11

Say I am installing a new CPU in my desktop motherboard and I am interested in grounding myself (by touch or one of those nifty bracelets) to avoid damaging my components.

What is the nature of grounding? Does grounding effectiveness correlated to the size of an object, the conductance of the object, its connection to the soil-earth, a combination of these or other factors?

I'd imagine touching a matchbox car with plastic wheels has a different effect than shaking hands with a life-sized gold statue of Michale Faraday, half embedded in the earth and contacting bedrock.

Dent7777
  • 229
  • 2
  • 6
  • 1
    For what it's worth, I've never damaged a component yet never use those wristbands. Simply making sure you're touching the case before you start working is sufficient. Oh, and those weird black plastic bags things come in are to help diffuse any potential difference before you get to the component itself. – phyrfox Jun 22 '18 at 21:00

3 Answers3

32

If you are installing a CPU in a motherboard, you don't really care about grounding - what you really want to do is to make sure that you, the CPU, and the motherboard (and any tools) are all at the same potential. If you have one of those grounding bracelets, you should connect its ground lead to the computer case or motherboard Ground - whether that is "Really Ground" or not is not particularly relevant.

Peter Bennett
  • 57,014
  • 1
  • 48
  • 127
  • 3
    If the CPU and the motherboard are at a potential different from ground, it's going to be difficult to keep yourself at that potential. – Acccumulation Jun 22 '18 at 14:54
  • 1
    @Acccumulation: As I said, you don't have to be at "Ground", as long as you and your work are at the same potential. A "Ground strap" between you and the computer will ensure that. – Peter Bennett Jun 22 '18 at 15:06
  • 1
    How should you ensure that the CPU is at the same potential as your body and the motherboard before touching it, though? – user986730 Jun 22 '18 at 18:18
  • @uset986730 the cpu is attached to the mother board with metal pins. They’re pretty much at the same potential. When you ground yourself to the motherboard, you’re also at the same potential. – jkd Jun 23 '18 at 07:17
5

Connectivity to earth is what matters, not the size of the object.

Size only matters if we are going to get nitpicky...if the wire connecting you to earth is so tiny that it vaporizes when current flows, for example.

In your example, the matchbox car is useless as a grounding object, while the statue is excellent.

The idea is you are looking for a way to remove excess charge from yourself so it cannot discharge through a circuit and harm it. An isolated metal object no matter the size (within reason) is not going to help with this. You simply need a conductive path between yourself and earth.

Edit: Earth is simply a convenient reference. You have access to earth in most places with electrical power outlets. As others have pointed out the important thing is not to have excessive charge with relation to what you are working on. If what you are working on is not connected to earth, you should ground yourself to it instead.

evildemonic
  • 8,698
  • 2
  • 25
  • 44
  • 1
    Note that some clothing can hold charge even if you are properly grounded. working with short sleeves is a good policy (e.g. cotton t-shirt) and cheaper than specialized clothing (usually woven conductive wire in the fabric) – MAB Jun 21 '18 at 21:33
  • 1
    @MAB I feel I should follow your note with the note that while there are cheap lab coats for ESD environs, my personal experience with these cheap coats is horrendous. They work about as well as a non-ESD polyester winter coat, possibly because cheap fabric has oxidised fibers, or maybe something else. Not all clothes are created equal, I suppose. Adding that just in case someone now goes off and buys a $9 coat and assumes safety. – Asmyldof Jun 21 '18 at 21:38
  • 3
    You also want something that has some resistance in it so that there's not a large current flow (i.e. you want a gentle drain, not an immediate discharge, at least when you first make the connection). This is noted in the answers to at least one other question (https://electronics.stackexchange.com/questions/59770/grounding-metal-object-to-eliminate-static-electricity-using-electrical-ground). – JAB Jun 22 '18 at 01:30
  • 5
    In the case of installing a new CPU, the differentials between your hand, the CPU, and the socket it's plugged into are what matters. Grounding yourself to the computer's case is generally sufficient in environments where turning the computer on would be safe. – Perkins Jun 22 '18 at 01:41
  • 13
    -1 for *Connectivity to earth is what matters.* It's absolutely what doesn't matter, though it's a convenient way to get it. What does matter is that components, tools, target PCB are all at same potential. So in this case, clipping to your PC case / mobo ground is the correct answer. – Neil_UK Jun 22 '18 at 06:37
  • 1
    @Neil_UK I will edit my answer to be more clear. I was trying to answer the question in the title, the size of the metal object does not matter. – evildemonic Jun 22 '18 at 15:01
2

Connectivity to "earth" doesn't matter, regardless of size, at least for the purposes of ESD (electrostatic discharge).

The reason we use grounding equipment is to avoid damaging the parts we're working on, because there is a chance for a rather high voltage difference between you and that part - and the second you touch the part, that voltage equalizes with a ZAP, sometimes letting out all the magic smoke.

What ESD grounding equipment does is help remove that difference. If you're using an ESD table, it will have a resistive top, and a metal point to hook a wrist strap to (though resting your hand on the table may work, as well). When you touch the table, you and the table will quickly reach the same voltage potential. It's not that tables magically drain away energy, or that anything grounded to a big metal pole stuck in the ground will always be safe, but rather that the part on the table and your fingers are tied to the same voltage. If you have your parts in a plastic bucket, they will probably be at a much different potential than you, regardless of what ground you use, simply because the plastic acts as an insulator.

As a note, professional ESD grounding equipment isn't just a wire; it also has a resistor, usually in the megaohm to 1 gigaohm range. Having a high resistance means the voltage potential drains creates a lot less current than a near-zero-resistance wire would. Running a thousand volts through a megaohm resistor makes 1 milliamp (I = V / R, where I=current in amps, V=voltage, and R=resistance in ohms); running a thousand volts through a 1 ohm wire makes a thousand amps. It's only for a split second, but that's enough to do some serious damage to a circuit board!

So to answer your question: the grounding object can be any size, as long as the parts you are working on are also connected to that object, and if you use a resistor rather than a straight wire, you'll limit the damage that equalizing potential will do. Size doesn't matter - if you're working on something size of a grain of rice, well, that Matchbox car may work just fine.

One additional note: ESD grounding and system grounding are not the same. ESD grounding is designed for high voltage, low current static electricity, to safely equalize the voltage difference without damaging a part. System grounding, on the other hand, is designed to direct current from dangerous shorts or eddy currents to an earth ground, away from users. For system grounding, you want a solid connection to true earth, with the lowest resistance possible.

ArmanX
  • 539
  • 3
  • 4
  • 2
    The other reason for that high resistor in the grounding strap: A low impedance ground path to a conductive bracelet that touches the veins in your wrist is going to make accidental contact with 110V or 240V AC hot an extremely noteworthy experience. Actually, a potentially very deadly one if your OTHER hand connects to hot - hand to hand and steered to a blood vessel is the perfect heart stopper. – rackandboneman Jun 22 '18 at 22:24