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### Voltage During Short Circuit Condition

07/13/2012 10:14 PM

Current is the rate of flow of charge and volatge is the measure of energy carried by the charge .

voltage is said to be at ZERO potential during a short circuit but what i cant comprehend is how current exist without voltage being present? doesnt the current need voltage to carry it along the cable towards the fault location?

now imagine a Generator(source) supplying distribution transformer 11/0.433kV supplying a switchboard. lets say a 3 phase fault happened within the switchboard and the main circuit breaker of the switchboard failed to trip and upstream protection protecting the 11kV line failed to trip as well. just imagine all protections have failed to act.

Now at this point of time what will be the voltage on the primary and secondary side of the distribution transformer and the volatge at the (source) generator.

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#1

### Re: voltage during short circuit condition

07/13/2012 11:03 PM

Please note that the voltage is ZERO at the point of short circuit and not at the source.

Now going by your logic, let us say, you have a single phase 50 Hz, 240V AC supply feeding to two loads which drop 120V each across them. Would you say that the 240 V of the source has alreday been dropped 120V each at the two loads and hence 240 - (120+120) is ZERO?

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#5

### Re: voltage during short circuit condition

07/14/2012 3:13 AM

Hi

thanks for replying. pls see attached as an example. if a fault occurs at the motor end what is voltage is expected at the secondary side of the transformer.

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#2

### Re: voltage during short circuit condition

07/13/2012 11:18 PM

Bravo! You have a very useful piece of insight here.

Normally one can consider wires and voltage sources as ideal devices that have no resistance because the load of a circuit will be the most significant resistance. However, a short circuit condition is the most frequently found abnormal condition. When a short circuit happens the small resistance of the wires can easily be the only significant resistance. Additionally during a short circuit, the voltage source can no longer be considered an ideal voltage source with no output impedance. Instead one should consider the Thevenin equivalent voltage source of an ideal voltage source in series with a resistor. Now how one calculates or measures this resistance is complicated and as you seen the shower of sparks that happens with a short circuit a potentially dangerous process.

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#3

### Re: voltage during short circuit condition

07/13/2012 11:56 PM

I do not care to imagine an all protective devices failure. Unless something trips, transformers and overhead lines will burn up. As energy generated will be dumped somewhere. Then, generators will be protected, or fail, spectacularly.

But the whole scenario is a feverish imagination of a student, who does not know a real short circuit from zero resistance. I am offering him \$100, if he can show up on my doorstep with a 0 Ohm short. Waiting, waiting, crickets.

Commingling the never-never land of theory with reality is really a no-no, that ought to be clear to a first year technician.

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#4

### Re: voltage during short circuit condition

07/14/2012 2:30 AM

It's simple math, when a Isc occurs, your terminal voltage is not actually zero but a very small value close to 0. If as you say voltage was 0, then yes, current will seize to exist, however, and as the case would be during a Isc, voltage tend to be a very small value close to 0 for a very short period of time allowing a spike in current in the path of minimal resistance.

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#6

### Re: voltage during short circuit condition

07/14/2012 3:22 AM

thanks for your reply. i guess im not understanding it right. from the attached sketch. if a fault were to happen close to the motor ( as indicated). what is the voltage available at the transformer secondary. say that the circuit breaker feeding the cable fail to open.

thanks

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#7

### Re: voltage during short circuit condition

07/14/2012 4:18 AM

What is the impedance voltage or impedance percentage of the transformer? based on that, you can perform a Isc calculation and determine the voltage required to drive full load current in your secondary winding terminal.

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#9

### Re: voltage during short circuit condition

07/14/2012 4:46 AM

thank you

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#8

### Re: voltage during short circuit condition

07/14/2012 4:34 AM

To answer your question, the voltage dip in the secondary depends on the impedance of the transformer. As you know impedance limits current variation, for so, the higher the transformer impedance, the lower the decrease in voltage, and vice versa.

So you can also say, the lower the transformer impedance, the higher the level of the Isc fault

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#10

### Re: voltage during short circuit condition

07/14/2012 7:10 AM

In the usual way of use, a motor circuit cable will not have zero resistance. In fact, since 80% volts at motor terminal is often quoted as a minimum and starting current is, say, 6 times full-load, the resistance could be that high.

Typical impedance of transfo is about 4 % - hence a short circuit at transfo secondary gives 25 times [transfo] full load. Now thinking very simply, if 6 times motor full load gives 20% volt drop, then 25 times full load gives 20% x 25/6 = 83% rated volts - all assuming motor full-load current is about transfo full-load.

So the voltage drop across motor cables is very significant and cannot be ignored. A correct calculation needs data on motor cable resistance and transfo impedance, even switchgear contact resistance.

In actuality, the arc at the fault will not have zero resistance - arcs tend to a "constant" voltage drop (a few tens of volts) rather than resistance. There are formulae for long arcs as you get at high voltage lines, which have higher volt drop, which is not always negligible compared to Multi-kilovolt sources.

It is normal practise to assume shorts are zero ohms - this ensures real currents are less than calculated and switch gear is never under-rated for short-circuit capacity.

It would be possible to have a "superconducting" portion in the circuit with zero volt drop. Remember that current is the movement of electric charge - early electric experiments used mechanical means - the Van der Graaf generator is a modern version. Think of the analogy of resistance to motion of a rocket in water, air and vacuum.

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#11

### Re: Voltage During Short Circuit Condition

07/14/2012 11:13 PM

You choose a complex circuit for an elementary question.I should face problems first like ¿what happens if a coil is suddenly open when it was conducting? ¿whats the difference between potential changes in a resistor and a f.m.e.? "voltage" in fact is a wrong expression to me,useful but wrong when you analize circuitry.Better expressions to study are L•di/dt or L/R ...

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#12

### Re: Voltage During Short Circuit Condition

07/15/2012 12:46 AM

I'm just going to post something to jar your memory a bit.

1. Sometimes ZERO is very hard to obtain.

2. Sometimes 1E18 is about the same number as 3 E18

3. inductance of a piece of wire 1/4" sometimes can be ignored. In high frequency work, it cannot be.

4. Sometimes you can assume the resistance of a wire is zero, other times you cannot.

I can tell you that I've measured the resistance of a piece of paper and you might tell me I'm nuts. Ill tell you it's value depends on the moisture content. Pure drinking water is different than pure water if your a semiconductor manufacturer. Normally, you can run wires near each other, but don't do it with HV transmission lines.

Watch what you assume.

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#13

### Re: Voltage During Short Circuit Condition

07/15/2012 12:55 AM

basically, theory is separate and different from practice, there is NEVER a zero voltage short condition and is usually a Low voltage short ckt condition in place because nobody uses superconductors in a ckt. therefore electrons follow the path of least resistance and the circuit fails.

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#19

### Re: Voltage During Short Circuit Condition

07/15/2012 11:26 AM

Be very careful when using absolute terms like NEVER and nobody.

I'm trying to get an experiment to work properly that uses a YBCO superconductor electromagnet. The wiring is cooled by LN2 and a Helium cryogenic refrigeration system to keep the superconductor well below quenching temperatures. To my surprise I've found that Joule self heating of superconductors does happen with voltage differentials of zero volts. I do not understand this phenomena despite the literature I've read and pioneers I've talked to.

Also electrons follow every path that they can travel not just the lowest impedance path. If they didn't then power would not get distributed and there would be no need to calculate parallel impedances.

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#14

### Re: Voltage During Short Circuit Condition

07/15/2012 12:57 AM

Voltage is defined as electromotive force. It is the PRESSURE that induces the current to flow in the wire. Voltage can be present with zero current flow. Voltage often IS present with zero current flow. We often pretend a wire is an ideal conductor with zero ohms resistance. In reality that is never true. If you have a short, the DIFFERENCE in POTENTIAL still exists, albeit reduced because of internal resistance in the voltage source. Voltage is often referred to as a difference in potential because you need a DIFFERENCE in voltage to induce current flow. no current will flow between two equal voltages. The more I think about it, the harder this is going to be to explain to someone with almost no electronics or electrical engineering background.

The U. S. Navy NEETS modules are available online in PDF form and a great way to learn electronics without an instructor. You need to clear up your basic misconceptions.

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#15

### Re: Voltage During Short Circuit Condition

07/15/2012 7:07 AM

Ohm's Law has stood the test of time.

I =E/R, or E = I x R, or R = E/I.

Just insert any two known values. This will give you the third. Funny things happen when R = 0.

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#16

### Re: Voltage During Short Circuit Condition

07/15/2012 8:12 AM

When two conductors connect without any resistance,(theoretical) ,it is commonly referred to a a "short". The reason is because it behaves like a very short section of the conductor.

When this happens, the only thing limiting the current in the circuit is the impedance of the supply and the resistance of the conductors.

If you had a very sensitive meter, you could measure a voltage drop across the junction.(Which is actually a very low resistance).

This characteristic is used frequently to measure current flow in very high current DC situations.In these cases, they use a very precisely calibrated shunt, which looks like a section of bus bar, but it will have a value stamped in it.I have seen them marked as low as .0001 ohms and lower.Pretty close to zero, but enough to measure a voltage drop in a high current situation:1000 amps =.1 volts.

This voltage is amplified and sent to a control circuit for regulation of the desired process, whether is a generator or speed control of a process.

This is considered "Old School" now by many young engineers, with the advent of Hall effect sensors,but before their advent, it was the best method of measuring DC current.

A/C current in high current applications has used basically the same device for many years:A Current Transformer, which is simply a multi-turn transformer shaped like a doughnut, with the measured conductor passing through the center.

Magnetic field will be proportional to current flow.

You can see some of these on industrial metering.Next time you go to a commercial building, look at the incoming power near the meter base.You will see them.They are used by the meter to measure current flow of the conductors without having to build a meter capable of carrying the full current.

So, when you have a short circuit, your load resistance has been removed,and replaced by a very low resistance, which increases current in the circuit, (up to the ability of the circuit to provide this current.)This ability is determined by the total circuit impedance, which has been explained by others.This capability has to be considered when replacing fuses or circuit breakers.The AIC (amperes interrupting capacity) will be specified on the fuse or breaker.For instance, a control voltage transformer for a home HVAC system will have very low requirements because the transformer can only supply a limited amount of currrent.

A battery powered fork lift, on the other hand, although only 36 volts, has a very high current capability, which is why you may see some very special and expensive fuses, (such as Shawmut,etc.) in these applications.

So when referring to a short circuit, remember, it is simply a very low value resistance..

The only place a real zero resistance is found is in a superconductor circuit, and this is utilized for transmitting enormous amounts of power,such as the Canadian/US power grid connection, and MID USA grid connections.Japan uses superconductors to power some islands, and some states also use it.

The advantages of a superconductor are obvious at first glance, but there are other features that make it attractive as well, such a phase synchronization between multiple grid systems.When A/C travels 1000 miles, and must synch with a local grid,the phases have to be matched.This problem is very simple with superconductors carrying DC; it is inverted and synched simultaneously(not going into that here!).

There are many other factors at work, and I shall not attempt to explain them all, but I hope I am able to illuminate your understanding of a ("Short Circuit").

Did this help?

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#17

### Re: Voltage During Short Circuit Condition

07/15/2012 10:37 AM

Remember R=pL/A? where R is Resistance, p = resistivity, L is the length and A is the cross-sectional area

p(rho) is a materials property in units of ohm-cm or similar. Thus, you cannot obtain Zero ohms.

FWIW, you can buy a zero ohm resistor, but what it means is it's a wire in surface mount or say 1/4 Watt leaded resistor used as a jumper. It's resistance is not ZERO.

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#18

### Re: Voltage During Short Circuit Condition

07/15/2012 11:17 AM

in a load cable voltage short circuit what will happen. load cable current fault occur what will happen in circuit.

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#20

### Re: Voltage During Short Circuit Condition

07/16/2012 4:09 AM

In the various statements, the issue of resistance at the point of the short circuit has not been considered. In order to overcome the contact resistance there has to be a potential difference between the two conductors which are causing the short circuit.

In other words, there should be a measurable voltage between the two conductors at the point of short circuit, however small. Otherwise, there will be no short circuit!

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#21

### Re: Voltage During Short Circuit Condition

07/16/2012 6:30 AM

In a previous post of mine, I stated:

"If you had a very sensitive meter, you could measure a voltage drop across the junction.(Which is actually a very low resistance)."

The "junction" I am referring to is the point of the short.Perhaps something is lost in translation.

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#22

### Re: Voltage During Short Circuit Condition

07/16/2012 8:54 AM

1- There is a voltage that is driving the current in a short circuit:

The source voltage will remain there, even if distorted or affected by the conditions of the short circuit.

The voltage at the output terminals of the transformer will be negligeable, BUT it is not this voltage that drives the current. The Whole circuit is made of the SOURCE Voltage Vo, The internal impedence of the Source Zo and the Outside Load impedence ZL.

If ZL =~ 0, the circuit insiside is still there and its internal Zo will limit the current, while the Vo is still the driver,(Even if it is distorted by the Short...).

This is as simple as it could get.

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#23

### Re: Voltage During Short Circuit Condition

07/16/2012 2:24 PM

The whole point of adding circuit protection devices, Murphy, is to protect the wiring when one of these short circuits occurs, so there is no damage or fire. Do you not understand that?

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#24

### Re: Voltage During Short Circuit Condition

07/17/2012 7:11 AM

Hi suresh8. Just a little help about your confusion. See my drawing below.

A power supply (depicted by the dashed line) includes an ideal voltage source (V) in series with a resistor (Rs) (i.e. the internal, output resistor of the power supply). By varying the load (RL) you can vary the output current and the power consumed by the RL. We get the maximum power on the RL when RL= Rs. There are two cases, though, that we get no power on the RL:

1) When RL=∞ (i.e. open circuit), as no current is flowing through RL (P=I2R=0). Also, the voltage on the P.S. output is V (i.e. we get the maximum voltage).

2) When RL=0 (i.e. short circuit) as no voltage exists at the output of the P.S. (P=VPS2/R=0). In this case, we get the maximum available current from the P.S. (i.e. Imax=V/Rs), while the whole power is consumed on the Rs (i.e. inside the P.S.).

(In every other value of RL we have various values of voltage, current and power on the RL. You can easily find them by considering the voltage divider which is formed by the Rs and RL.)

I hope that it helped.

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#25

### Re: Voltage During Short Circuit Condition

07/17/2012 8:39 AM

Just a small correction on (2) in my previous post (i.e. in the case of short circuit):

I wrote: P=VPS2/RL=0. However, this is wrong because VPS=0 and RL=0, so P=VPS2/RL=02/0=0/0=undefined

It's better to say that P=I2.RL= I2.0=0 (where I=Imax=V/Rs)

Yeah, that's better...

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#26

### Re: Voltage During Short Circuit Condition

07/17/2012 11:43 AM

A similar correction must be done in (1) of my initial post too (i.e. in the case of open circuit):

I wrote: P=I2RL=0 This is, also, wrong because I=0 and RL=∞ so we have:

P=I2RL=02.∞=0.∞=undefined

The correct is: P=VPS2/RL=V2/∞=0 (or even better: lim(x→∞)V2/x=0)

Just to be accurate...

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#27

### Re: Voltage During Short Circuit Condition

07/17/2012 1:15 PM

Actually, your 1) entry was not wrong: 0.∞ =0 (it is not undefined)

Your 2) should have been: P = Vps²/(RL+Rs) where RL = 0 in short circuit. Since there is no Load, The power will be dissipated in Rs (this is theoretically correct if RL = 0). Therefore, Rs will be the limiting factor to the Short circuit current. ==> P ≠ ∞ and determined.

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#28

### Re: Voltage During Short Circuit Condition

07/17/2012 1:38 PM

Also it might seem that 1) is the perfect place for L'Hôpital's rule on limits here. Particularly because as RL→∞ I→0, I and R are not independent variables. The power equation with either single variable (P=V2/R or P=VI) clearly shows that as one considers either single term the power always approaches 0 as either RL→∞ or I→0 as long as V remains finite.

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#29

### Re: Voltage During Short Circuit Condition

07/18/2012 3:21 AM

Hi LAA Lucke... 0.∞ is undefined (and not 0)... I'll try to prove this to you:

First of all: 0/0=undefined (eq.1)

(It's easy to prove this: 0/0=x => 0.x=0 The latter is true for [x=any number] hence x=0/0=undefined)

Now let's suppose that 0.∞=0 which gives 0/0=∞ (eq.2)

As you see, eq.1 and eq.2 give different results. eq.1 is proven (i.e. true) and eq.2 is "under question". Also, they cannot both be true. So, it's obvious that eq.2 cannot be valid. Hence: 0.∞≠0

It is true that 0.n=0 no matter how big the n is. So, we tend to consider (by simple logic) that even if n=∞ this should, also, be true. But it's not. The concept of "∞" is a very tricky one. Let's put it another way: We assume that if we add an infinite number of zeros the result should be zero. But we'll never be able to confirm this, as we should add zeros for ever...

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#30

### Re: Voltage During Short Circuit Condition

07/18/2012 12:46 PM

You are absolutely right if we want to split hairs in the math theory of limits...

Since we are at it, what do you make of:

x/x = 1 then both x varying at the same rate → ∞, will ∞/∞ = 1?

Then, going the other way, both x → 0 will give 0/0 = 1 at the limit?

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#31

### Re: Voltage During Short Circuit Condition

07/18/2012 3:15 PM

The sentence below is true.

The sentence above is false.

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#32

### Re: Voltage During Short Circuit Condition

07/19/2012 2:48 AM

Actually, x/x=1 no matter how big or how small x is. But, again:

0/0=undefined

∞/∞=undefined

When you say that something tends to 0 or ∞ is different from saying that something is exactly 0 or ∞.

The meaning is that we are not able to know the results of such operations. So, -e.g.- the expression could be like this: 0.∞=Unknown=Not allowed=Undefined

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#33

### Re: Voltage During Short Circuit Condition

07/19/2012 9:41 AM

Sigh. This apparent paradox was solved and published three hundred and sixteen years ago by the French mathematician Guillaume de l'Hôpital. His paper on this was so convincing that this solution is now known as L'Hôpital's rule. The Wikipedia link IMHO is a little dry and uninformative, yet accurate on the mathematics. The history though is informative and is why I used the link. I prefer Wolfram for a better presentation of the mathematics.

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#34

### Re: Voltage During Short Circuit Condition

07/19/2012 2:57 PM

0.∞=0 is not treated as an equation but rather a statement since you cannot state, as a deduction, that ∞ = 0/0 ( which =1 and ∞ cannot be = 1).

Another way, since ∞ here should be looked at as an undetermined value, like a variable: 0.x = 0 cannot be carried out to deduce x = 0/0 unless the x variable =1 and only 1.

Also: 0.25 = 0 for example, does not imply that 25 = 0/0 which becomes absurd and has nothing to do with any type of mathematics.

0 . any number, even ∞ is allways going to be reduced to 0 (Zero) in any mathematical equation, deduction etc. The linits theories has nothing to do with it here. It is a well defined accepted value = 0

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#35

### Re: Voltage During Short Circuit Condition

07/20/2012 3:53 AM

Sorry, you are wrong again. I can easily prove to you that 0/0≠1. Here it is:

Let's suppose that 0/0=1. Then we have:

0/0=1 => (0+0)/0=1 => 0/0+0/0=1 => 1+1=1 => 2=1 (which is obviously wrong)

So, the initial statement "0/0=1" is wrong.

That's why 0/0 is stated as "undefined". In fact, also, x/0=undefined (where x=any number) [although some others believe that x/0=∞ (where x=any finite number) but this is, also, wrong]. The correct statement is that x/0 (i.e. the division by zero of any number, including 0 and ∞) has no mathematical meaning and is not permited. And that's why it is stated as undefined.

Also, the expressions ∞/∞ and 0.∞ are stated as undefined.

Moreover, even the expresion n/∞ is undefined (although some people believe that n/∞=0). Here is the proof: if n/∞=0 then it must be 0.∞=n which is wrong, because (as I have already said and is proven in previous post) 0.∞ is actually undefined. So, n/∞≠0 and actually it is, also, undefined. However, the expression lim(n/x)(x→∞)=0 (for n=any finite number) is absolutely correct.

As I said, the numbers 0 and ∞ are very tricky and must be handled carefully.

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#36

### Re: Voltage During Short Circuit Condition

07/20/2012 7:25 AM

Sorry, also, but nobody is wrong here.

As I said before, in another way, this kind of playing with entities 0 and ∞ is futile and at best can lead to conclusions that do not match the reality in practice. for example, your line:

0/0=1 => (0+0)/0=1 => 0/0+0/0=1 => 1+1=1 => 2=1 (which is obviously wrong)

can be: 0/0=1 => 0 + 0/0 = 0+ 1 = 1 and by yours, 0/0+0/0= 1+1 =2, two different outcomes! therefore, mathematically becomes undetermined the moment you start speculating and go wild with the grey cells.

(... 0/0+0/0+...+0/0 ===> ∞ ???) or (0/0=0 . 1/0 = 0 )? all becoming useless juggling.

Simple rule (practically in the real world ) zero x any value from -∞ to +∞ will result in Zero. In philosophy it can be anything you like. We were trying to explain a simple electrical phenomena involving current and voltage and real practical pieces of wires and transformers. We are not trying to explain the theory of state transition or limits in pure math.

sorry for all the ....

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#38

### Re: Voltage During Short Circuit Condition

07/20/2012 9:01 AM

In every way, (in real life) we are trying to avoid weird mathematical expressions (such as 0/0 or 0.∞ or ∞/∞). That's why I made the posts [#25] and [#26] (as additions to my initial post [#24]), in order to avoid such things (i.e. in [#25] I avoid the "0/0" and in [#26] I avoid the "0.∞" by using alternative ways for finding the value of P).

However, concerning the "0.∞=0" I still disagree with you (see proof in post [#29]). In strict maths it has no meaning and we can't get any reasonable result (i.e. that's why it's undefined).

In the same way, in post [#35] I stated that n/∞=undefined (for n=any finite number). However, some others (even mathematicians) state that n/∞=0 and -in a first place- it seems that it makes sense. (I think that there is a kind of disagreement between the mathematicians about the result of "n/∞".) However, even the "common sense" is not a good advisor in such tricky concepts.

E.g. a 'mind game' concerning the n/∞=0: If you have -e.g.- 10 apples and you divide them by 5 you'll get 5 groups with each group containing 2 apples. If you divide by 100 you'll get one tenth of an apple in each one of the 100 groups... Then divide by 1000... then by 1000000... and so on... In every case, you can find even a tiny piece of apple inside each group... But what about, dividing by ∞ ??? By saying that n/∞=0 (in our case: 10/∞=0), actually you are saying that if you look inside any one of these (infinite) groups you'll find absolutely nothing. Wherever you look there is... nothing. So, our 10 apples are vanished. How is this possible? At first, "n/∞=0" looked nice by "common sense", but then it looks bad, also, by "common sense"...

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George
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Join Date: Dec 2008
Location: Long Island NY
Posts: 7000
#37

### Re: Voltage During Short Circuit Condition

07/20/2012 8:29 AM

George,

First, you should understand that the blue words in my above reply are hyperlinks that will take you to citation pages. So I have provided reference material that supports my statements.

Second, you are correct that arithmetic alone will not define 0/0, 0*∞, n/∞ and other arithmetic combinations like these. When one reaches these numbers through an applied function then limit theory does define if 0/0=1, 0/0=0, 0/0=∞ or some other possible value. This is a very important process that must be known and applied in control theory and filter design to determine if a system will be stable. This is not an esoteric mathematics exercise with no real application.

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"A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering." Freeman Dyson
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Join Date: Mar 2007
Location: Greece / Athens
Posts: 575
#39

### Re: Voltage During Short Circuit Condition

07/20/2012 9:40 AM

Redfred, thanks for the info about what... blue words are...

(I haven't looked at those links yet... but I will... thanks...)

Concerning, all the rest that you wrote, I totally agree with you...

Anyway, the initial issue has, already, slipped in other 'paths'... But it was fun...

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George
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