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short circuit idea

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devigan

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hi there,
I am a newbi looking for a circuit to detect current in 230 volt Ac neutral wire to cut power - switch off live current simply detect short circuit
help will be appricated Thank you
Dave :roll:
 
Dave, the way you worded your question is not how a circuit works.
Current flows out from the active wire and BACK through the neutral wire. With AC, this current reverses its direction 50 times a second (50Hz).

So, there HAS to be a current flowing in the neutral wire for anything connected to the circuit to work.

What the safety devices detect is a current IMBALANCE. In other words, exactly the same current that flows out has to flow back, if not there is an earth leakage somewhere.

During a short circuit (active to neutral) it is still the SAME current that flows in both wires. However, it is excessive and could melt them or the equipment connected to them.
What you need to do then is measure if the current is excessive and trip a circuit breaker. As Nigel said, the simplest way is a fuse.
Klaus
 
When you say cut power, I think you mean cut power as in a power outage? Or somebody turns off your power.

What do you want to do? Make a device that kicks in when the power goes out? The simplest way to do this would be to use a DPDT relay. When there is power (say a couple volts from a wallwart supply), you could make it so that the relay would be open, and when there is power, the relay would close.
 
Since you dont explain WHY you want to do such a thing, all I can say to you is buy a fuse. A fuse has numbers on it which are maximum ratings. I think the numbers are in amps, but it depends on the manufacturer too.

As soon as current/voltage exceeds the maximum current/voltage listed on the fuse, the fuse breaks and therefore, you have an answer.

I think this may be the safest answer.

BUT, DO NOT stick a fuse in an austrailian (or wherever you are) socket. You will get yourself in trouble for that. Your best bet is to connect a lightbulb the socket, and a fuse all in series (in a loop). so that the lightbulb shines until you unplug the circuit or the fuse blows.
 
How is the maximum current rating of a fuse affected by the voltage applied to it compared to what its specified voltage is?

Say we have a 2A 32V fuse but we subject it to a 10V line, will it still blow if the 10V reaches the fuse-rated current limit of 2A, or will it take a higher current to blow it because of the lower voltage?

What if that 2A 32V fuse were subjected to 64V, would it blow immediately or only blow when a certain level of current were drawn through it, even if it's less than the 2A its rated for?
 
Let me rephrase my last sentence.

If the current injected into the fuse exceeds the maximum current specified on the fuse, the fuse may blow.

If the voltage injected into the fuse exceeds the maximum voltage specified on the fuse, the fuse may blow.
 
A 10A fuse will blow at 10A (or ther about) thats all there is to it with fuses!!!

The voltage rating is purly for isolation purposes, you put a 2A 30V fuse in a 100V environement (ie 100V across it) then it will arch!!! and thus the fuse wil not blow at the designed amps since an alternative path for the current now exists,
 
Styx said:
The voltage rating is purly for isolation purposes,

Correct, but that condition would only happens after the fuse had blown, not before. The fuse is like a short circuit all the time before it had blown. You cannot have large voltage across a good fuse even if it is use in 20KV circuit.

Styx said:
you put a 2A 30V fuse in a 100V environement (ie 100V across it) then it will arch!!!

Probably nothing happens and it will not arc. But if it is blown due to overcurrent, then it can not hold off the voltage across its two ends and that's dangerous.

Styx said:
and thus the fuse wil not blow at the designed amps since an alternative path for the current now exists,

The fuse will definitely blow at its design amps but it can not be rely upon to isolate the load from the source. That's what the max. voltage rating really means.
 
Yes true, soz drunk at the time.

What i ment (now I am sober) is the amp rating of the fuse is the amperage that is will blow at (speed? dependant on its I^2t rating)

The volt-rating is to do with the voltage that a blown fuse will gurantee to support.

When a fuse is in place the voltage across it will be small (since the resistance is very-low). IF you over-current it (or heat it up and then pass some current through it), it will blow and open-ciruit that section and a voltage will develope across the fuses terminals - since htere has to be a forcing voltage to initially supply the current.

IF it was say a 30V fuse in somewhere that when blown can see 100V across it, then it will arc and the current will still be established thus the point of the fuse is negated
 
Yes, that's it. Your explanation is very accurate.

For information only, two other fuse ratings electronic hobbyist often not care about is AC/DC use, which make a big difference in fuse construction. The other is kA rating which said how many amps that the fuse can safely interrupt without itself being destroy into smithereens.
 
eblc1388 said:
For information only, two other fuse ratings electronic hobbyist often not care about is AC/DC use, which make a big difference in fuse construction. The other is kA rating which said how many amps that the fuse can safely interrupt without itself being destroy into smithereens.

But neither of these have much relevence to a hobbyist, they are only really industrial concerns.
 
Yes, Nigel. Thanks for pointing that out.

I meant "need not care about". Problem with english being my second language. 8)
 
eblc1388 said:
Yes, Nigel. Thanks for pointing that out.

I meant "need not care about". Problem with english being my second language. 8)

Your English is quite good - better than some of the younger ones for whom it is their first language! :lol:
 
Their english is as good if not better but they seem to have crappy keyboards.

Most of thE TImes the sHIFt keys are NOT WORkinG, CAP LOCK STUCK oR their fingers are much fastr thn ther kybord can catch up with. 8)
 
interesting topic and lots of replies but nobody noticed that he wants to put fuse on _neutral_ wire...
 
panic mode said:
interesting topic and lots of replies but nobody noticed that he wants to put fuse on _neutral_ wire...

Actually he said he wanted to detect in the neutral, and cut the live - but I don't think he really knows what he's doing, or what he wants?.

He also seems to have disappeared?.
 
eblc1388 said:
Their english is as good if not better but they seem to have crappy keyboards.

Most of thE TImes the sHIFt keys are NOT WORkinG, CAP LOCK STUCK oR their fingers are much fastr thn ther kybord can catch up with. 8)

I resnt that. im only 24 but that dsnt men i rite txtspk styl. i thnk shrtng the langge is gr8 and its esy 2 undstd wot im ritng i thnk, but evry1s a fascst & wont undstd me. They thnk im lzy or cant spel or hva the iq of a mnkey. Yur jst old-fshnd & wnt 2 stp yng pepls prgrss. Wer jst fndng r own styls wich hapn 2 b betr thn yurs.


p.s. Just in case the direction of my post isn't clear, I must point out that I cannot stand so-called 'txt spk' and would happily chop off the hands of anyone who uses it to prevent them assulting my brain with any further monkey-gibberish. There's no excuse for it and it takes people with normal brains 5 times longer to divulge the meaning of their writing style than if it was written normally. Personally, I equate such writing with what I'd expect to get from a monkey made to sit in front of a keyboard.

/RANT (+mode 'I feel better now')
 
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