Circuit completion : is it necessary?

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In summary: In your example, one of the terminals of the battery (most likely the negative) was (edit: probably) fastened to the chassis framework of the car to use it as an intentional return path back to the source (the battery). It found it's way back to the battery through the metal parts of the car... It didn't drain itself into the ground through the tires, I assure you.
  • #141
sophiecentaur said:
Even a half decent transformer in a low power equipment power supply will have an electrostatic screen.
Why does that elecrostatic scrreen exist? Because electric fields (and other reasons) are never eliminated. The screen only reduces that current. Primary currents leak by various means into the secondary. An electrostatic shield only reduced currents from the 33,000 volts side appearing on the secondary. Does not eliminate it.

Second, all wires can have two different voltages. Demonstrated were different currents creating 240 volts and 33,000 volts on the same secondary wire. To better understand this, learn two basic electrical concepts - longitudinal and transverse currents.

So that 33,000 volts does not appear on the 240/120 secondary, the secondary must be earthed somewhere.

Third, another example. Suppose lightning strikes the primary (33,000 volt) system. This is a lower energy event. That lightning strike can short transformer primary to secondary. The resulting plasma leaves utility 33,000 volts shorted directly to 240/120 volt power consumers. This 'follow-through' current means high energy connects into a homes and factories. Lightning is the low energy event. A high energy source that may cause much more damage exists if the transformer is not properly earthed.

If a plasma short is not created, then currents on the primary side are not shorted to wires on the secondary side. Another example of why earthing is essential for protection - in this case for lightning protection.

Fourth, earthing a secondary is necesary for numerous reasons both for normal operation and for safety during fault conditions. Elecricians, for example, would not know this. These installers spend years learning only what must be connected to what. Code also only says what must be connected to what. Neither would learn or say "why" this connection is made. Neither would know or discuss two important concepts - longitudinal and transverse currents.

Studiot kept asking "why" a transformer must be earthed. Instead, with frustration, he got "what" must be earthed. If a secondary is not earthed, then 33,000 volts will exist on same wires that also provide 120 volts.
 
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  • #142
Third, another example. Suppose lightning strikes the primary (33,000 volt) system.

Just how could this happen without striking the casing first?

Surely the casing of a 33Kv txfmr will be earthed in its own right, regardless of other earthing arrangements?
 
  • #143
sophiecentaur said:
My post was specifically about the 'transformer statement'. I, of course, go along with the notion that grounding is useful for all the other reasons you give (I have written as much already).
However, I have a feeling that any high power transformer that is likely to be used in a serious power distribution system will be designed 1. With the HV ends of the primaries away from the secondary windings and 2. with a grounded electric screen in between the primary and secondary windings (eliminating capacitative coupling). Even a half decent transformer in a low power equipment power supply will have an electrostatic screen.
well you won't need a screen if you ground the secondary, and you get added benefit of protection against any equipment (or thunderstorm) leaking high voltage onto secondary. But yes it is not too hard to prevent primary --> secondary strike. Just extra protection (and its validation) is much less necessary if secondary itself is grounded.
 
  • #144
Studiot said:
Just how could this happen without striking the casing first?
Kilometers of wire down many streets. A lightning strike to those wires (that are often highest and most exposed) is a direct lightning strike to the transformer - inside its case. Transformer mounted on Earth must have a dedicated and short connection to a superior Earth electrode. Similar to what transformer atop poles require.

A late 1970s IEEE paper discusses this also with a figure. Figure shows an extremely rare 100 kA strike to distant wires. 40 kA of that surge go to Earth via transformer grounding. 20 kA attempts to destroy home appliances. Another 40 kA is earthed by other consumers. That earthing is essential even to protect appliances and factory machines.

Lightning is only one reason why transformers are earthed. Primary and secondary must be as close as possible to make the transformer efficient. Lightning that cannot be stopped by 3 miles of sky will just as easily short 33,000 volts (utility power) into any nearby house IF that always required and critically important earthing does not exist.

Again, most techs (ie electricians) are only told what must be connected. Most would never understand why a utility transformer (or one inside some factory for a special machine) also must be earthed. Most only know "what" must be done. Never learn "why" it must be done. And never learn the difference between "what" and "why". Even utility linemen will claim that earthing does nothing because they never learn the difference between "observation" (also called junk science) and "why".

Demonstrated is only one reason "why" transformers must be earthed. Another reason "why" was presented previously. Your question demonstrates one who seeks "why" as well as "what".
 
  • #145
Thank you for the information.
 
  • #146
The NEC clearly states that grounding SHALL BE done in such and such a way in order to protect against lightning, etc...

I interpret that as an answer to "why" we do it... but I guess your mileage may vary.

And also, I don't think anybody was saying that grounding was unimportant... The point that was being made was that it doesn't affect how the circuit works under normal conditions... There always has been and always will be confusion on this. People tend to think that electricity from power lines (as opposed to static electricity, like lightning) has this inherent tendency to "dissipate" itself into the earth, when in actuality the tendency is just to return to the source any way it can. There is no giant electron sink in the earth...
 
  • #147
Evil Bunny said:
I interpret that as an answer to "why" we do it... but I guess your mileage may vary.
That only says "what" must be done. Where, for example, does it discuss wire impedance? Where does it discuss "why" a grounding wire must be shorter rather than thicker? Where does it discuss "why" a grounding wire must not be inside metallic conduit for lightning protection? Where does it even say "why" grounding is so important to lightning protection? All examples of the "why" that is not found in code.

Also not discussed is "why" both longitudinal and transverse currents are relevant. Does either relevant word appear in the code? Basic electrical concepts are required to understand "why". But not required for "what".

If code says “why”, then each above question is answered by quoting directly from the code.
 
  • #148
westom said:
Why does that elecrostatic scrreen exist? Because electric fields (and other reasons) are never eliminated. The screen only reduces that current. Primary currents leak by various means into the secondary. An electrostatic shield only reduced currents from the 33,000 volts side appearing on the secondary. Does not eliminate it.

Second, all wires can have two different voltages. Demonstrated were different currents creating 240 volts and 33,000 volts on the same secondary wire. To better understand this, learn two basic electrical concepts - longitudinal and transverse currents.

So that 33,000 volts does not appear on the 240/120 secondary, the secondary must be earthed somewhere.

AN electrostatic screen is a sheet of conductor wrapped around (not joined - so it's not a shorted turn) the inner windings and earthed. Any electric field on the secondary winding will be a severely potted down version of any field due to the primary - your 33kV wouldn't appear by capacitative coupling. The screen cuts out common mode signals on the primary - such as interference or unbalanced volts due to lightning. The core, itself, is grounded and also acts as an electric screen between windings.

You will need to explain "longitudinal and transverse currents".

You have not explained any mechanism for this 33kV appearing on the secondary - you have merely asserted that it will be there "and other reasons". You could say, with justification, that there may be a DC (resistive) path between the windings but how much current is likely to get through this Earthed screen and appear on the secondary? Do you some actual numbers to back up your assertion?

There are many reasons for earthing a supply to a house, of course, but the workings of the transformer seems to be one of the less relevant ones.
 
  • #149
Going back to the discussion on parasitic capacitance:

Assume you have a 12 V battery connected to a parallel-plate capacitor whose plates are 12 meter aparts. Take a voltmeter, and put its lead on the wire leading to one plate, and its other lead you put on the other wire leading to the other plate. We all agree we measure 12 V. Now take one of the leads of the voltmeter off of one wire, and instead place it in the empty space halfway between the plates. Do we all agree there should be 6 volts [ V=Ed=(12V)/(12 m)*(6m)=6V ] , but the voltmeter instead reads 0 volts?

Now consider 120 volts AC that has not been grounded. Would there be 60 volts between each wire and the ground? However, if you touch one of the wires and the ground, not much current will flow through you to the ground, just as the case that no current flows in the voltmeter in the above example because there is no completed circuit?

This might be a dumb question, but if there is 60 volts between you and the ground, but the ground doesn't complete a circuit with the source, how do you know how much current will flow through you in that brief instant you touch the wire and the ground? Is it enough to be dangerous?
 
  • #150
If you have 6V/m then you would measure 6V half way across. This assumes the instrument is ideal.

Edit...Owch, that's rubbish. I mean 1V/m.
 
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  • #151
sophiecentaur said:
If you have 6V/m then you would measure 6V half way across. This assumes the instrument is ideal.
You have also assumed transformers are ideal. Parasitic components inside transformers are why a primary and secondary are never fully isolated. A floating secondary could be anywhere between 0 and 33,000 volts. Another reason why the secondary is earthed.
 
  • #152
RedX said:
This might be a dumb question, but if there is 60 volts between you and the ground, but the ground doesn't complete a circuit with the source, how do you know how much current will flow through you in that brief instant you touch the wire and the ground? Is it enough to be dangerous?
Not a dumb question at all. What counts here is the Capacity involved, which wouldn't be much for just a human body. If you were joined to a long transmission line or a large steel building then there may be enough charge to harm you when it's passed through you.
If you touch a Van der Graaf ball, you just feel a small prickle - a man on a high voltage cable, doing live maintenance, needs to wear a Faraday suit to make sure that the charge doesn't flow through him when he connects up, even though he's on an insulated platform. Similar volts but different capacities - so different charges.
The volts in your example are very low so the charge would be miniscule and harmless. But you can die from a 60V shock under the right conditions so, perhaps with a supercapacitor but no complete DC circuit. . . . . .
 
  • #153
westom said:
You have also assumed transformers are ideal. Parasitic components inside transformers are why a primary and secondary are never fully isolated. A floating secondary could be anywhere between 0 and 33,000 volts. Another reason why the secondary is earthed.
You are just asserting something on the grounds of "parasitic components". If you want parasitics inside then you have to have parasitics outside too. Leakage between windings and Earth are involved on both sides - particularly if a screen is involved. Until you can quote some actual figures relating to a suggested mechanism then your 33kV is just a made up number. Any statements you make on this sort of forum need, really, to be backed up with a good reference or some convincing sums if you want to be believed. I am always open to being convinced - honest I am.
 
  • #154
westom said:
Where, for example, does it discuss wire impedance?

It doesn't. Is it important to the grounding discussion? I would think the lower impedance the better... that's the whole idea behind a conductor in the first place, right?

westom said:
Where does it discuss "why" a grounding wire must be shorter rather than thicker?

It doesn't. Can you explain the relevance?

westom said:
Also not discussed is "why" both longitudinal and transverse currents are relevant.

Right again... Not discussed. Can you expand on this for us?

You brought them up, so I'm assuming they must be important points... Would love to hear more.
 
  • #155
sophiecentaur said:
What counts here is the Capacity involved, which wouldn't be much for just a human body. If you were joined to a long transmission line or a large steel building then there may be enough charge to harm you when it's passed through you.
If you touch a Van der Graaf ball, you just feel a small prickle - a man on a high voltage cable, doing live maintenance, needs to wear a Faraday suit to make sure that the charge doesn't flow through him when he connects up, even though he's on an insulated platform. Similar volts but different capacities - so different charges.
The volts in your example are very low so the charge would be miniscule and harmless. But you can die from a 60V shock under the right conditions so, perhaps with a supercapacitor but no complete DC circuit. . . . . .

The formula for charging a capacitor is:

[tex]I(t)=\frac{\epsilon}{R} \left(e^{-\frac{t}{RC}} \right) [/tex]

120 V can be dangerous to the human body if you touch the hot and the neutral at the same time (assume the neutral is not grounded for simplicity). But now say you only touch just the hot wire. Then you form one plate of a capacitor, and the other plate would be the outer surface of the neutral wire itself. I assume that the capacitance between you and the outer surface of the neutral wire is really small. So using the formula for the current, the emf (120 V) divided by your resistance is enough to kill you, but the only thing that could possibly save you is the exponential decay of the current. A low capacitance would drop the current really fast, but the resistance R of the human body is high and this counteracts that (the time constant is RC, so high resistance can make up for low capacity). Moreover, for small time intervals, the exponential doesn't drop off by much (at time=0, the current flowing through your body is the full current, which is enough to kill you).

My question is:

1) Am I correct to model a human who touches just the hot wire as a resistor and capacitor in series with the power source, with resistance being the resistance value of a human, and the capacitance being between the human and the outer surface of the neutral wire? So would you assume a human is a spherical capacitor of radius equal to the waist size, and the other "plate" of the capacitor is a long cylindrical tube (the outer surface of the neutral wire), and the dialetric between them is air?

2) Using the RC formula, is the low capacitance C enough to overcome the high resistance of the human R in the exponential, and even if it is capable of doing so, then at least for some small time interval doesn't the human experience the full, capable-of-killing current: emf/(human resistance) ?

My understanding of the Van der Graff ball is that the ball will run out of charge so that it can't maintain it's voltage, and also by having a fixed amount of charge the current is limited. But in the example I have above, the emf will always maintain its voltage of 120 V. So I'm not sure the Van der Graff ball applies.
 
  • #156
It is the charge that does the damage (ionising vital bits inside us). A brief high current is not particularly harmful. The Van DDR Graaf is an example of a harmless jolt because the Capacity is small. Leyden Jars on a Whimshurst are far more dangerous because their capacity will hold a more "dangerous" quantity of charge. Not used in Schools any more!
Btw it's not the capacity between body and neutral wire that counts. It't total capacity to ground, which is more. Also, the Capacity does not "overcome" the resistance; it just prolongs the time for which current flows.
 
  • #157
RedX said:
1) Am I correct to model a human who touches just the hot wire as a resistor and capacitor in series with the power source...

I don't think so. I think the capacitance here would be so small that we can essentially consider it an open circuit. You would not be capacitively coupled as you would with a "real" capacitor circuit that you describe above.

I don't have any fancy equations or engineering models to back this up... only personal experience. I have on several different occasions, been in contact with a 120V "hot" wire without being in contact with the return and have not felt so much as a tingle.

Usual disclaimer... 120V can kill you so do not play games with it!
 
  • #158
Evil Bunny said:
I don't think so. I think the capacitance here would be so small that we can essentially consider it an open circuit. You would not be capacitively coupled as you would with a "real" capacitor circuit that you describe above.

I don't have any fancy equations or engineering models to back this up... only personal experience. I have on several different occasions, been in contact with a 120V "hot" wire without being in contact with the return and have not felt so much as a tingle.

Usual disclaimer... 120V can kill you so do not play games with it!

Equations aren't "fancy" my boy. They are just a way of stating things in a way that is least likely to be misinterpreted. If you want an arm waving forum, then this is not one of them.:biggrin:

You are right that the capacitance between your body and the Earth is not great but, under many circumstances, you PLUS a lot of other conducting stuff may be joined together and then the capacity is more significant. The same principle applies whatever Volts and Capacity are involved - it's just that the numbers make a difference to your experience.
Q = CV, the initial current is V/R and the energy stored is CV2/2 whatever the circumatance. Those formulaearen't "fancy" are they?
 
  • #159
Seems to me that if you want to figure out the "real" capacitance between yourself and a conductor, especially with "a lot of other conducting stuff" around... the equations involved would probably be pretty fancy :rofl:

I'd like to see someone figure out the "actual" capacitance in such a situation.

Maybe it's simple... But to a lowly arm-waver like me, it would look pretty fancy!
 
  • #160
Calculating the AC current into capacitor, directly from basics.
lets say capacitance is 100pF = 1E-10 F, let's say amplitude is 120*sqrt(2) volts (the 120v is rms), then the voltage is
v=120*sqrt(2)*sin(t*60*2*pi)
and it's time derivative is
dv/dt=120*sqrt(2)*60*2*pi*cos(t*60*2*pi)
Current is charge per time. Charge on capacitor is voltage*capacitance.
So the current is
I=c*dv/dt
or
I=120*sqrt(2)*60*2*pi*1E-10*cos(t*60*2*pi) = about 6.4 micro amper peak.

Or you can use the formula explained here:
http://www.allaboutcircuits.com/vol_2/chpt_4/2.html
reactance = 1/(2*pi*f*c)
and to find the AC current for sine wave you take AC voltage and divide by reactance.
You can even calculate things for arbitrary circuits consisting of capacitors, resistors, and inductances, the same way you would for resistor network, but using complex numbers for 'resistances'.

Safety notice: the current in mains is not quite sine wave, and there may be high frequency components leaked by some circuitry (such as cheap PC power supply, or perhaps circuit trip and all the connected inductances shooting the voltage up). Meaning it is not safe to touch mains even if you are well insulated.

Note for guessing capacitance: the capacitance of 1 m^2 plates at distance of 10cm is about 88 pF.
 
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  • #161
What about the calculation for capacitance between a human and a wire... and possibly some other metal stuff that might be around?
 
  • #162
I understand there is more capacitance between the ground and a person just touching the hot wire (and insulated from the ground) than there is between the person and the neutral wire, but I assumed for simplicity that the neutral wire was not grounded. So if you wanted to connect the person, the power source, and the ground in one big circuit, you'd have to include the resistance between the neutral wire and the ground [tex]R_{n-g} [/tex], which decreases the overall current:

[tex]
I(t)=\frac{\epsilon}{R_{\mbox{human}}+R_{\mbox{n-g}}} \left(e^{-\frac{t}{R_{\mbox{total}}C}} \right)
[/tex]

since the first factor would be decreased with the increased resistance R between the neutral wire and the ground.

In reality, the neutral wire is grounded, so there would be no resistance between the neutral wire and the ground, so you would consider the capacity between the Earth and the person touching the hot wire rather than the person and the neutral wire.

In other words, if there is no grounding, the least resistance is offered if you consider the capacitor as being between the human and the neutral wire.

I think what confused me is I've seen charts that state the amount of current it takes to kill you, but it's really charge and not current, so you don't have to worry about being a capacitor if you touch just one wire.

You might need to worry when the voltage is really high before it has been stepped down, since the voltage is 1000 times more. In fact someone just did a calculation in post #160 (using AC) and got 6.4 microamps. 1000 times more voltage would be 6.4 milliamps which looks to be on the threshold of dangerousness:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html

So I guess it's not dangerous to just stand on the ground beneath a power line (on a real power line the neutral is grounded), since the capacitor would be between you and the cylindrical hot wire. But if you touch the hot wire on an insulated platform, it would be between you and the earth.

Anyways, the formula I used for current is actually only good for DC power, so I guess it doesn't apply for AC. Although if the current dies off faster than 1/60 seconds, then maybe it would be a good approximation.
 
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<h2>1. Is circuit completion necessary for a circuit to function?</h2><p>Yes, circuit completion is necessary for a circuit to function properly. Without a complete circuit, electricity cannot flow and the circuit will not work.</p><h2>2. What happens if a circuit is not completed?</h2><p>If a circuit is not completed, electricity will not flow and the circuit will not work. This can result in a device or system not functioning properly or at all.</p><h2>3. How can I tell if a circuit is complete?</h2><p>You can tell if a circuit is complete by using a multimeter to measure the voltage and resistance of the circuit. If there is a complete path for electricity to flow, the multimeter will show a reading. You can also visually inspect the circuit for any breaks or disconnected components.</p><h2>4. Can a circuit be completed in different ways?</h2><p>Yes, there are different ways to complete a circuit. For example, a series circuit is completed when electricity flows through each component in a single path, while a parallel circuit is completed when electricity flows through multiple paths. Additionally, circuits can be completed using different materials such as wires, conductive metals, or even water.</p><h2>5. Why is circuit completion important in electronics?</h2><p>Circuit completion is important in electronics because it allows for the flow of electricity and enables devices to function. Without a complete circuit, electricity cannot flow and devices will not work. Additionally, understanding circuit completion is essential for troubleshooting and repairing electronic devices.</p>

1. Is circuit completion necessary for a circuit to function?

Yes, circuit completion is necessary for a circuit to function properly. Without a complete circuit, electricity cannot flow and the circuit will not work.

2. What happens if a circuit is not completed?

If a circuit is not completed, electricity will not flow and the circuit will not work. This can result in a device or system not functioning properly or at all.

3. How can I tell if a circuit is complete?

You can tell if a circuit is complete by using a multimeter to measure the voltage and resistance of the circuit. If there is a complete path for electricity to flow, the multimeter will show a reading. You can also visually inspect the circuit for any breaks or disconnected components.

4. Can a circuit be completed in different ways?

Yes, there are different ways to complete a circuit. For example, a series circuit is completed when electricity flows through each component in a single path, while a parallel circuit is completed when electricity flows through multiple paths. Additionally, circuits can be completed using different materials such as wires, conductive metals, or even water.

5. Why is circuit completion important in electronics?

Circuit completion is important in electronics because it allows for the flow of electricity and enables devices to function. Without a complete circuit, electricity cannot flow and devices will not work. Additionally, understanding circuit completion is essential for troubleshooting and repairing electronic devices.

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