# What actually causes a BOOM after a Short Circuit?

I've already read many answers about short circuits and I know when electricity takes shortcut through a path with very low resistance, is called short circuit.

But the question is which factor actually causes fire or a blow. It's like a MCQ, but I hope someone will answer my question.

When I short a 1.5v torch battery nothing happens.

If I short Van de Graaff with 100,000v, just a tiny spark.

But if I short a 12v car battery, BOOM!

Again if I short a 220v socket in my wall, again BOOM!

It seems that voltage is not the factor that causes that BOOM.

So, my question is which factor actually causes that BOOM?

1. Voltage.
2. Current.
3. Power.
• Power, so..all of them. – Wesley Lee May 22 '17 at 23:40
• If the magic smoke is under pressure, it can get released rapidly! – Cort Ammon May 23 '17 at 6:55

So, as Wesley suggests, it's sort of all of them.

The "BOOM" is usually the air heating up and expanding very fast, causing a pressure wave which propagates until it hits your ear, sounding like a boom. To make an actual sound rather than just a wave of heat, the air has to heat up very fast. This means that the power level has to be very high, and become high very quickly.

Another possibility, depending on what you short it with, is the solid short heating up very fast and expanding, eventually rupturing and causing a boom sound. Again, this requires a high power and rate of power increase.

In electrical terms, power can be expressed as a combination of Voltage and Current: P=I*V

This is why you need both high current and high voltage to make a boom.

In your examples, a car battery is capable of supplying a lot of current, so even at a lowish voltage of 12V, if you short it it can still push out a lot of power. It can probably supply more current than your wall socket, but your wall socket is at a higher voltage, ans so can still put out a lot of power.

Your VDG generator, although it has a very high voltage, has an extremely limited maximum current output, on the order of milliamps, and so wont make a big bang.

An AA battery will only put out a few amps, so that's only a few watts.

A 12V car battery with an internal resistance of 0.005 ohm... (it must have very low internal resistance to put out a huge amount of current to crank your car in the cold winter)... well you're looking at 2400 amps short circuit current, needless to say this is quite a bit of power...

Same for a big capacitor at the output of a power supply: a 50 mOhm cap charged to 30V will output tons of power into a low-impedance load.

Now, if your short circuit is exactly 0 ohm, then it won't dissipate any power, so nothing will blow. In theory. But in reality, the bit of metal that shorts the battery or capacitor has internal resistance, plus it does not make perfect contact, and that contact has resistance too.

The 220V outlet in your wall will have an enormous short circuit current too (like >500 amps at 230VAC, that's lots of watts) before the breaker trips, which is a process involving mechanical parts moving, so it takes time, at least several milliseconds.

Example 1

2x 2700µF very low ESR caps charged to 30V = 2.5 joules, not a lot... Then there is a SO-8 dual MOSFET which runs DC-DC conversion to 5V. My scope probe slips and shorts one gate. Both FETs conduct simultaneously, shorting the supply caps. ESR = 0.1 ohm total Current = 300A Power = 9kW Result: MOSFET explodes, leaving a nice crater in the package. Bits of molten stuff end up encrusted into my safety glasses... because I was looking at it closely, mindful of not having my probe slip, ahem...

Example 2

I cut a cable with wire snippers. This causes a flash and a bang... The cable was under 230VAC power. Oops. I must've switched the wrong breaker off... I always close my eyes when I do that, and it's a good thing, because the bits of metal which were vaporized from the wire cutter must have ended up somewhere...

Example 3

Now if you really wanna get a deep sunburn, do it like this guy. Industrial power panel, high voltage, thick wire, and very slow breakers (because high current/high voltage breakers have big slow mechanical switches) ... remember an arc is a very efficient means to turn electrical power into harmful UV/IR and molten metal projections...