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Computer Maintenance - Safety, Cleaning and ESD

Discussion in 'General Hardware Issues' started by Digerati, Jun 15, 2007.

  1. Digerati

    Digerati Super Moderator Techie7 Moderator

    Safety First. It is common practice, as taught in any electronics course, to ensure safety first, so that's what we are doing here. 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. Anything that plugs into the wall can kill! We must never assume a power supply unit (PSU) is safe and working properly, that the wall outlet is wired correctly, or that the building has a good "ground" ("Earth" in some countries).

    There is some confusion as to whether it is necessary to unplug the PSU (power supply unit) from the wall when doing maintenance inside a computer. When doing any kind of maintenance (routine, preventative, or unscheduled) when potentially lethal voltages are present, safety dictates that it is essential to unplug when digging around inside.

    There are only three exceptions:
    1. IF the PSU has a master power switch on the back of the PSU (many do not) - but that assumes the power supply and $.50 switch are not defective, or damaged
    2. When necessary to have power applied for troubleshooting
    3. When inspecting fans to ensure they are spinning
    The problem with those exceptions is they all 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 should 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 corrupt third world governments. Some by children in slavery. :( You cannot trust the quality of the materials and components used in construction, the training or skills of the assemblers, or the integrity of their quality control. And remember that even name brand, quality PSUs can fail, or be damaged.

    It is necessary to unplug before doing maintenance even with a perfectly functioning power supply to prevent damaging the motherboard or other components. 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 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. If the PSU does not have a master power switch, and many PSUs do not, Standby Mode is enabled whenever the power cord is simply connected!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.

    While it is true that the exterior (output) side of a properly working PSU does not present a hazard to humans, if the supplied +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:
    1. The greater the voltage the greater the chance of arcing
    2. The smaller the gap the greater the chance of arcing
    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 circuit. 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 to ensure a simple distraction or less than rock-steady hand does not cause that +5VSB to contact the wrong point.

    I know what safety and electronic technical/service manuals say, but in researching for this post, 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 and video card makers who feel unplugging the PSU is necessary. I note a few examples:
    Abit AT8 32X Motherboard Manual, Page 2-1:
    ASUS P5LD2 Deluxe Motherboard Manual, Page 2-1, Before you proceed (their bold emphasis):
    ATI Radeon 1600 Graphics Card Manual, Page 8:
    Gigabyte GA-945P-DS3 Motherboard Manual, Page 9:
    Intel D945GCCR Motherboard Manual, Page 30:
     
    Last edited: Mar 13, 2018
  2. DJNafey

    DJNafey UK site moderator

    re: Unplugging PSU during Maintenance - Safety, Cleaning and ESD

    Bill,

    Thanks very much indeed for your enlightening post. This is certainly a topic of mixed opinions. I consider myself to be a fairly experienced system builder / maintainer (although with no background in electronics or engineering) and, until relatively recently, I always disconnected the power cord when working inside a system. But, since reading conflicting suggestions in the forum here, I've started to leave it connected on the basis that a connection back to the earth terminal of the mains power socket is the best 'ground' that you can get. I always wear an ESD strap as well of course. Since reading your post, I think I'll go back to the way that I used to work! :)
     
  3. Digerati

    Digerati Super Moderator Techie7 Moderator

    re: Unplugging PSU during Maintenance - Safety, Cleaning and ESD

    It is - but shouldn't be. Safety dictates the correct procedures. That is exactly why I went into such depth. I wanted to touch all bases, even at the risk of being too wordy.

    Thanks.
     
  4. Digerati

    Digerati Super Moderator Techie7 Moderator

    re: Unplugging PSU during Maintenance - Safety, Cleaning and ESD

    I recently came back to this thread after having a discussion about PSUs with a colleague on another forum. I have edited my post above to explain further the perils working inside a computer with the power cord connected, and added comments about the option to using a ground strap instead of a 50 cent power cord that also contains lethal voltages.

    I decided to make it a sticky - at least for now - as more and more folks are doing their own maintenance in facilities not setup for maintenance on electronics. Sadly, too many folks buy the cheapest power supplies they can find, instead of buying from a reputable maker. That's no guarantee the name brand maker does not use inferior parts, but we can hope they are not made by unskilled children in slave labor camps, and have gone through some quality control check. Nevertheless, it is never wise to assume when it comes to safety around deadly voltages.
     
    Last edited: May 29, 2009