Isolatied Line Voltages? Do they kill?

  • #1
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Main Question or Discussion Point

I am being confused at a basic grounding question.
I think the only reason, why you get a shock if you touch a live wire is because the neutral is already grounded back in the distribution transformer, so you are effectively touching the live and neutral at the same time, completing the circuit. For the same reason, the circuit breaker breaks off the supply, if the live wire comes in contact with the grounded body of a instrument.

So, why not just avoid grounding in all place. If the neutral isn't grounded in the distribution transformer, then nothing will happen if you touch the live wire. Nothing to worry if the live wire comes in contact with (unearthed) metal casing of instruments, because touching it won't do anything.
Well, I do have a answer myself though:
If the earth is left isolated from the system, then yes above mentioned things happens. But If some crazy student connect a 100000V (w.r.t neutral) wire to the earth, then the whole world comes at 100000V and, if someone tuches the live wire (after reading the above paragraph:) he is gone to heaven.
Is this the only reason (yeah, its a great and enough reason) ? I just want to make sure I am not missing anything.


All of these makes sense to me and are written just for my background info. I got confused, when thinking about UPS. Since the UPS generates Isolated AC, I shouldn't get shocked by touching only one of its output. But Before, touching it, I tested it with a Tester (A screw driver like thing, which you push into socket to check presense of mains, you have to touch its rear metal part to provide grounding necessary for the light to glow), and it glows. I was expecting it to not glow. What do you think?
 

Answers and Replies

  • #2
200
4
I am being confused at a basic grounding question.
I think the only reason, why you get a shock if you touch a live wire is because the neutral is already grounded back in the distribution transformer, so you are effectively touching the live and neutral at the same time, completing the circuit. For the same reason, the circuit breaker breaks off the supply, if the live wire comes in contact with the grounded body of a instrument.

So, why not just avoid grounding in all place. If the neutral isn't grounded in the distribution transformer, then nothing will happen if you touch the live wire. Nothing to worry if the live wire comes in contact with (unearthed) metal casing of instruments, because touching it won't do anything.
Well, I do have a answer myself though:
If the earth is left isolated from the system, then yes above mentioned things happens. But If some crazy student connect a 100000V (w.r.t neutral) wire to the earth, then the whole world comes at 100000V and, if someone tuches the live wire (after reading the above paragraph:) he is gone to heaven.
Is this the only reason (yeah, its a great and enough reason) ? I just want to make sure I am not missing anything.


All of these makes sense to me and are written just for my background info. I got confused, when thinking about UPS. Since the UPS generates Isolated AC, I shouldn't get shocked by touching only one of its output. But Before, touching it, I tested it with a Tester (A screw driver like thing, which you push into socket to check presense of mains, you have to touch its rear metal part to provide grounding necessary for the light to glow), and it glows. I was expecting it to not glow. What do you think?
Your thinking of 1st part is correct... The neutral of the UPS might be connected to earth internally. How bright is the tester glow?... sometimes, the non linear waveform generated by the UPS causes high frequency harmonics which may leak through the tester to make it glow, albeit dimly. Leakage currents and stray capacitances might also contribute to the glow.
 
  • #3
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So, why not just avoid grounding in all place. If the neutral isn't grounded in the distribution transformer, then nothing will happen if you touch the live wire. Nothing to worry if the live wire comes in contact with (unearthed) metal casing of instruments, because touching it won't do anything.
...............................

I just want to make sure I am not missing anything.
Yes indeed you are missing the essential point.
Additionally it is far from true to say that nothing will happen if you touch a live wire or casing that has become live.

Here is a simple example of why sensible regulations require exposed metalwork to be earthed.

Let us suppose you are in the kitchen boiling making a cup of tea.
Let us further suppose that your stainless steel kettle has developed a fault, unbeknown to you, such that the internal insulation has failed and the case has become live.
If you now hold the kettle under the water tap to fill it, simultaneously grabbing the tap handle to turn it on with the other hand, you will have hold of earth in one hand and mains in the other and a large current will flow through your body.
Worse it will take the most damaging possible path, through your heart.

On the other hand if the metalwork had been earthed the fault current would have disconnected the mains supply as soon as the fault developed.
 
  • #4
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Yes indeed you are missing the essential point.
Additionally it is far from true to say that nothing will happen if you touch a live wire or casing that has become live.

Here is a simple example of why sensible regulations require exposed metalwork to be earthed.

Let us suppose you are in the kitchen boiling making a cup of tea.
Let us further suppose that your stainless steel kettle has developed a fault, unbeknown to you, such that the internal insulation has failed and the case has become live.
If you now hold the kettle under the water tap to fill it, simultaneously grabbing the tap handle to turn it on with the other hand, you will have hold of earth in one hand and mains in the other and a large current will flow through your body.Worse it will take the most damaging possible path, through your heart.

On the other hand if the metalwork had been earthed the fault current would have disconnected the mains supply as soon as the fault developed.
Why should current flow to earth (through my body) as the neutral isn't connected to earth at any place. Live connected to earth won't complete the circuit to let the current flow. I think you are missing the assumption I made for the second part --that the neutral isn't grounded (earthed) in the distribution transformer.
 
  • #5
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Your thinking of 1st part is correct... The neutral of the UPS might be connected to earth internally. How bright is the tester glow?... sometimes, the non linear waveform generated by the UPS causes high frequency harmonics which may leak through the tester to make it glow, albeit dimly. Leakage currents and stray capacitances might also contribute to the glow.
No chance for that. I completely unpluged the UPS from the mains and Run it in isolated mode. The glow is quite bright. I really don't know how to quantitatively measure its brightness. So, I used DMM in AC voltage mode. I inserted one probe into the outlet and experimented with the other probe floating in air, or pressed against the floor carpet. ( I felt completely silly trying to measure voltage of the outlet w.r.t. mid air or my carpet floor, both being such well known insulator.) The DMM meausred around 25 Volts. I am really surprised at the result. 25 volts between the outlet and mid-air??? Can I hook a 24 Volts lamp between those points and expect it to glow? Of course not. Then what the heck is that voltage? I am going completely dumb here.

I even did similar test on the mains outlet, this time I only got around 7 volts between the outlet and the mid-air. So, does this means the UPS output is more dangerous the the mains?
 
  • #6
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The idea of connecting the earth to the neutral is to let the use of a control circuit to detect that there is a fault somewhere and not to protect directly from a shock, since as you mentioned above if the ground is connected to the neutral and a contact with the live terminal happens then we will get shocked.
In the presence of a control circuit which detects the current passing through the grounded terminal the control board will know that there is a fault and the circuit breaker will trip.

If the neutral is not connected to the ground and if you touch the live terminal you will get shocked, this is due to the capacitance between the wire and the ground you are on.
 
  • #7
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It is not legal to connect the neutral to earth in many countries and also a bad idea in terms of electrical engineering, since if everybody did this there would be seriously out of balance return current issues for the supplier.

Why should current flow to earth (through my body) as the neutral isn't connected to earth at any place. Live connected to earth won't complete the circuit to let the current flow. I think you are missing the assumption I made for the second part --that the neutral isn't grounded (earthed) in the distribution transformer.
I'm not sure you understand what 'completing a circuit' means.
It is not necessary for a complete loop for current to flow.
Current can (and indeed will) flow in any path where there is a potential difference.

The earth acts as a current sink of effectively infinite capacity. No loop is necessary for currents to flow to earth.
A certain Mr B Franklin demonstrated this 250 years ago.

You provide that path in the case concerned.
(Un)fortunately the resistance of the body is such that mains voltages cannot drive enough current through to activate protective disconnection devices, but the current that does pass can be sufficient to kill.
 
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  • #8
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Let me explain it the way it was explained to me. Let's say you have a boiler which is electrically heated. There is a pressure switch in the boiler to keep the pressure from getting too high and causing the boiler to explode. The pressure switch is tied back to the circuit feeding the heater and will turn off the heater when the pressure reaches a limit. None of the grounds at A, B or C exist. The circuit is floating.

Years go by and eventually the insulation deteriorates and eventually a ground develops at point A. Nothing happens. The fuse doesn't blow and in fact nobody even notices the ground. More years go by and another ground develops at point B. Care to guess what happens next?

However if the return is grounded at C, then when the first ground develops at point A or B, the fuse blows and the electrician investigating why the fuse blew discovers the ground and fixes it before a disaster can occur.
 
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  • #9
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It is not legal to connect the neutral to earth in many countries and also a bad idea in terms of electrical engineering, since if everybody did this there would be seriously out of balance return current issues for the supplier.

I'm not sure you understand what 'completing a circuit' means.
It is not necessary for a complete loop for current to flow.
Current can (and indeed will) flow in any path where there is a potential difference.


The earth acts as a current sink of effectively infinite capacity. No loop is necessary for currents to flow to earth.
A certain Mr B Franklin demonstrated this 250 years ago.

.
Yeah, I am totally off now. Thanks for your concern. Sorry, for my previous post.
So, you mean to say, if I put only one of the terminal of a large battery (500 V?), or only one of the terminal of a generator on earth, then huge current flows? And it flows for long time (infinite)? Do, you mean to say, that the earth acts as a huge capacitor and it continiously draws charging current (never charges up)?
Can you please expand on this please? Its totaly counter to what I used to think.
 
  • #10
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Let me explain it the way it was explained to me. Let's say you have a boiler which is electrically heated. There is a pressure switch in the boiler to keep the pressure from getting too high and causing the boiler to explode. The pressure switch is tied back to the circuit feeding the heater and will turn off the heater when the pressure reaches a limit. None of the grounds at A, B or C exist. The circuit is floating.

Years go by and eventually the insulation deteriorates and eventually a ground develops at point A. Nothing happens. The fuse doesn't blow and in fact nobody even notices the ground. More years go by and another ground develops at point B. Care to guess what happens next?

However if the return is grounded at C, then when the first ground develops at point A or B, the fuse blows and the electrician investigating why the fuse blew discovers the ground and fixes it before a disaster can occur.
I couldn't understand the circuit. What does the black line going from beneath the switch to the Circle (boiler) represent?
If it represents nothing, then I don't see any difference between point A and B, so grounding A, or B or Both is indifferent (when the circuit is On, i.e. switch closed)

Since, you are asking this question so seriously, I guess the Black line has something to do.

I understand why gorunding both A and C blows up the fuse, because it bypasses the heater resistance resulting in a direct short.

Also, if ground develops at A, then it is against what studiot's point to say nothing happens. He says, huge current will flow and the fuse blows up.
 
  • #11
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7
I do wish teachers at elementary science level would stop promoting the idea that there must be a complete loop or return path for electric current to flow.

this is just not true and leads later to many students having difficulties unlearning this misconception.

Here is another student with the same problem.

https://www.physicsforums.com/showthread.php?t=490440.

There are three ways to cause a current to flow

1) Mechanical as in a Van De Graaph generator

2) Thermal agitation as in thermionic emission

3) Under the influence of an electric field or potential

Any of these will convey charge (current) from A to B. If there is no return current then charge will build up at B.

Now consider this experiment.

You have a mains testing screwdriver. You touch the prod to the line of the mains. The bulb in the handle glows.

Why?

Because a small (safe) current flows from the line to earth through your body.
There is no return path.

Now consider this in the light of my statement about charge build up above.

Yes there is therfore a charge buildup at point B (the earth) but it is insignificant compared with the size of the body so cannot measurably change the potential of the earth.

This is what I mean by saying the earth acts as an infinite current sink.

Alternatively you can say that an earth is a circuit element that does not change its potential, no matter how much current flows into (out of) it.
This definition is quite useful as you can incorporate it into circuit theory for calculation purposes.

go well
 
  • #12
28
0
I agree that we get shock only due to current flow, whick means there should be return path (neutral grounded at distibution transformer).

keeping isolated system is a little risky. let us assume in a 3 phase system, neutral is not grounded.

phase to ground voltage = 400
phase to neutral voltage = 230

now a fault occurs at one phase to ground at some persons house, That will not trip any MCB as system is not grounded. now other two phases would have voltage 400 w.r.t. ground. that will surely kill a person. thats not what we want, normally with system grounded at neutral maximum voltage between ground and phase would be 230

Your Experiment: for measuring voltage of UPS i would measure between earth wire (or GI pipe of water supply) and UPS output wires. sometimes output is not solidly grounded but some leakage currents are there (may be MOV connected between UPS output and ground). if you get some low voltage like 24V that should be due to leakage. connecting a lamp (220V 100W lamp should be ideal) would cause voltage to drop to Zero.

dont rely on neon testers they glow for very low currents.

and remember current as low as 8mA can be fatal
 
  • #13
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I couldn't understand the circuit. What does the black line going from beneath the switch to the Circle (boiler) represent?
It's just a shorthand method of showing that the pressure switch controls the power switch.

If it represents nothing, then I don't see any difference between point A and B, so grounding A, or B or Both is indifferent (when the circuit is On, i.e. switch closed)
If the circuit is completely ungrounded, then grounding at A or B, but not both, won't make any difference. However if you should get accidental grounds at both A and B, then when the pressure switch senses the pressure limit and tries to turn off the power to the heater, it can't. The pressure will build up to the point that the boiler explodes.

I understand why gorunding both A and C blows up the fuse, because it bypasses the heater resistance resulting in a direct short.
Correct.

Also, if ground develops at A, then it is against what studiot's point to say nothing happens. He says, huge current will flow and the fuse blows up.
Perhaps Studiot had something else in mind. I'll let him explain.
 
  • #14
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It's just a shorthand method of showing that the pressure switch controls the power switch.


If the circuit is completely ungrounded, then grounding at A or B, but not both, won't make any difference. However if you should get accidental grounds at both A and B, then when the pressure switch senses the pressure limit and tries to turn off the power to the heater, it can't. The pressure will build up to the point that the boiler explodes.
Nice puzzle. :)

What you said are all fine.
We are now just talking over-- Does huge current flows if Live touches the ground even when the neutral isn't grounded (in distribution transformer) ?
Studiot says, yes. I am looking for a more complete thoery (models?) on why it flows, how much current flows and for what amoount of time?

Also to studiot, you say, current flows when live (230 V) touches the earth, because earth is 0 potential, live is 230 V and current flows as long as there is potential difference. But no-matter how much current flows, the earth's potential won't rise above 0, so current flows continiously.

That partly makes sense but the conflict is live is 230V w.r.t neutral. I thought voltage are always w.r.t something, not abosolute . So, we don't know what live is w.r.t earth so, how can you be sure current flows from live to earth?
 
  • #15
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Well, a transformer secondary does not have 'neutral' and 'live'. It generates potential difference between wire 1 and wire 2 , sine wave 50hz (in US/ 60hz). If you don't have anything what so ever connected to transformer, connecting either wire to ground will not result in massive current.
However, in the real situation, the transformer is connected to all sorts of equipment, that will be leaking current onto ground - both through resistance, and through capacitance (remember, it is AC). Then, touching either wire while being connected to ground by your feet is not safe.

The reason why one of the wires is grounded is that it allows to a: get rid of some insulation, b: prevents charge buildup (that could eventually break insulation between primary and secondary), minimizes risk of lightning strike damage, c: does not decrease safety as for safety evaluation it has to be assumed that eventually one of the wires will leak onto ground massively due to fault in some equipment, which will go unnoticed. Or worse still, some high voltage equipment - e.g. tvs, or photocopiers, leak high voltage into the circuit. You cannot assume that different equipment would balance out. All the photocopiers of same brand, and probably indeed of almost all brands will leak with same polarity.

Furthermore, imagine that the transformer develops fault between primary's live (several KV, one side grounded) and secondary. Then, if neither side of secondary is grounded, you will have several kilovolts on the secondary, which will break the insulation and possibly kill someone. If one side of the secondary is grounded, then the current would flow through secondary windings to the ground, blowing the fuse on primary.

To summarize: grounding one side of the secondary on distribution transformer ensures that the potential on either wire stays within the range (±340v max in 240v places) from the ground. Then the insulation is designed to withstand this voltage, with enough safety factor.
Non-grounding the secondary would not let you assume lack of current flow in the ground fault (as the ground fault may be already present). and would not even give you upper bound on the potential from either wire to ground. You may end up with live 1 at 6000v and live 2 at 6000v + 240v rms sin wave , for example, which would break down the insulation in most of the equipment.
 
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  • #16
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There are significant differences between modern US and UK/European practice.

Mains supply in the US is provided by at the consumer via a split phase secondary at 120 - 0 -120 volts.
In general there is a supply transformer at or near the consumer's premises
The centre zero provides the neutral. It is also the consumer's duty to earth this point.

In the UK mains supply is 230 volts single phase with the supplier providing separate line, neutral and earth connections to the consumer.
In general the supply is derived from the phase to neutral voltage of a three phase transformer at a local substation.
It would be illegal for the consumer to connect any of these to his own earth.

Unfortunately the colour coding of the conductor covering is also very different.

I have never said that a large current will definitely flow.

I said the amount of current depends upon the path and the driving voltage.

We often arrange these parameters to ensure a large current flowing in fault conditions to ensure rapid operation of the safety disconnection device.
This is why the UK wiring regulations require an earth bonding wire of sufficient cross section to accomodate the expected fault current, rather than one of lower cross section that would just provide equipotential bonding.
 
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  • #17
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2
I do wish teachers at elementary science level would stop promoting the idea that there must be a complete loop or return path for electric current to flow.

this is just not true and leads later to many students having difficulties unlearning this misconception.

Here is another student with the same problem.

https://www.physicsforums.com/showthread.php?t=490440.

There are three ways to cause a current to flow

1) Mechanical as in a Van De Graaph generator

2) Thermal agitation as in thermionic emission

3) Under the influence of an electric field or potential

Any of these will convey charge (current) from A to B. If there is no return current then charge will build up at B.

Now consider this experiment.

You have a mains testing screwdriver. You touch the prod to the line of the mains. The bulb in the handle glows.

Why?

Because a small (safe) current flows from the line to earth through your body.
There is no return path.

Now consider this in the light of my statement about charge build up above.

Yes there is therfore a charge buildup at point B (the earth) but it is insignificant compared with the size of the body so cannot measurably change the potential of the earth.

This is what I mean by saying the earth acts as an infinite current sink.

Alternatively you can say that an earth is a circuit element that does not change its potential, no matter how much current flows into (out of) it.
This definition is quite useful as you can incorporate it into circuit theory for calculation purposes.

go well

I was wondering that myself. Why does the small current flow through you? If you touch main by hand nothing happens, assuming you didn't touch the neutral as well.

And (i just started learning 3 phase and 1 phase systems) isn't neutral connected to the generator?
 
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  • #18
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7
If you touch main by hand nothing happens, assuming you didn't touch the neutral as well.
If you touch a conductor at (significantly) higher voltage than your body what happens depends upon just how well insulated you are from the rest of the universe.

If you are perfectly insulated, nothing happens.

If you are connected via a high resistance path (as with the testing screwdriver) a small current flows.

If you are connected via a lower resistance path a larger current flows.
With my kettle and sink example the path's resistance may be low enough to pass enough current to kill.

There is an old saying

Its the volts that jolts
But the mils that kills.
 
  • #19
677
16
I have never said that a large current will definitely flow.
I said the amount of current depends upon the path and the driving voltage.
Hi,
Can you tell me (order of magnitude) the current that flows if the isolated mains 230V (generated by isolated UPS) if it touches my carpet floor directly or through my body?
And you din't reply my question regarding the concept of 'absolute voltage' you have been using.
 
  • #20
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I meant main main, like in wire that you have at home. Does my body represents low or high resistance path?
 
  • #21
510
1
Hi,
Can you tell me (order of magnitude) the current that flows if the isolated mains 230V (generated by isolated UPS) if it touches my carpet floor directly or through my body?
And you din't reply my question regarding the concept of 'absolute voltage' you have been using.
well if you touch one line of UPS, and the other line is isolated, the current would be very tiny. It'd be on the order of 2*pi*f*c*v where f is frequency (50Hz), c is capacitance between your body and the other line (may be between your body and earth, and between earth and the wire), and v is the voltage. So, I'd guess, some tens to hundreds nanoampers.
If you are standing barefeet on ground, it'd be larger (2x perhaps, assuming the other lead is as big as you are, and capacitance between you and other lead is negligible) as the capacitance between other lead and ground would be larger than between other lead and you. The real world UPS might have high frequency harmonics, so the current would be larger.
 
  • #22
948
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I would like to go to bottom of this. Accumulating posts to try to answer my question is pointless when I need some basic knowledge. Studiot do you have any book recommendations where this issue or problem of neutral and ground is worked in detail?

I would really want to know this as future electrical engineer.

Thanks
 
  • #23
5,439
7
Studiot do you have any book recommendations where this issue or problem of neutral and ground is worked in detail?
I would suggest the best source for you would be practical guides for practical technicians.

In the UK this might be

The Electricians Guide to the (16th or 17th) Edition of the Wiring Regulations and Part P of the building regulations.

By John Whitfield

Since this book was written there have been several other guides, but all have the same compact format and are written to explain the thinking behind the regulations as well as the practical implementation of the regs themselves.

Additionally many trade associations and companies publish 'yearbooks'. You may be able to get an older one from an electrician. These again are a mine of useful practical information.

Finally IMHO, every electrical engineer should have a copy of Hughes on his shelves.
Even if he is at PhD level the book provides so much useful information, diagrams, techniques and formulae all aimed at people actually doing the job day in day out the book is worth its weight in gold.
This book has seen many editions, almost any will do the job.
Unfortunately there is no real safety section in this book however.

Electrical Technology
by
E Hughes
 
  • #24
sophiecentaur
Science Advisor
Gold Member
24,801
4,616
I can't help thinking that the US system is a bit of a dog's dinner. If they had started off with a sensible supply voltage, instead off about 100V then they wouldn't have been embarrassed by a heavy current demand by washing machines etc., which requires a special 'double voltage' supply for some equipment. The European system makes a lot more sense as it doesn't involve multiple voltages in a normal domestic premises.
@studiot
I know many many 'PhD level' engineers who I wouldn't trust to wire a plug correctly! Everyone needs a guide to correct practice.
 
  • #25
AlephZero
Science Advisor
Homework Helper
6,994
291
The earth is a conductor, therefore it acts as a capacitor just like any other conducting sphere.

The capacitance is a lot smaller than you might think, of the order of 700 microfarads. You can go to any electronics store and buy a capacitor an inch long which has a much higher capacitance than the whole earth, but at mains frequency of 50 or 60 Hz, the impedance of the earth is about 4 or 5 ohms.

So if you connect even 100V AC mains to "earth" you wll get a current of about 20 or 25 amps, which is 1000 times bigger than it takes to kill a human.

That is the basic explanation of what happens when mains wiring is "earthed", compared with what happens if you connect one terminal of a battery (DC) to earth and leave the other one unconnected, which produces no obvious effect at all (unless you make careful measurements of what happens at the instant you make the connection).

Note: People who know EM theory will realise there are some wild assumptions being made in those calculations, but at least they show the order of magnitude of the effects.
 
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