Grounding
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From: John on 14 Jan 2008 06:19 My daughter is about to replace a memory module. (UK based where we have 3 pin grounded wall sockets (mains receptacles)) What is best - - Leave the PC plugged into the grounded wall socket (but switched off) and keep in contact with the case of the PC to remove static? or - Unplug the PC and keep in contact with the case (on the understanding that she will be at the same potential as the case - but not necessarily grounded to earth)
From: Paul on 14 Jan 2008 06:55 John wrote: > My daughter is about to replace a memory module. (UK based where we have 3 > pin grounded wall sockets (mains receptacles)) > > What is best - > > - Leave the PC plugged into the grounded wall socket (but switched off) and > keep in contact with the case of the PC to remove static? > > or > > - Unplug the PC and keep in contact with the case (on the understanding that > she will be at the same potential as the case - but not necessarily grounded > to earth) > The best way, is to use one of these. This is a disposable wrist strap. On one end, is a shiny copper section, with an adhesive applied to a portion of it. You stick that end, onto a shiny screw that bites into the chassis metal somewhere. (I use a screw or rivet on the PSU.) The idea is, the copper end of the wrist strap is at chassis potential. I've actually used the same disposable wrist strap a number of times, so you don't have to throw it away until it gets grubby. (Order one when you buy memory...) http://www.crucial.com/store/partspecs.aspx?imodule=CTWRISTSTRAP With the other end wrapped around your wrist, there is a high resistance connection between you and the computer chassis. Perhaps somewhere around 1 megohm or so. The purpose of the high resistance, is to slowly drain ESD off your body. (A fast discharge = high current flow, slow means a lower magnitude current during equalization.) Then, pick up the ESD packaged DIMM, open the package and install it. If you don't have a strap, you can... 1) Put on a pair of summer shorts. This leaves bare legs. 2) Unplug computer, lay it in your lap. Remove the side panel, lay panel on the floor. By the computer laying in your lap, part of the chassis touches bare skin, bringing you to the same potential as the chassis. Touch shiny metal on the chassis to be doubly sure. 3) Snip open DIMM package. Hold DIMM by edges (not the contacts). Finger oils and salt are not good for the contacts. Insert DIMM into slot. Check clips have snapped into the correct position. 4) Put the side panel back on. Plug in and test. 5) If DIMM is not detected, or programs like memtest86+ fail almost immediately, you can... 6) Put the computer in your lap again. Make sure you are at the same potential as the rest of the chassis. (Computer is unplugged, to ensure no +5VSB is present.) Then, remove and reinsert the DIMM about five times. The removal and insertion will help clean manufacturing dirt from the socket contacts. The aqueous wash at the factory doesn't necessarily remove all debris. Avoid the temptation to use other cleaning techniques. Removing precious gold from contact surfaces is self defeating. 7) Put the side panel on again and retest. A DIMM slot is powered (indirectly) via +5VSB. That is so, when a computer is in sleep mode, the memory has power to help keep it refreshed. You don't want to plug a DIMM into a slot which is electrified. Yes, you can flip off the switch on the back of the computer, and *most* of the time there will be no DC power inside the chassis. We've had one report here, where a cheap supply continued to run, even when the switch was flipped to the off position (a broken switch). Even though that is a remote possibility, unplugging the computer guarantees there is no DC present inside the chassis. Wait at least 30 seconds after unplugging, before inserting the DIMM. My computer needs about 30 seconds for all power to be drained by the standby loading. Then it should be safe to work on a DIMM. HTH, Paul
From: wasbit on 14 Jan 2008 09:00 "John" <john.plant90nospam(a)ntlworld.com> wrote in message news:UyHij.15938$a61.3064(a)newsfe3-win.ntli.net... > My daughter is about to replace a memory module. (UK based where we have 3 > pin grounded wall sockets (mains receptacles)) > > What is best - > > - Leave the PC plugged into the grounded wall socket (but switched off) > and keep in contact with the case of the PC to remove static? > > or > > - Unplug the PC and keep in contact with the case (on the understanding > that she will be at the same potential as the case - but not necessarily > grounded to earth) > After seeing the way that memory strips are thrown into a box and rifled through at computer shows I doubt there is anything worse that you could do. Although I no longer bother to take precautions when working on my own computers apart from turning off the power, it would be wrong of me to advise you to do the same.
From: kony on 14 Jan 2008 09:23 On Mon, 14 Jan 2008 11:19:16 GMT, "John" <john.plant90nospam(a)ntlworld.com> wrote: >My daughter is about to replace a memory module. (UK based where we have 3 >pin grounded wall sockets (mains receptacles)) > >What is best - > >- Leave the PC plugged into the grounded wall socket (but switched off) and >keep in contact with the case of the PC to remove static? > >or > >- Unplug the PC and keep in contact with the case (on the understanding that >she will be at the same potential as the case - but not necessarily grounded >to earth) > The basic strategy is as follows: 1) Case should be connected to ground through the power cord. (this is optional, but the easiest way when PSU has an on/off switch). 2) User should be connected to ground through high impedance (ESD strap or touching metal with serial megaohm resistor connected to ground). 3) User should not be touching the system case when the memory module is picked up, but should be electrically connected through the ESD strap or megaohm resistor to ground... to earth ground if the system is connected to earth ground, or to system chassis ground if the system is not connected to earth ground (like if the AC power cord has to be unplugged because the PSU has no on/off switch). There are three "bodies" here all possibly at a different potential. Human, system, and memory module. Having the system grounded to earth through the AC electrical system is sufficient, so what remains is whether the memory module itself is at same potential as the system, at ground potential - but it is not enough to be touching the system case because the case and memory module may not be at the same potential yet. If earth ground through the AC outlet is not possible, then connecting the ESD strap or megaohm resistor to the system case serially and then touching the memory will be sufficient. What will not be safe is to have the case grounded, be directly touching the case (with or without the ESD strap) while picking up the memory module. So earth ground is not important, but to slowly bring the memory module to the same potential as the system ground is important. Earth ground is just a very convenient reference.
From: Mike Walsh on 14 Jan 2008 09:26
Many people recommend that the PC be unplugged. This is primarily because when the PC is "switched off" there can still be power on the motherboard. It is better to leave the PC plugged in so that it is grounded and remove all power by turning off power at the power supply, power strip, or UPS. John wrote: > > My daughter is about to replace a memory module. (UK based where we have 3 > pin grounded wall sockets (mains receptacles)) > > What is best - > > - Leave the PC plugged into the grounded wall socket (but switched off) and > keep in contact with the case of the PC to remove static? > > or > > - Unplug the PC and keep in contact with the case (on the understanding that > she will be at the same potential as the case - but not necessarily grounded > to earth) -- Mike Walsh |