Above: An abortion of a mess of untidy wiring.

A number of years ago a friend gave me an old computer that had “just died” in part-exchange for one of mine (Not my own build.) that I was selling. She said that her husband had been tinkering with it and had upgraded the graphics card. (He works at IBM; but in the office department, not the practical computer maintenance department.) It worked after he had been fiddling with it – Until it had been moved to a different location; after which it had been totally dead from then onwards.

On opening it up the reason became apparent: All the unconnected power leads from the PSU were hanging around loose and swinging around as it was moved. It appeared that one of the the power-carrying contacts of one of the loose Molex connectors, which had become somehow extended out from the plastic sheathing, had touched the inside of the case.

A Quick Lesson in AC Power

The electricity supplied to your computer is in the form of AC (Alternating Current) electricity, at between 110 and 260 volts, depending upon where in the world you are and what time of day it is*. The job of the power supply unit in your computer is to convert this AC voltage into a very accurate number of lower DC (Direct Current) voltages with which it supplies the various components of your computer.

AC electricity is made up of a waveform which alternates in electrical polarity between negative and positive. A complete alternation; where the waveform swings from zero-volts to positive, through zero volts again to negative, and returns to zero volts again, (Also known as a 360-degree phase.) is referred to in electrical terminology as a cycle. The number of cycles per second are measured in hertz, and are the frequency of the AC waveform.

In the UK the AC mains electricity supply is approximately 220 to 260 volts (Depending upon the time of day and the area of the UK.) at 50 Hertz (Hz). (It’s actually over 400 volts before it flows through a transformer on every home’s distribution-board, which reduces the voltage. (The unmanned electrical substations you see dotted around the UK reduce the voltage to around 450 volts from several thousand volts.)) Due to the dual-polarity of AC power, it is commonly conveyed at a number of different (high) voltages using a single cable or wire, right up as far as the domestic power unit attached to each house in the UK. At that point another wire is introduced; that being the neutral (blue) wire. It is connected to a well insulated electrical earthing point. (Not to the same one as the safety electrical earth (Green/yellow) wire.) When electricity is used by an appliance in the UK the power flows from the live wire, which is indirectly connected to the supply via the transformer I mentioned, through the appliance, and to earth via the neutral wire. The safety electrical earth; which is totally unconnected to and completely separate from the neutral earth, is only there to prevent people getting electrified if the live wire somehow becomes connected to the metal case of a faulty appliance. In modern homes there are resettable circuit-breakers fitted to the power-distribution board that will cut the power to a whole section of the home’s wiring if such a fault is detected.

The computer’s case is connected to the safety electrical earth in the UK. Although it’s not the same as the neutral connection it works in the same way; carrying electricity safely to earth and away from metal objects. Although the DC power rails of the power supply unit in a computer seem totally unrelated to the AC mains electricity in your home; that AC mains electricity is nevertheless the source of that DC power that your computer uses. (The power is stepped down in voltage using transformers inside t6he computer’s PSU, changed from AC into DC current by a number of devices called rectifiers, stabilised and regulated by various high-wattage integrated circuits and other regulation devices, and supplied to your computer’s components via the supply rails from the PSU. Allowing one of these supply rails to touch the case, therefore connecting it to electrical earth, completes a circuit, and electricity flows through that circuit to earth. Due to the nature of the build of the PSU combined with the nature of the circuit created by doing so, the power supply unit overloads and stops working. (I could explain it in detail; but neither time nor space allow currently.)

Amateur Upgrade

That’s just one scenario; the one that had happened in the case of this computer, where my friend’s husband had left the power connectors lying loose around the inside of the case after he’d changed the graphics card: Moving the computer to another location had caused a protruding connector on one of the Molex plugs that should have been insulated and inside its plastic sheathing to touch the outside of the case; therefore overloading the PSU and the computer stopped working as a consequence. (In this case I just changed the PSU and the machine worked fine.)

It could have been worse of course: The connector could have touched a metal case or a connector of something on the motherboard; rendering the motherboard useless as well as the PSU itself.

To be fair my friend’s husband wasn’t an engineer or technician: A colleague at the office had told him how to change a graphics card after learning it himself from somewhere, and he’d done as instructed. It worked; but an unforeseen snag caused a problem. He had no idea of how a computer works, and no ideas as to electricity: He followed instructions, correct instructions, and fitted a new graphics card. The power connectors were tied up and tidied; but were in the way of what he needed to do; so he moved them. He just didn’t bother to tidy them up again afterwards.

Nightmare

The sight of loose power connectors (Molex or SATA) lying around inside a computer case make any engineer or technician’s hair stand on end on sight. All unused power connections should be secured by a twist-wire or cable-tie, attached to (But obviously not connected to) the chassis or to other wiring, and kept away from anything metal. Ideally they should be bunched and woven into bundle with the used power leads and secured well away from all surfaces. Anything else is just tempting negative providence and asking for Murphy’s Law to come into play.

Although it can be difficult to make the power leads secure and also look nice at the same time; that’s the constructor’s way of doing it. Practice makes perfect. Power leads/Molex & SATA power connectors (Used and unused.) should be easy to find and/or to trace back to or from the PSU itself by any repair technician or engineer at any time. They should also be safely secured, separate from other wiring, and restricted from movement to as great an extent as is possible. Wherever possible they should be kept away from and not cross the path of other leads/ data transmission leads. (Although this is often unavoidable in many cases, at least to some extent.)

Keep it tidy, keep it traceable, keep it safe.