V=0, I=0: Open or Short Circuit?

[swayam007]

When Voltage across two points is 0 & current flowing through that connection is 0. Is it a short circuit or open circuit?

 

[Studiot]

Short circuit means there is zero impedance (resistance) between the two points.
This may lead to zero voltage and no current or a lot of current.

Open circuit means that there is infinite impedance between the two points.
Current flow is always zero but the voltage is indeterminate and may be any value including zero

The condition v=0, I=0 can be either so it is impossible to determine which without knowledge of the connecting impedance.

 

[mistermotown]

'When Voltage across two points is 0 & current flowing through that connection is 0'
That means I have an amp meter, and a volt meter. I measure the voltage between two points, and the voltage is zero. I then take my amp meter and measure the current that flows through both those nodes, and that current is zero.

That means your circuit is off buddy, you could just as well have a wire lying on the table, not connected to anything and measure that. You could measure V=0 and I=0 off a glass cup.
:P

If you are talking about circuit theory:
then V=0 is a short circuit AND there can be current flowing through it
I=0 is an open circuit AND there can be a voltage across the two non-connected wires.
But those ONLY count when your circuit is ON in some way.

What are you trying to do? Are you trying to calculate resistance of a circuit, or using super position?

 

[sophiecentaur]

If R=V/I and both V and I are zero then you get
R = 0/0
which is indeterminate. You need to specify either V or I.

 

[Ratch]

swayam007,

--------------------------------------------------------------------------------
When Voltage across two points is 0 & current flowing through that connection is 0. Is it a short circuit or open circuit?

A short or open is determined by physical contact, not by what voltage is across, current through, or resistance between the contacts. So if the contacts physically touch each other, then you have a short circuit across the contacts.

Ratch

 

[sophiecentaur]

swayam007,



A short or open is determined by physical contact, not by what voltage is across, current through, or resistance between the contacts. So if the contacts physically touch each other, then you have a short circuit across the contacts.

Ratch

True but, if all you know is that measurements gave you V=0 and I=0, you cannot determine anything about the resistance - infinite / zero / 347Ω.

 

[Ratch]

sophiecentaur,

True but, if all you know is that measurements gave you V=0 and I=0, you cannot determine anything about the resistance - infinite / zero / 347Ω.

If you are able to insert an ammeter between the contacts, then you should be able to determine whether they are touching or not, and thereby know whether they are open or closed. The quality of the contacts, such as whether they are dirty or corroded does not mean they are open or shorted. Only a physical inspection can determine that for certain. The resistance of the contacts are a a good indication, but not a certainity of their status.

Ratch

 

[sophiecentaur]

sophiecentaur,



If you are able to insert an ammeter between the contacts, then you should be able to determine whether they are touching or not, and thereby know whether they are open or closed. The quality of the contacts, such as whether they are dirty or corroded does not mean they are open or shorted. Only a physical inspection can determine that for certain. The resistance of the contacts are a a good indication, but not a certainity of their status.

Ratch

According to the title of the thread, your Ammeter reads zero and your voltmeter reads zero. If the only info you have is those readings, then you can tell nothing. You are going for a very practical way of finding out the condition of one particular pair of contacts. Fair enough but unless you have some way of finding current and volts, you can't be sure.

 

[Ratch]

sophiecentaur

...but unless you have some way of finding current and volts, you can't be sure.

Well, assuming a perfect ammeter and voltmeter, the ammeter is going to short out the switch so the voltmeter will always read zero whether the switch is open or closed. So, knowing the current and voltage is no sure way to determine the switch status. For me, physical inspection is the definitive determination.

Ratch

 

[sophiecentaur]

sophiecentaur



Well, assuming a perfect ammeter and voltmeter, the ammeter is going to short out the switch so the voltmeter will always read zero whether the switch is open or closed. So, knowing the current and voltage is no sure way to determine the switch status. For me, physical inspection is the definitive determination.

Ratch

But who, in their right mind, would ever connect an Ammeter in PARALLEL?

 

[Ratch]

sophiecentaur,

But who, in their right mind, would ever connect an Ammeter in PARALLEL??QUOTE]

I sure wouldn't, but from the OP's description, I envisioned an ammeter in series with the contacts, and a voltmeter across the contacts, both at the same time. That makes the ammeter in parallel with the voltmeter.

Ratch

 

[sophiecentaur]

sophiecentaur,

But who, in their right mind, would ever connect an Ammeter in PARALLEL??QUOTE]

I sure wouldn't, but from the OP's discription, I envisioned an ammeter in series with the contacts, and a voltmeter across the contacts, both at the same time. That makes the ammeter in parallel with the voltmeter.

Ratch

Are you suggesting that the 'original' circuit consisted of just a loop of wire, joining each of the switch contacts together? What sort of a circuit would that be and what possible use could it be?
I was naturally assuming that the contacts were part of an existing circuit and that they happened to have zero volts across them with no current flowing through them (quite a possibility, in fact it could be part of a balanced bridge circuit, for instance).

 

[Ratch]

sophiecentaur,

Are you suggesting that the 'original' circuit consisted of just a loop of wire, joining each of the switch contacts together? What sort of a circuit would that be and what possible use could it be?
I was naturally assuming that the contacts were part of an existing circuit and that they happened to have zero volts across them with no current flowing through them (quite a possibility, in fact it could be part of a balanced bridge circuit, for instance).

Nope, I make no assumptions about whether the switch is connected to a circuit or stands alone. All I am saying is that you cannot definitively determine whether the switch is open or closed by just examining the voltage across it or the current through it. A physical examination of the contacts is required to definitively determine whether the switch is open or closed.

Ratch

 

[swayam007]

actually It is about a Galvanometer in a balanced wheatstone bridge . Current passing through it & Voltage across it is zero. Is it open circuit or short circuit?

 

[sophiecentaur]

sophiecentaur,



Nope, I make no assumptions about whether the switch is connected to a circuit or stands alone. All I am saying is that you cannot definitively determine whether the switch is open or closed by just examining the voltage across it or the current through it. A physical examination of the contacts is required to definitively determine whether the switch is open or closed.

Ratch

So all those 'black box' exercises we did at School and all the measurements done, daily, on components are a waste of time? We have to see what it actually looks like before we can tell the impedance of a circuit element.

Interestingly, it turns out that the scenario was, as I suspected, the situation in a bridge circuit.

 

[Studiot]

It is neither open circuit nor short circuit.

I have already outlined the conditions for both and your circuit meets neither.

Note the conditions do not depend on voltage or current.

 

[Ratch]

sophiecentaur,

So all those 'black box' exercises we did at School and all the measurements done, daily, on components are a waste of time?

The exercises were theoretical problems. The measurements were determining highly probable conditions. The observation of the contacts are the definitive status.

We have to see what it actually looks like before we can tell the impedance of a circuit element.

You can determine the impedance from the theoretical position of the switch, or what you think it is.

Ratch

 

[sophiecentaur]

I'm not sure of your point here. There are many examples of systems into which we cannot look. Opening them up could even destroy them or alter their state. A philosophy of measurement surely can't be based on what we infer from the outward appearance of a component.
The point was made in a very early post that 0/0 is indeterminate and that's really an end to it. You need to have at least a finite value for one or the other in order to infer anything about the resistance of a component. 'Visual' observation is just not reliable.

 

[Ratch]

sophiecentaur,

I'm not sure of your point here.

Just what I said before. You cannot be sure of the status of a switch just by knowing the current existing through it or the voltage across it. I hope the last sentence was clear and concise.

There are many examples of systems into which we cannot look. Opening them up could even destroy them or alter their state.

True that is, but lack of accessibility does not change the uncertainty.

A philosophy of measurement surely can't be based on what we infer from the outward appearance of a component.

The philosophy would have to include a caveat stating that the status of the switch cannot be determined with complete confidence from the current/voltage measurement previously mentioned.

The point was made in a very early post that 0/0 is indeterminate and that's really an end to it.

And my point was that any value of current/voltage other than 0/0 still has a degree of uncertainty.

You need to have at least a finite value for one or the other in order to infer anything about the resistance of a component.

An inference is not the same as knowing for sure.

'Visual' observation is just not reliable.

Visual inspection of the contacts is 100% reliable. It is not always available or practical.

Ratch

 

[sophiecentaur]

Surely it is obvious that you need to know both current and PD in order to measure the resistance. Resistance is actually defined in terms of the two variables taken together.
You can know nothing, "for sure". One can only infer, from appropriate measurements.
Your visual inspection issue is a total red herring. It is not part of the Set of 'electrical measurements'. Visual inspection of contacts is not sufficient at all. They could be made of a shiny insulator, for all you could know. (And, in any case, the OP did not introduce the state of a switch into the argument). You would need extra chemical analysis, at least. On the other hand, an electrical measurement of V and I would tell you how your test subject will behave 'electrically', which is what we want to know (because we are seeking to find the resistance).
You are welcome to have the final word on this as I'm not sure whether you are 1. Disagreeing with any or all of the above or 2. Agreeing ditto.

 

[Ratch]

sophiecentaur,

Surely it is obvious that you need to know both current and PD in order to measure the resistance. Resistance is actually defined in terms of the two variables taken together

For resistance yes, but a switch that is supposed to be closed can be defective and have a high resistance. Similarly, an open switch can be shorted. So resistance by itself is not a positive determining factor as to the status of the state of the switch.

Your visual inspection issue is a total red herring. It is not part of the Set of 'electrical measurements'.

Correct, but electrical measurements are not the defining factor in determining the state of the switch.

Visual inspection of contacts is not sufficient at all. They could be made of a shiny insulator,

Physical positioning of the contacts is the absolute determining factor of the switch state. A switch is not manufactured with insulators for contacts.

(And, in any case, the OP did not introduce the state of a switch into the argument).

Actually he did. In post #1, he asks about whether there is a conduction path or not between two points.

You would need extra chemical analysis, at least.

Why?

On the other hand, an electrical measurement of V and I would tell you how your test subject will behave 'electrically', which is what we want to know (because we are seeking to find the resistance).

That would involve isolating the switch from the circuit, applying its rated current and knowing its specified voltage drop. That is what the factory quality control does, not me. You can measure the resistance with an ordinary ohmmeter and determine the state of the switch with a high degree of confidence. But to truly know, you have to look at the contacts.

Ratch

 

[sophiecentaur]

When Voltage across two points is 0 & current flowing through that connection is 0. Is it a short circuit or open circuit?

@ratch
Does that (original) post in any way imply a 'mechanical switch'? Why are you obsessed with your special case of a metallic switch with 'visible' contacts? What has that got to do with the OP?

When will you ever find even the mechanical switch in your argument where there is a pure short circuit? Resistance may be relevant for any switch you care to choose so what has the position of a lever got to do with the question.
We have agreed that you need to know V and I to be certain of the electrical characteristic so I still don't know what extra point you are trying to make that has any bearing on the OP.
We can either discuss the Maths of 0/0 or discuss the business of electrical measurement without any power supply but they both end up with the same conclusion. That conclusion is not allowed to contain the concept of visual inspection because that is just one special case.

 

[Ratch]

sophiecentaur,

Does that (original) post in any way imply a 'mechanical switch'?

Yes, it does. When he talks about open or short across a connection, that implies a switch.

Why are you obsessed with your special case of a metallic switch with 'visible' contacts?

Because that is the way physical switchs are made.

What has that got to do with the OP?

He asked the question.

When will you ever find even the mechanical switch in your argument where there is a pure short circuit?

Defective mechanical switches sometime short out.

Resistance may be relevant for any switch you care to choose so what has the position of a lever got to do with the question.

Certainly, but it is not necessarily definitive as to whether a defective switch is open or closed.

We have agreed that you need to know V and I to be certain of the electrical characteristic so I still don't know what extra point you are trying to make that has any bearing on the OP.

I never agreed that the electrical characteristics determine definitively whether the switch is open or closed. The OP asked about whether the electrical measurements could be used to determine whether a two contacts were open or shorted. That question ties the OP to the statement I make.

We can either discuss the Maths of 0/0 or discuss the business of electrical measurement without any power supply but they both end up with the same conclusion. That conclusion is not allowed to contain the concept of visual inspection because that is just one special case.

Without visual inspection of the contacts, it cannot be determined with certainty what the status of the state of the switch is.

Ratch

 

[sophiecentaur]

So a piece of wire is a switch? (That's a short circuit)
The OP is about two points in a circuit. You are merely implying that he means a switch (and, if you read later, he is definitely not talking about a switch.)
Switches don't always do what you expect from their appearance but the will always have some value of resistance. The proof of the pudding is in the eating. If I find that a sealed switch happens to have a low resistance then for my purposes, the state of the switch is closed and the circuit that it is put into is likely to agree with me.
Why are you so much in love with switches? They are only extremes of a variable resistance. You can't determine anything about either without applying some I or some V.
It helps to stick to the point when possible rather than to chase wild geese.

 

[Ratch]

sophiecentaur,

So a piece of wire is a switch? (That's a short circuit)

No, how did you deduce that from what I said?

The OP is about two points in a circuit. You are merely implying that he means a switch (and, if you read later, he is definitely not talking about a switch.)

The OP has more or less described a switch.

Switches don't always do what you expect from their appearance but the will always have some value of resistance. The proof of the pudding is in the eating. If I find that a sealed switch happens to have a low resistance then for my purposes, the state of the switch is closed and the circuit that it is put into is likely to agree with me.

Likely yes, but not for certainty.

Why are you so much in love with switches? They are only extremes of a variable resistance. You can't determine anything about either without applying some I or some V.

Besides being an important electrical component, it is what we are talking about here. You can determine their state by visual observation of the contacts.

It helps to stick to the point when possible rather than to chase wild geese

Yes, that is what I am doing. If it is not interesting to you, that does not mean if is off point.

Ratch

 

[sophiecentaur]

The OP said nothing about switches and, later, specifies what is happening in a balanced bridge circuit. Switches just elbowed their way into the thread.
A piece of wire is electrically the same as a closed switch - does that make it a switch? Two points in a circuit have zero resistance between them. Does that imply there must be a closed switch joining them? That was my point.

There are millions of switches (electronic / relays / marine grade etc. etc.) where you cannot observe their contacts. Observability is not a universal attribute of switches. BUT their resistance definitely is. The state of a switch (measured electrically) IS its state because that is how it is behaving in a circuit. Seeing is NOT necessarily believing. If you have ever done serious fault finding, you will know that.

After a while on PF , you will find that people who post questions on the forum really appreciate answers and discussion points that converge on what they want to know. Some of the questions are posted by absolute beginners who cannot distinguish between wheat and chaff. Continually dragging this thread in the direction of switches when the poor OP wanted to know about bridge circuits is not fair to him.
"We" are talking about switches only because you won't let them drop and address the main subject of the thread, which is about two unspecified points in a circuit.

Why not start a separate thread entitled something like "Is a switch really 'on' when I measure zero ohms across its contacts"? Then see what people have to say.

 

[Ratch]

sophiecentaur,

The OP said nothing about switches and, later, specifies what is happening in a balanced bridge circuit. Switches just elbowed their way into the thread.

You mentioned that before, and I answered. Instead of repeating it, why don't you address my answer?

A piece of wire is electrically the same as a closed switch - does that make it a switch? Two points in a circuit have zero resistance between them. Does that imply there must be a closed switch joining them? That was my point.

You only went halfway with with your assertions. The scenario described by the OP also took into consideration that the pathway could be open.

There are millions of switches (electronic / relays / marine grade etc. etc.) where you cannot observe their contacts. Observability is not a universal attribute of switches. BUT their resistance definitely is. The state of a switch (measured electrically) IS its state because that is how it is behaving in a circuit. Seeing is NOT necessarily believing. If you have ever done serious fault finding, you will know that.

Anytime you cannot observe a switch's contacts, you can never be absolutely sure what its state is. I expounded on this before. Please do not repeat this again until you discuss my previous answer. Seeing the switch's contacts is believing in this case.

After a while on PF , you will find that people who post questions on the forum really appreciate answers and discussion points that converge on what they want to know. Some of the questions are posted by absolute beginners who cannot distinguish between wheat and chaff. Continually dragging this thread in the direction of switches when the poor OP wanted to know about bridge circuits is not fair to him.

And it could also be said that they appreciate new knowledge or a different perspective. Absolute beginners have to ask more questions or be more clear. The OP did not initially ask about a bridge circuit, and only mentioned it long after the thread was under way and we were discussing switches. If the OP mentioned it earlier, I would have said that the bridged was balanced, and it both contacts were at the same voltage. Therefore, open or shorted was irrelevant.

"We" are talking about switches only because you won't let them drop and address the main subject of the thread, which is about two unspecified points in a circuit.

At the beginning of the thread, switches were a valid topic from the information given.

Why not start a separate thread entitled something like "Is a switch really 'on' when I measure zero ohms across its contacts"? Then see what people have to say.

I believe I have made my point already. Anyone can chime in if they please.

Ratch

 

[sophiecentaur]

Oh boy, this won't lie down will it? Does every open circuit, you come across, consist of an open switch and does every branch, with zero resistance, consist of a closed switch?
The OP asked a general question because it applies in many cases and he was, presumably, trying to get a general answer. Switches are definitely not the most general of components that one could consider. They are not even the only components with binary states.

Let me just ask you this. If a switch is faulty, is its 'state' what is marked on the front or is it the resistance that it presents to a circuit? Which 'state' will the circuit respond to, the actual one or the nominal one? Which state actually counts?

I suggest that you should read a few more threads, posted by beginners and you may get to realize that people don't always know what actual question they should be asking. A sensitive answer will try to find this out.

 

[Ratch]

sophiecentaur,

Does every open circuit, you come across, consist of an open switch and does every branch, with zero resistance, consist of a closed switch?

No, but if the circuit could be either way, that means "switch" to me.

The OP asked a general question because it applies in many cases and he was, presumably, trying to get a general answer. Switches are definitely not the most general of components that one could consider. They are not even the only components with binary states.

I don't know how "general" his question was, but I gave him the best answer I could.

If a switch is faulty, is its 'state' what is marked on the front or is it the resistance that it presents to a circuit?

Its state is whatever the contacts say it is. If the contacts are closed, but the switch does not conduct current, then the switch is on, nevertheless.

Which 'state' will the circuit respond to, the actual one or the nominal one?

In the above example, where the switch is on, but no current can pass, the circuit will respond to an off condition.

Which state actually counts?

I depends on what "counts" means. Does it mean circuit response or state indication? In either case the switch is faulty and should be replaced.

I suggest that you should read a few more threads, posted by beginners and you may get to realize that people don't always know what actual question they should be asking. A sensitive answer will try to find this out.

I don't consider that suggestion beneficial. I always give what I consider to be a literally true answer, usually with an explanation. If they don't understand or disagree, they can engage me further.

Ratch

 

[sophiecentaur]

If they don't understand or disagree, they can engage me further.

Ratch

Fair enough.