The same way you would determine the water pipe friction so that you do not have a main break!!!!
What impedance determines is if you do have a short circuit, how bad it (short circuit current)could be. It is not something you intentionally insert (except for very rare cases..not the subject here)to avoid the accident.
I'm not sure what you are trying to achieve exactly. The resistance and impedance of a wire will depend on the frequency, tempeture, componsition, size and length. Most factors involved in short circuits in a breakdown of the insulation, either because of an object being injected into the circuit, or over voltage puncture of the insulation, or over heating of the insulation, or some change of the insulation material.
Given a car starter is something close to a short circuit, the cable is sized to prevent overheating in the time it is intended to be active.
I think you might be looking at this problem from a diff perspective. As cranky mentioned I am not really sure what you’re asking but ill try to help out anyway. From what I understand this is a pretty fundamental question (Ohms Law). Try to look at your “short circuit” as ampacity or current in your circuit. Your current will vary based on your system voltage and your system impedance (I=E/Z for a simple circuit). For example, lets assume your circuit has a piece of equipment with 5 ohm impedance and 20V supply. Your current through the wires will be 20/5 = 4Amps. Now in case of a “short circuit” (two wires touch before the equipment) your impedance will be much lower lets say 0.5ohms in which case your current will now be 20V/0.5 ohm = 40Amps. The higher ampacity is not desirable due to a lot of obvious reasons so you want you over-current device to trip and open up the circuit before any damage takes place. This of coarse is a very basic description of the Ohm’s Law and for all electrical systems there is much more to consider, especially when dealing with “short circuit analysis” which I think you are not concerned with?
I hope this is what you were referring to with your question.
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
Your first (original) question was not technically accurate, meaning you cannot have a circuit which will not get you a short circuit. The first line in my first response was a sarcastic statement.
Your second question is too broad to get you started in a right direction. You need to start with some very basics, such as google "basic short circuit calculations", it wil throw up many links. Some more complex than others. See any of them make any sense to you. If you are confused, I would suggest you leave electrical issues to electrical professionals.
May be you can narrow down your question to the specific situation you have at hand, rather than trying learn something that can get very complex very quickly.
rbulsara,
i'm a mechanical engineer. I know basic electrical concepts like most engineers, but i am increasingly interested in learning more. Can you recommend a good web site where I can learn more?
A short circuit is the same as any other circuit, except it is an unintended circuit arrangment. The same ohms law applies. However with AC power there is an angular component associated with the voltage, current, and impedance.
When you do a short circuit study you have to include some factors that you normally would not consiter, like source impedance (impedance of batteries, or generators), and impedance of ground (normally consitered near zero).
If I knew a good reference, I'd give it to you, but I don't have one.
I do not know of a website that will teach you electrical short circuit calculations, as it has so many variables.
If you really want to get started on a right footing (and dealing with 3 phase AC circuits), you need borrow or buy some good books. You will still need a good electrical engineer's help to understand those.
You can start with IEEE Red Book and IEEE Buff Book. Look up at
The sites are easy to find if you just type in what you are looking for. A lot depends on what exactly you are looking for, how basic do you want? You can download courses from a lot of colleges also.
