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The experimental computer has been shutting down and then rebooting for a couple of days by itself last night it shut down and would not restart. This morning I put it on my bench and opened it up cleaned up the dust bunnies and plugged it in at the bench to check the fans everything was working. brought it back in and hooked it up no power nothing came on. replaced the power cord with the one at my bench it now works. When I checked out the power cord that did not work, hot side was good, common side was good, checked out ground and had infinite Resistance. Can some one tell me why a computer needs ground to run?
dinga

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nubimax wrote:

Can some one tell me why a computer needs ground to run?

It doesn't. I'm fairly sure that something is wired wrong somewhere. What you describe indicates that the computer is getting its common through the ground rather than through the designated common wire.

What could explain the behaviour is if the power supply common is actually connected to the ground connector at the power cord plug rather than the common connector. That would mean that the computer is getting its common over the ground connector, and doesn't work if the ground connector is bad.

There are other possibilities that involve the socket you are using not having a good ground or common, but check the computer first.

Counter check: use the cable that tested bad, and see if the computer runs with that on the work bench. If the problem is still there, the socket is most likely ok. If the problem is not there on that socket with the bad cable, the computer is probably wired properly, but the socket where the computer normally is (or the powerboard or double adapter? ) may have an issue.

Just to be sure, the plug is the "Type B" discussed here, right?
plugs and sockets type A and B

The relevance being, in that case it is not possible (assuming everything is wired correctly) to get hot and common mixed up. If it were possible to get those two mixed up, a whole new set of interesting possibilities come into the game.

Coming from Australia, I thought it was a given that hot and common could not be mixed up. However, here in the Land of precision, we use this plug
Type F : Schuko

which can be plugged in either way around. This means, when you screw out the globe in a table lamp, pull the plug out first. The switch generally only switches one of the two wires, so you never know for sure if the socket is live or not.

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Yes it is a type B socket. The common and the hot wire are working correctly. My meter shows that the ground wire is broken so it should not be doing anything. That is what is confusing me. I am much more use to using DC wiring then AC wiring.
I have wired a lot of boats but that is mostly DC. Some of the larger boats had generators for running large AC spotlights to find buoys in the night.
dinga

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nubimax wrote:

My meter shows that the ground wire is broken so it should not be doing anything.

We are talking about the ground wire in the cable between the wall socket and the computer here, aren't we?
Also, the wire is apparently broken now, but wasn't necessarily always broken. I am assuming it was not broken until the computer started playing up, and it being broken is possibly the cause of the problem.

I had the following paragraph at the end. The text has got so long, however, that I decided to put the "summing up" at the start:

Suffice to say that the ground and the common in your socket in the wall are almost certainly connected somewhere in the house. This means, if the common and the ground connector are swapped in the device (in this case your computer), the device will find the "common" connection via the ground wire. If the ground wire in the cable being used then breaks, the device will stop working (obviously).

And now the long-winded explanation:

There is not really a big difference in how to wire DC and AC. Because a lot of the stuff that "normal people" as DC see is 12 V in cars, or 24 V in trucks, the wires are often a little thicker than for the AC applications that normal people encounter. This is because, to get the same work done, i.e. power (Watt), you need a higher current (Ampere) when the voltage is lower, and that needs a thicker cable.

Otherwise, the principle is the same: volts applied to one side of the device cause a current to flow through the device and work to be done. For that, you only need two wires. One to the device from the source, and one back to the source from the device.
In DC circuits the wires are referred to as "plus" and "minus". In AC they get called things like "hot" and "common", "hot" and "cold" or whatever.
The principle is always the same: the "hot" wire applies the potential (Volts), and the "common" is the way back to the source.

The ground has nothing to do with the operation of the device. It is purely a safety feature, providing a "way of least resistance" back to ground should the volts get onto a part of the device where they shouldn't be, like the housing, for instance.
The "way of least resistance" means, hopefully, the stray volts will go that way to earth (and hopefully trip a fuse) rather than through a person that touches the part of the device where the stray volts are.

Generally, the ground is connected to the housing and metallic chassis parts of the device, but mostly not the operational circuit in the device. There are exceptions, for instance professional audio gear mostly does have a connection between signal common and earth.

Wiring up a boat or whatever is relatively simple in as much as you see the entire circuit: starting at the plus pole of the battery, a wire goes to the device (with probably a switch on the way, and a fuse maybe) and from the other side of the device a wire goes back to the minus pole of the battery.

In principle, an AC circuit is the same. Volts come into the house from the power plant, go through the devices (and fuses and switches) and flow (theoretically) back to the power plant. Two wires. The only difference really, is that the potential (Volts) in the "hot" wire changes polarity regularly (60 Hz in USA, I believe). Therefore, the current in the circuit changes direction at the same frequency, hence "alternating current".

The third wire in your plug is the ground, which, as I said, is simply a safety feature that has nothing to do with the operational circuit of the device.

What you don't see, unless you wire your house up yourself from scratch, is that the common and the ground are generally joined at the fuse box of the house, and connected to a stake in the ground. The wiring from the power plant is, as far as I know, only the "hot" side of the circuit.

The "common" is present in the wiring in the house, but generally not in the power distribution network. The power lines you see along the sides of the road are only "hot" wires. There are usually three because power is generated and distributed as "3 phase power". In that system, the "common" is provided for each phase by the other two phases.
Wikipedia

The "common" is literally connected to the earth, i.e. a big fat metal stake in the ground, at the power plant and at the house. If I understand it correctly, no real current flows. It is more like an anchor, so that the Volts have a stable reference that they want to flow back to.

So, once again, the summing up:
Suffice to say that the ground and the common in your socket in the wall are almost certainly connected somewhere in the house. This means, if the common and the ground connector are swapped in the device (in this case your computer), the device will find the "common" connection via the ground wire. If the ground wire in the cable being used then breaks, the device will stop working (obviously).

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Ok got all that and that makes me think that either the hot or common wire is broken and when the power cord is bent the internal wire is pulling apart where the brake is or internal corrosion is happening.
Are you ready for a laugh mick? The new dog just came in went under the computer table and lay down on the some of the cords under there, the wife's Wi fi went down and this computer shut down. I need to clean up the wiring mess under the computer desk. thanks for helping me to understand all this.
dinga.

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nubimax wrote:

...I need to clean up the wiring mess under the computer desk. ...

Uuummmm, yes, it sounds like it. 🤣

Bear in mind, a poor contact can get hot enough to start a house fire. Don't leave the clean-up too long...

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audiomick wrote:

nubimax wrote:

Can some one tell me why a computer needs ground to run?

It doesn't.

I haven't seen an ungrounded desktop computer power cord in the States in several years. Technically you're right, they don't need to be grounded. I don't think installing an ungrounded outlet in your house is even legal anymore.

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I've never seen a desktop that didn't have a ground connection. Laptops I don't know for sure, but I don't think I've seen one there without a ground on the power supply either.

but...

as I wrote earlier, that is purely a safety factor. It is not necessary to have a ground for the computer to run (as long as the common is there...).

This applies to all electrical equipment. One proof of this is a last-ditch measure to stop humming noises in audio equipment: separate the ground contact on all but one device in the chain. Everything still runs, but it is not to be recommended because the safety factor is a great deal less.

If anyone is interested, that is about circumventing this problem
Wikipedia)

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A lot of white box computers from the 90's and first decade off the shelf low end models were ungrounded. My first computer, a Compaq before HP bought them, was ungrounded. As a general rule, if it was before 2010, in the States, and had a power supply less than 100 watts there was a fair chance it wouldn't be grounded.

The only laptop I've ever seen with a ground is my Asus G73.

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Here, so we know what we're talking about...

Appliance classes in Wikipedia

And this one
Appliance_couplers

So, desktops, at least here, are going to be Class I as in the first link. Here, the connector for the power cable on the machine is always C13 / C14 as in the table in the second link.

Laptops here (real ones, not fruit...) very often have a C5 /C6 connector on the wall socket side of the power supply.
I recall at least one older one (of mine) that had, I believe, a C7 / C8 connection on the power supply. That one obviously was not Class I, as that connector doesn't supply an earth. It, and the Laptop, must have been Class II.
For Laptops that use the C5/C6 connector, I think the minus of the DC that goes to the Laptop from the power supply often is connected to the ground on the power supply.

Basis for the assumption is not having measured the connections with a multi-meter, but rather that a Laptop that is connected to audio equipment without a transformer will often hum if it is not on its' power supply. If the power supply is connected, the hum often stops (or at least changes). This is a sure sign that the machine is getting a ground through the power supply.

A lot of household stuff (hair dryer, shaver, small kitchen appliances ) is Class II, but bigger stuff tends to be Class I.

And remember: I'm talking about here in Germany, and in Australia. Things are pretty much the same in both countries. I can't comment on the USA, as I've never been there. 😉