Why do computers have ground cables when the entire case is ground?

[Mark Kaine]

1 hour ago, Lurking said:

The car power source is DC and is the battery/alternator

[snip]

i think that's the best explanation we'll probably get, thanks!

 

and yeah, I knew DC is different but i completely forgot about this, especially in this context,  i just thought of the fact that a car isn't connected electrically to the "earth" lol, although some are ...

 

not sure about EVs actually.

 

 

[Mark Kaine]

18 minutes ago, wseaton said:

Ive actually tested this and found many MBs mounted in such a way, typically with with washers and standoffs that they are intermittently insulated from the case.

ive actually seen stand offs made out of some clear rubber like plastic once,  that was certainly weird... i first wanted to throw them away, it was clearly just trash to me lol...

 

20 minutes ago, wseaton said:

I applaud his caution, but all you have to do is touch the PC frame to ground yourself before touching RAM or other sensitive PC components. 

yup always just touch the case while doing something on pc components...

 

 

 

[stipuledfatcat]

3 hours ago, Lurking said:

The car power source is DC and is the battery/alternator. So the electrons go from battery plus (red cap) via cables to the device, and back through the the frame to the negative of the battery. The only reason the frame is used is to save weight and money. They also could have ran separate wire. The ground (as in actual Street) isn't part of this circuit.

 

A PC has 2 "loops" the DC side (3.3 - 12V) works like in the car, except it goes from PSU and use 2 wires instead of using the chassis. I suppose one could use the case, but that may be made of plastic, or not closed, or the connections aren't great. And the case also is part of the AC ground (see below)

 

The AC side (from your outlet) is separated from the DC side by the PSU (for good reason since it is lethal). There is the line, and the neutral wire that run the current. It is AC, so it isn't +/-, but from a wiring point it works the same. The middle contact in your outlet is the ground and typically does NOT carry a current. This goes through the panelboard and the E-service to the actual ground. A building or service entrance typically has a ground rod or is bonded to steel in the structure. It isn't used for actual power and you could omit it and some old outlets don't have it. But don't omit it, it is a safety feature. That ground is connected to the metal case of the PSU, which is connected to the case. If the line voltage wire (120 or 230V!!!!!) would be damaged and touch the PSU case, your case would be lethal if you touch it! The ground will run the current to the ground (instead of through your body!). That will lead to high current and typically trip the breaker. So please don't use those outlets without ground, even if the PC "works" without. 

 

At the service entrance the neutral and ground actually bond together. But up to that point ground doesn't carry current (except when there is an issue). A common mistake is to install a sub-panel and bond neutral and ground in the sub panel as well. Don't do that!. Because in that case the grounds wire between the panels would carry a current under normal conditions. 

 

The tricky thing about electricity is, you can do things wrong and they appear to work. But you will have stray voltages and safety issues that are not immediately apparent. They become apparent when it is too late. So unless you really know what you are doing and are educated, never work on the high voltage side of a PC. Never open a PSU unless you plan to condemn it (or you actually are an expert).

 

Probably the best explanation, better than mine that went off on a few tangents. Grounds and voltages are weird and sometimes do funky things if you don't know all the intricacies involved.

 

Also fun fact if you are a physicist or a hard core academic electron goes from - to +.

[RejZoR]

Electricity always takes the shortest route through objects. If you grab the electrical contacts with both hands it'll pass through both your arms and through the chest where heart is, making it most common and deadliest electrocution type.

 

If you step on the exposed contact points, it'll run through your legs. It'll probably still hurt like hell, but since it goes rather far away from heart, chances are it won't be anywhere as lethal.

 

When it ocmes to computer cases, grounded chassis means electricity will ALWAYS prefer to go through much better conducting wire to a grounded wall outlet than through you wearing shoes, slippers and standing on carpet or laminated flooring. In addition to that, modern homes use FID breakers which also detect current leakage further enhancing protection.

[Dujith]

9 hours ago, Mark Kaine said:

and yeah, I knew DC is different but i completely forgot about this, especially in this context,  i just thought of the fact that a car isn't connected electrically to the "earth" lol, although some are ...

AC isnt different from DC in the way a circuit acts. Instead of a positive you'll have a phase and a common (which is tied to earth alot of times).

When you create a floating circuit like a car it will act the same way and you can "safely" touch it (if everything is in working order). 

Since there is no path from the generator/battery through you to ground.

 

[mariushm]

Some other reasons why there's grounding on computers are:

 

1. Computers emit electronic radiation which can affect AM radios, analogue tvs, CB radio, lots of things that work with radio frequencies ... so you have the case made out of metal with small enough ventilation holes to not leak radiation - the whole computer case is a Faraday Cage ... it works better if this faraday cage is grounded.

 

2. There were external drives and devices (hard drives, floppy drives, printers) which connected through SCSI or parallel port to the main computer, and some of these devices used high voltage AC motors inside and also lower voltages... some consumed so much power that they had to be powered from a different AC socket, which may have been on another circuit . So it made sense to have a ground as a common wire between the two devices so that you're on same potential , same voltage level ... so that when a computer sends 0v for 0 and 5v for 1 through the cable, the other computer doesn't see that 0v as 2v.

 

This is still seen these days in audio amplifiers, sometimes you get ground loops, you get humming or buzzing when connecting an audio amplifier to the PC using analogue jacks, something you don't get if you use optical outputs because that's light going through plastic "wire"

 

But again, like I said, external devices could have high voltage motors or fans in them, and just in case that motor failed or shorted or did something wrong and you got 110v going through the parallel or scsi cable to your computer, you didn't want 110v AC on the outside of your case, or going through the PS/2 cable to your keyboard and get shocked by the metal case of your keyboard (some keyboards had metal bottom, for weight and rigidity, not plastic like now)

 

Cases these day with  plastic or glass panels are less about acting as Faraday cages to blog electric interference and people care less about it, because nobody listens to AM radio or analogue TVs anymore ... but it's still important to have ground wire in your AC cable.

 

[Lurking]

16 hours ago, stipuledfatcat said:

Probably the best explanation, better than mine that went off on a few tangents. Grounds and voltages are weird and sometimes do funky things if you don't know all the intricacies involved.

 

Also fun fact if you are a physicist or a hard core academic electron goes from - to +.

Actually the electrons go from positive to negative. Because what we call positive, actually is negative (in relation to electrons). the reason we now say it wrong is that when electricity was discovered, it was assumed the positive parts move. 

 

See around minute 7

 

(I hope i didn't confuse myself and didn't do a double-negative - pun intended