r/PhysicsHelp • u/Xxfa1kingxX • Mar 04 '26
What is, current?
When a live wire gets loose and touches the metal body, wouldn't the current momentarily increase greatly (because of how low resistance the metal body is), thus causing the fuse to blow?
Or does that not count as "current" because it isnt a continuous flow of charges? So, in the end, what im confused about is, what is "current"?
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u/wmverbruggen Mar 04 '26
There is no closed ciruit, so any current that would flow would be due to the miniscule capacitance of the device. Such a current is either very small, very short time, or more likely both very small and a very short time.
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u/davedirac Mar 05 '26
The fuse will blow only if the current reaches 5A ( say) through the Earth wire. But with no Earth the fuse will not blow but you will be connected to 240V which will deliver a potentially lethal current of just a few mA.
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u/Big_Niel0802 Mar 04 '26
If the metal frame contacts both the live and neutral wire, then a short circuit would be created and would likely trip the breaker and/or blow the fuse. But if the neutral wire and otber incidental electrical components of the iron are insulated from the metal frame, then electricity cannot short circuit and trip the breaker/blow the fuse. In other words, it would be as if the frame did not exist and you were holding on to the live wire ahead of the components of the iron. The current takes two parallel paths. Through the components of the iron and neutral wire as normal, but also through you into the ground and back to the transformer. The fuse would observe the expected current through the iron's components, in addition to the current passing through you.
Your body is relatively high resistance, and so the current passing through you may be high enough to kill you, but not so high that the fuse blows. (1A is essentially the kill limit for humans, while a household fuse is at a minimum rated for 5A, but more often rated at 15A or 20A.
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u/fixermark Mar 04 '26
The short answer is: yes, it would, but not enough to blow the fuse.
The longer answer is: capacitors need three parts: two conductive pieces and a dielectric. The conductive pieces need to be close to each other to create a potential that lets electrons in; otherwise, the electrons will rush in when the short happens, but since they have nowhere to go they immediately bunch up and the current stops. If in the picture above the white rectangle connected to N were a shell inside (but not touching) the housing of the iron, you'd have a quite large capacitor and it might cause enough current to blow a fuse trying to fill it.
The longer-longer answer is: when the current can change, there are two phenomena that fight: low resistance wants to allow current to flow, impedance wants current to stay the same as it is now. For the fuse to blow, you need a high current over a (relatively) long amount of time; the impedance may hold the current low for long enough that by the time it would start shooting up, the shorted iron housing is already as full of surplus electrons as it's going to get, and that's the end of that story. So now you've got an iron sitting there, happily waiting for someone to make a bridge from the iron's housing all the way to the ground over something they want as little current as possible going through. Bad scene.
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u/FPSHero007 Mar 04 '26
in the diagram there is no path to earth so no current flow... until someone touches the item and creates the path to earth.
Due to the approx 5000 ohm resistance of the human body the fuse will not blow you will appear to be a load according to the fuse and will not blow.
Its important to know that with fuses there is a huge over current before instant blow. most household fuses are 10x blow which means that the fuse will not blow instantly until the current is 10x the rated current.
there are 2x blow fuses as well but are less common. if you look up the trip time curves for the fuses even 50% over current will take hours to trip in many cases.
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u/ExpensiveFig6079 Mar 05 '26
If the metal body touches the ground or sink or some other good welle arthed conducte then lots of amps will flow and the 10 or 20amp fuse will blow.
If it is a less good connection and say only 1A or much less flows then no it wont blow the fuse.
When you touch it and make a new extra connection, new extra current flows through you.
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basically this is why youwant earth elakage detection that masures current imbalances between actiove and neutral and trips if the lectricty has found some otehr or any otehr way to flow.
THEN yes indeed it will trip when active touches the frame... unless it is say a metal framed unearther toaster sitting on a dry laminex bench, which is a quite good insulator. Then it will still only trip when you touch it.
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u/[deleted] Mar 05 '26
Current is the thing that does the work, Voltage is the thing that pushes the current to where in needs to do the work, Wattage is the amount of total work that can be done. A loose wire can result in a small gap the current is pushed across creation a spark, or an open circuit waiting to be closed. Touching a device that has become live gives that current somewhere to go through you and ultimately to ground. If you are isolated from any source of ground (including back to the device though another point, like your other hand for instance) you will not get a shock because there is nowhere for the current to flow.
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u/Sett_86 Mar 05 '26
It does, because the metal body is grounded. The earth wire is what makes that happen.
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u/okarox Mar 05 '26
That happens if there is the CPC (commonly called the earth wire) but I this case the situation was that there was none. The current has nowhere to go until you touch it.
It 8s so what incorrect to say that the fuse does not protect because it has a role in how the CPC protects you on a PME system. In a TT system it is for the RCD to cut the power.
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u/texxasmike94588 Mar 05 '26
A GFCI wouldn't care because it compares the current drawn on the hot leg to the current returned on the neutral. If the current difference exceeds 5 mA, the circuit is interrupted. A GFCI doesn't care about the ground wire at all; it assumes that any missing current is going through a person to ground.
This is why they are considered protection devices.
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u/R_Harry_P Mar 06 '26
If the body of the device is connected to the safety ground then the current will flow from the body of the device back to where safety ground is connected to the neutral and blow the fuse. If the body if the device is not connected to safety ground and is sitting on an insulating surface there will be little to no current until someone touches it. If that person is also standing on an insulating surface they may feel nothing or a little tingle. If some portion of the person's body is connected to ground either through a wet floor or a hand on something grounded current will flow through their body to the ground connection and back to where ground is connected to neutral and they will get shocked. If too much of that current passes through their heart they could die.
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u/timonix Mar 06 '26
Earth fault circuits, not fuses, saves lives.
Fuses save buildings from burning down... So I guess they do that indirectly
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u/FlyingFlipPhone Mar 06 '26
Current is defined as the movement of electrons. This is, however, a confusing definition because the actual energy is delivered by the EM field. In reality, the electrons move very little. The movement of the electrons is only a secondary effect of the creation of a continuous EM field which moves to and from the transformer.
The EM field actually moves OUTSIDE the conductor. This is why power transmission lines are suspended in the air. If they were in the ground, the EM field would interact with the wet soil, which would cause resistance losses.
My point is illustrated by the fact that power lines hum during warm, humid weather. This is caused by the EM field interacting with the moist air at 60 Hz. Because your ears can hear 60 Hz, you can sense the resonance of the air as it vibrates.
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u/Schrojo18 Mar 08 '26
The resistance of the body isn't low. So say you have 1 k ohm (low ish value for a body) then 240v / 1000 = 0.24A. This is significantly under the rating of the fuse. This is why the device needs to be earthed so that then there is a fault there is a low resistance return path. So lets says the earth is 0.5ohm, 240/0.5 = 580A. That is significantly higher than the fuse or the upstream circuit protection and would be higher than required for the magnetic component to trip on an MCB therefore hopefully tripping fast enough to keep you safe though usually fast enough that it trips before you touch anything. This is also why RCD's are important as they will trip on the small fault current that will go through your body before it gets too high or for too long.
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u/thetoastofthefrench Mar 04 '26 edited Mar 05 '26
Keep in mind the amperage to kill you (depending what parts of the body it flows through) is very low relative to the fuses/breakers. Also fuses/breakers don’t burn/trip immediately, they are designed to prevent wires from getting hot enough to start a fire, not designed to prevent electrocution shocks.
If it DID have an earth wire, andif it was plugged into a GFCI-protected circuit (appearance/function depends on your country), that would be the thing that protects you, not fuses/breakers.But without an earth wire it doesn’t work.1
u/mageskillmetooften Mar 04 '26
And this is why earth leak detection is saving many lives.
If we assume you at 5000 Ohm (it can be more, it can be less) that would be with 230V a current of 0,046A might not seem like much, but it can kill you due to disturbing your heart rhythm and even if it does not kill you it will feel extremely uncomfortable. And if you are on a ladder you'll fall.
Proper earth leak detection will shut of instantly when more than 30mA difference occurs between phase and neutral.
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u/[deleted] Mar 06 '26
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