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December 9, 2009
Short Circuit Currents

I talk a lot about short circuit and over-load protection. However the other day I was talking with an OptiFuse distributor salesperson who didn’t understand how short circuit currents could get so high (maybe 10 - 1000 times higher than the normal circuit current).

Allow me to offer up the following easy to understand explanation:

There are very few "laws" governing electricity, but one of the most important and most used is "Ohm’s Law".  This principle is named after a German Physicist, Georg Ohm, who in 1826 described the relationship between voltage and current.

His law basically states that the voltage in any circuit is equal to the resistance of the circuit multiplied by the current flow.

The following diagram illustrates Ohm’s Law

Ohm’s Law

If you know the voltage (like 120 V coming out of the wall in most homes in North America), and the internal resistance of the device being plugged into the wall (say 12 ohms for your big screen television).  Then you can compute the current that will flow through the power cord and through the big screen TV (10 amps).

So now let’s consider the short circuit event...

A short circuit occurs when the normal load is bypassed with a lower less-resistive path.  This can happen possibly if the power cord is accidently cut with a pair of metal scissors allowing the two wire to touch each other.  When this happens, the higher resistance (big screen TV) is replaced with a much lower resistance (metal scissors touching copper wires).

Here is what a short circuit might look like:

Ohms Law - SC

The resistance (from the metal scissors touch the copper wires) is now only 0.12 ohms rather than the 12 ohms of the big screen TV.  The current now will flow through the least resistive path (the metal scissors) rather than the higher resistive path (TV).

Ohm’s Law is still applicable in the case of a short circuit...

That is:

Voltage = Resistance x Current or

120 Volts = 0.12 Ohms x 1,000 Amps

Where the normal current was 10 amps, it is now 1,000 amps when a short circuit occurs (in this example)

A 1,000 amp current will now try to travel through wires and other circuit elements designed only to carry 10 amps.  The wire and the other circuit elements will quickly fail (generally within a few milliseconds) and will have a high probability to inflict great harm to equipment and people.

A protective device, such as a fuse or circuit breaker, must be placed into the circuit to prevent this damage from occurring.  The fuse or circuit breaker is specifically designed to fail (it actually works when it fails) opening the circuit very quickly and preventing further harm.

I apologize to to my engineering friends who read my Wednesday blogs on a regular basis for their more technical slant. I realize that you are already fully aware of many of the principles discussed, however I think that education is good for those not so technically trained as well.  Thanks for your patience this week.

To everyone...you very much for your continued support of OptiFuse where we attempt to bring education to everyone.

Jim Kalb


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