is simple actually. The key is to remove the "difference in potential" between the two conductors - you, and the device.
Notice I did not say you have to "ground" the static. I said you must "remove the difference in potential between the two conductors". What this means is that it is not necessary to have the computer case connected to the facility ground to effectively eliminate static. Just as you can eliminate static in your body by touching the doorknob of an ungrounded, wooden door (or the dog's nose 
), simply touching the case will work just fine. When you touch the case, you equalize and eliminate that difference, thus eliminating the chance of a static discharge (arc) to a sensitive component. Think of a "floating" ground - such as used in avionics - (airplane electronics). Continually touching the bare metal case (through a wrist strap, or dangling little finger) will prevent static caused by wiggling around in your clothes, moving on the carpet, sliding on the chair cover, etc. from ever building up. If you can't stay in constant contact with the case, frequently touching the case to discharge any build up before it reaches damaging potentials is generally considered a sufficient precaution, as long as you are self-disciplined enough to discharge yourself often.
Heat, Cleaning & ESD.
Heat is the bane of all electronics. To control heat, computers use fans to pull cool air in, and expel hot air out. Unfortunately, this same process also draws in heat trapping dust, dirt, hair, dander, microscopic critters that eat dander, and microscopic droppings from those microscopic critters.
Cleaning the insides of the computer presents a problem because sticking your hands or cleaning tools in the case without care can result in ESD damage. Yet it is a user responsibility to keep heat trapping dust from blanketing heat sensitive devices inside the computer. Today's computers generate a lot of heat. In particular, the CPU and increasingly, the GPU (graphics processor unit). Both can generate as much heat as 100 watt (or more) light bulbs!
I recommend inspecting monthly, and cleaning if necessary. While observing ESD precautions, I take my systems outside and use a soft paint brush and compressed air from an air compressor. With compressed air, you can blast out the PSU (make sure you do it from inside the case). WARNING: If using an air compressor, ensure it is of the oilless type, configured for use on electronics. It must be equipped with a suitable, in-line moisture and contaminant filter to prevent spewing rusty water and other contaminants onto the electronics.
Alternatively, you can use cans of compressed dusting gas to clean the PC. (Note: Even with cans of dusting gas, I still recommend cleaning outside. There's no need to blow the dust back into the room, ready to be sucked back into the computer by the fans).
With care you can use a vacuum cleaner and a soft paint brush. But it must be remembered that air and dust particles zipping past the nozzle of the vacuum will generate a large static charge in the nozzle which can destroy ESD sensitive devices. So using a vacuum does raise VERY SERIOUS and legitimate concerns. But so does heat! And using some sort of forced air is best at removing heat trapping dust, so if compressed air is not possible, vacuuming is the next best thing.
Because of greater risk of static buildup, it is absolutely essential ESD precautions are observed to prevent static buildup and damage from ESD, as well as physical damage from banging the nozzle into fragile devices, or scratching the motherboard. The process is simple, and with care, can be done safely.
With the computer powered down and unplugged, with a soft brush in one hand and the vacuum nozzle in the other, extend a finger on your nozzle holding hand out past the tip of the nozzle and plant that finger on bare metal of the case. Then simply use the brush to sweep the dust into the vacuum. With your finger(s) touching metal, and your hand wrapped around the nozzle, no static can build up, no ESD damage can occur. Every time you reposition the nozzle, touch metal with an extended finger before moving in close.
Note: With compressed air or vacuum, resist the temptation to see how fast you can make the fans spin. You can make them spin faster than designed limits and damage the bearings. I use wooden Popsicle sticks, available at any crafts store, to hold the fans stationary, including those in the PSU.
So the bottom line here is this: When doing maintenance (cleaning, installing and/or removing components) on your computer, ALWAYS remove the +5Vsb voltages from the computer by unplugging the power cord. Keep yourself in contact with the case to ensure there is no difference in potential to prevent/eliminate static buildup in your body. This will prevent standby voltages from destroying your components and provide efficient ESD control as well.
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A quick note about those "Warranty Void if Seal Broken" labels. It is important to understand that keeping the computer clean of heat trapping dust is a user responsibility. You must open the case to inspect, and clean when necessary. Adding RAM or swapping out the graphics card is an owner's Right! As long as done properly, the laws are on your side - you have the right to break that seal and open the case. It does NOT void the warranty. Those stickers apply only when there are no "user serviceable" parts inside. That would be the case with PSUs, or hard drives for example. But not the computer case.
A note about notebooks. Notebooks by their vary nature are difficult to maintain. Opening the case is not typically a user function. But notebook owners can still ensure vents and cavities/bays are clean.
For those who practice leaving the power cord connected to use as a ground wire, let me address that. Use a grounding strap designed to connect equipment to "station" or facility ground, and not a power cord that also carries lethal voltages! This old, bad habit came out of the old days when ground straps to station/facility ground were not always available to non-technicians. It was and still is a "shortcut" in safety protocols to use the ground wire in the power cord to put the case and facility ground at the same potential. That poor safety practice assumes both the cord and PSU are fault free - a possible fatal mistake! That bad habit came about when,
(1) An understanding of ESD was not complete, and
(2) The old "AT" Form Factor standard was the standard.
With AT Form Factor cases and PSUs, the front panel power switch was not a remote power switch like those used today on ATX systems, Rather, it was a hardwired switch that went straight back to the AT Form Factor power supply. When you killed power from the front panel power switch on old AT systems, all power was removed from the motherboard. With the old AT motherboards, there was no such thing as exposed standby voltages present across the motherboard, as there is today with the ATX standard.
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Edit History:
3/8/2018 - minor content edits - Digerati
3/7/17 - minor content edits - Digerati
10/4/16 - Updated links - Digerati
6/1/09 - More document formatting changes, minor content edits - Digerati
5/18/09 - Modified document formatting, minor content edits, added in-line air filter reference - Digerati
4/13/09 - Title Change - minor edits - Digerati
5/10/08 - added comments about ground straps, minor edits - Digerati
Edited by Digerati, 08 March 2018 - 12:06 PM.
With the astronomical cost of housing in the UK, what do you live on, pet food? 
Cheers,
Lee
Cheers,
Lee
