ELI5: How Do Short Circuits Occur? Need Real-World Examples

[Appletax]

Currently studying for the A+ cert and I'm having a terrible time trying to understand how a short circuit works in regards to actual, real-world issues.

I totally understand how a short circuit works with the example where there's a circuit and a low resistance wire is placed across a hot and neutral wire, but how do real-world short circuits occur at home?

Here's my confusion with a lamp as a real-world example:

If I have a grounded lamp, the electricity comes from the power station through the lamp's black wire (hot), goes into the lamp where the flow is controlled with resistors, and back out the lamp through the white wire (neutral), thus completing the circuit

A short circuit is uncontrolled electricity taking the path of least resistance

"A short circuit is caused when two or more uninsulated wires come into contact with each other, which interferes with the electrical path of a circuit.

The interference destabilizes normal functioning of electricity flow. The resistance generates a lot of heat in the wires and can lead to a fire. "

So if the lamp’s wire’s insulation breaks down and exposes both the hot and neutral wires and those two wires make contact, the electricity (hot) will skip the lamp and go straight to neutral. Neutral is linked to ground so this uncontrolled electricity will go to Earth rather than back to the power station?

How does that help prevent a fire?

With this short circuit there’s no resistors so where does the resistance come from?

Why does it matter if the electricity goes to ground or back to the power station?

Either way, the electricity is leaving the circuit.

A Friend Says

"An unwanted ground would be where your two lamp wires touched and it either blows the fuse or starts a fire from the unregulated amperage draw."

Don't understand how an unregulated amp draw would cause a fire.

Regarding PCs

A PC should be connected to ground to prevent extreme damage caused by a short.

How does a PC get a short (need real-world examples)?

How does ground help prevent damage to the PC when a short occurs?

PC Example

Mobo is touching the panel behind it. It could potentially decide to send electricity to the panel if it's metal and a path of least resistance. This would cause a short. The ground wire would wick away this uncontrollable and unwanted electricity and send it to Earth, thus preventing the uncontrollable electricity from flowing to other components and damaging them.

Accurate?

 

[Krynn72]

So if the lamp’s wire’s insulation breaks down and exposes both the hot and neutral wires and those two wires make contact, the electricity (hot) will skip the lamp and go straight to neutral. Neutral is linked to ground so this uncontrolled electricity will go to Earth rather than back to the power station?

How does that help prevent a fire?

With this short circuit there’s no resistors so where does the resistance come from?

Why does it matter if the electricity goes to ground or back to the power station?

Either way, the electricity is leaving the circuit.

A Friend Says

"An unwanted ground would be where your two lamp wires touched and it either blows the fuse or starts a fire from the unregulated amperage draw."

Don't understand how an unregulated amp draw would cause a fire.

First off, let me say I'm not even A+ certified myself and have no formal education in the topic, but from what I've learned on my own is thus:

The reason why unregulated amperage draw will cause a fire is because the extremely fast flow of electrons (high amperage) generates heat by friction. The electrons are bumping into the atoms of the wire very fast and transferring kinetic energy. The atoms of the wire can't move and so they dissipate that energy via heat rather than movement. The resistance comes from the wire itself, as everything, even conductors, have some small measure of resistance.

I don't really know how grounding something helps prevent short circuits though. I do know that ground wires and ground planes on PCBs tend to be very thick/large, and are usually capable of handling and dissipating heat better than the rest of the circuit.

Hopefully someone more knowledgeable comes in and gives a reply, as I'd like to know if my understanding is correct also.

 

[Appletax]

First off, let me say I'm not even A+ certified myself and have no formal education in the topic, but from what I've learned on my own is thus:

The reason why unregulated amperage draw will cause a fire is because the extremely fast flow of electrons (high amperage) generates heat by friction. The electrons are bumping into the atoms of the wire very fast and transferring kinetic energy. The atoms of the wire can't move and so they dissipate that energy via heat rather than movement. The resistance comes from the wire itself, as everything, even conductors, have some small measure of resistance.

Hopefully someone more knowledgeable comes in and gives a reply, as I'd like to know if my understanding is correct also.

Those extremely fast flowing electrons go into the lamp and the resistor (filament) in the bulb causes resistance...which is what happens with a short (electrons go into the neutral wire, which has some resistance), BUT, the filament has even MORE resistance.

Also, the electrons flow out of the filament back into the neutral wire (electrons in this short are also going into the neutral wire) = same.

So why doesn't a fire occur in the bulb itself given that the situation is the same as a short: fast electrons flowing and resistance.

Maybe the resistor inside the filament slows down the electron's speed before it exits into neutral so there's less friction inside the neutral wire?

 

[Krynn72]

Essentially there are two reasons why fire doesn't happen in a light bulb. First is that there is no fuel there to burn. When a circuit board catches fire, its the plastic and solder mask and other chips that are the fuel for the fire. Secondly the filament acts like a fuse. When it gets hot enough the wire burns through itself, snaps, and breaks the circuit.

If you were to put some kind of fuel in there, like a piece of paper, and use a thicker wire as the filament so it would get hotter without breaking, you absolutely would get fire from it.

Maybe the resistor inside the filament slows down the electron's speed before it exits into neutral so there's less friction inside the neutral wire?

Exactly. A resistors job is to reduce current. Light bulbs are a delicate balancing job where you make the filament wire burn red hot making light while limiting current with a resistor just enough to prevent that wire from burning through its self.

 

[ZenTree]

Essentially there are two reasons why fire doesn't happen in a light bulb. First is that there is no fuel there to burn. When a circuit board catches fire, its the plastic and solder mask and other chips that are the fuel for the fire. Secondly the filament acts like a fuse. When it gets hot enough the wire burns through itself, snaps, and breaks the circuit.

If you were to put some kind of fuel in there, like a piece of paper, and use a thicker wire as the filament so it would get hotter without breaking, you absolutely would get fire from it.


Exactly. A resistors job is to reduce current. Light bulbs are a delicate balancing job where you make the filament wire burn red hot making light while limiting current with a resistor just enough to prevent that wire from burning through its self.

Aren't lightbulbs filled with Argon gas? Without oxygen they cannot burn and reduces the impact of the heat on the filament. No oxygen so no fire.

 

[TAPtech]

So if the lamp’s wire’s insulation breaks down and exposes both the hot and neutral wires and those two wires make contact, the electricity (hot) will skip the lamp and go straight to neutral. Neutral is linked to ground so this uncontrolled electricity will go to Earth rather than back to the power station?

How does that help prevent a fire?

With this short circuit there’s no resistors so where does the resistance come from?

Why does it matter if the electricity goes to ground or back to the power station?

Here's my crack at it:

1) It doesn't, really. The breaker at the panel prevents the fire by tripping and cutting the circuit.
2) The wire is a resistor. Almost everything is a resistor. Take a small piece of that lamp wire (one strand) and connect the hot and neutral. Put that strand in a glass bulb. There you have it- a light bulb.
3) The neutral wire is grounded at the panel. The "uncontrolled" electricity will probably go back to the power station as that path is much less resistance than through the earth. I think the whole "dump it to ground" idea can be confusing.

I think this will help: remember the old BX electric cable, the hot and neutral inside that big flexible metal tube? Then all the boxes were metal as well? Now all the wires are encased in a metal system. If one of the wires breaks, it will touch that metal system and be scooted off to ground easily. So now the risk of shock when you touch a piece of metal, maybe the light switch or the light switch cover... is much less. That's the point of the ground. Big appliances and computers- lot's of electricity going on in there. So you ground the metal housing, just in case.

 

[Markverhyden]

Electrical shorts can cause fires in two ways. One way is because there can be sparking involved if the circuit has enough energy. Second way is by over heating the conductor because the energy draw is un-regulated. This can cause the insulation to melt away and the conductor can come into contact with a flammable substrate.

By the way a light bulb filament lasts because the gas it is encased in does not allow it to oxidize when current is flowing. Just break the bulb glass and watch the filament when you turn it on. That is basically what burning is - oxidizing.

Resistance is a function of the length and cross section of the conductor as well as the conductor material itself in relation to the current/load. You can make a network cable out of 18 gauge wire and it will work fine. Well maybe not gb performance because of the lack of twisting to control EMI effects. The opposite, such as using Cat5 to run standard AC, 115V, will not work.

 

[nlinecomputers]

How does that help prevent a fire?

It doesn't. That very scenario is what causes most fires. The amount of electrical current is so high because of the very low resistance that heat builds and the insulation or items near the wiring catch on fire. A fuse or a circuit breaker is designed to fail, in the case of the fuse, or expand from heat and open in the case of a circuit breaker and break the circuit before things catch on fire.

 

[B Trevathan]

If anyone would like to learn about electricity this site is a good site to start: www.allaboutcircuits.com

I don't think of a light bulb as a resistor I think of a light bulb as a load (A device that uses current) not as a device like a resistor that reduces the current flow. The only real resistance in a light bulb is the wires and tungsten filament. But of course everything has resistance even gold.

A circuit is basically just a closed loop, you can think of it like a race track. Things are added to the circuit loop such as power (source) and a bulb (load).

You can have a complete circuit without a ground, for many years buildings had power without grounds, there where only two wires, a hot and a neutral.

A ground was added mostly for safety so that people would less likely be shocked or killed (less than 1 Amp can kill). The ground wire should be a better ground than a persons body. Grounds should not be connected to neutrals, but most commonly are inside circuit breaker panels. The neutral wire should go back to the power company and the ground wire should go to the ground.

Wires are made from metal (conductor) and most computer cases or lamp frames have metal in them and manufacturers connect the cases or frames to the ground wire knowing that if there ever was a short the current will hopefully go to the ground wire and not a person touching the case or frame.

A short is when current is flowing through a circuit and bypassing any loads in the circuit loop. The fast flow of current without a load produces lots of heat and this heat can melt wire insulation or cause fires or explode devices.

Wires do not have to touch each other to have a short, electricity can jump (arc), sometimes this arcing is what starts a fire.

For safety a fuse or breaker is added to a circuit so if a short happens a fuse should blow because the short causes a larger current draw across the wires than what a sacrificial wire inside the fuse is rated for and the sacrificial wire blows in two or in the case of a circuit breaker the breaker breaks (opens or trips) to reduce damage and try to prevent a fire.

A short circuit is caused when two or more uninsulated wires come into contact with each other, which interferes with the electrical path of a circuit.

The interference destabilizes normal functioning of electricity flow. The resistance generates a lot of heat in the wires and can lead to a fire.

Also a hot wire can touch a grounded frame and cause a short. The heat is caused by the resistance of the wires to the fast flow of current through them.

How does a PC get a short (need real-world examples)?

Components like capacitors go bad and cause shorts, screws can get lost on or under the motherboard, liquid spills, etc..

How does ground help prevent damage to the PC when a short occurs?

The ground is mostly for safety but when any short occurs a fuse should blow or a circuit breaker should open to reduce damage and try to prevent a fire.

All that being said don't count on fuses and breakers, sometimes they don't always work fast enough to prevent damage and sometimes they don't work at all.

 

[fencepost]

It's also important to note that electrons "want" to be distant from each other (same charges repel) which is why a larger wire is relevant for higher capacity. When you run too much current through too fine a wire, it heats up - you may have seen experiments with that in school with fine wires, I have actually watched a transmission line melt*. Electrical transmission is also an important factor in design for distance in terms of tradeoffs between capacity before overheating and tensile strength.

* Was actually kind of cool. There was a storm going on and I was in an area that still had overhead lines - most stuff near me is buried. As I was driving along I noticed a glowing orange line across a parking lot, then watched the line just get brighter and start to droop, then finish stretching and laying across the hood of a vehicle and the parking lot. I suspect a line had been cut somewhere else and automated failover switching put too much current on the existing one, but didn't find out anything more about it. I did call the power company though, and they already had a variety of trucks in the area.

 

[Ktex]

Laptop plastic cases have a metallic finish inside them. Don't take out and lay a live motherboard on top, a short might occur. The only short I have created was with a live board and one of the antenna wire hookups touched it, I was trying to hook up the LCD cable while it was on. Chalk it up to experience.

 

[gadgetfixup]

Most of the questions have been answered but it should be pointed out the 3rd wire ground we use today does little to protect devices and circuits. It was created and mandated to prevent humans from being energized when devices fail or short. In a properly wired home or business with grounds correctly wired, energized devices would seek the path of least resistance and find earth through that ground wire rather than taking the path through a consumer to earth. 3rd wire grounds (typically green) don't protect devices, they protect humans.

How does a PC get a short (need real-world examples)?

Those of us from the old generation of computer power supplies can remember computers that would "bite" you if you touched the case while they were turned on. Typically by cheap power supplies that would leak AC current to the metal case. I've had a few that would really zap you while running.

A short in today's computers is mostly on the DC side of the power supply and not the AC side. Wires chaffing on sharp metal or by some "master builders" that thought it was a good idea to pinch wires under the motherboard between the board and backplate without realizing many of those solder points on the bottom of the motherboard are sharp as needles. Typically a short on the motherboard is overheated semi-conductors (power transistors).

 

[MDD1963]

Probably a very common example of a short is an extra mainboard mounting stud on the case where none should be.....

Contacts rear of mainboard, zzzzt, a bit of smoke, and....done!

 

[MDD1963]

And a good example of a near electrical fire/meltdown with NO short.....

Run huge amounts of current (think 208V running thru a 75 or 100 amp breaker supplied by one of three 150 KVA generators for an aircraft's electrical system) through a large diameter wire (think of wires the size of your middle finger) to an eyelet for connecting to a circuit breaker, but, have it not fully tightened, so that all of the current flows thru a small contact surface the size of a BB instead of the flat surface area of the eyelet.....it will glow red, and melt. (not a fun mess to clean, but, interesting to diagnose when everything was working fine, electrically, prior to the meltdown...; after powering everything off, opening the circuit breaker panel, and finding the eyelet's connection at the circuit breaker loose by 4-5 full screw driver turns told the tale.)