Ground the primary then. High voltage is not a concern anyway or potential transformers would be installed to protect against over voltage. Over current is the only concern and we read 10mA is lethal yet over current protection begins at 1500 X that at 15 amps.
Floating voltage is not at issue with center tapped secondarys as utility supplied voltage is often transmitted one phase, ungrounded, over long distances, then bucked, grounded, center tapped and sent to the meter.
Grounding creates shock hazard, reduces equipment and property loss by sinking faults to a common reference. Again, if preservation of life were the issue, systems would be ungrounded, high voltage would be isolated with movistors, potential relays, hv detectors,
Grounding provides a high current path which will trip breakers. Without the low resistance path, buildings would burn down before breakers would trip because even small electrical damaged assemblies can ignite long before a 15 amp fault would occur. The trade off is,Humans are sided against ungrounded conductors always a potential current path, in exchange for fault path available to mitigate fire hazard.
For those who say “…security cable, rg6, Rf shielded, all grounded…” Let me say I KNOW, however, it is shielded cable, used to isolate from myriad grounded AC systems, an effort to increase quality of signal harmed BECAUSE OF GROUNDING, a ‘fight fire with fire’ method.
Question:
What object has least resistance to current flow?
The primary is grounded anyway. How would a isolated transformer secondary differ from unisolated? Assume transformer is wired completely non-auto transformer, just primary and secondary exclusively inductively coupled.
In addition, do secondary windings accumulate primary voltage while the voltage constantly reverses polarity?
Yes, the transformer that is hanging on the pole has a grounded primary and a grounded secondary. The secondary would be connected to a 1:1 isolation transformer. The primary of the isolation transformer would be referenced to earth ground. The secondary of the isolation transformer would be floating.
In theory, no, but in practice since you have a floating isolated system the capacitance to ground can accumulate charge, thus my suggestion of a high resistance to ground. How hazardous this might be, according to this calculator 14 AWG has about 15 pF of capacitance per meter (center-center spacing of 5 mm), so 100 meters would have 1500 pF of capacitance, which can store up to 96 millijoules of energy at 11.3 kV (peak voltage of 8 kV AC), which can cause quite a shock but isn’t dangerous (at least not from the shock itself). On the other hand, a lot of electronics have noise-filtering capacitors on the AC input from each line to ground, usually around 2200-4700 pF (times two capacitors, not counting an additional capacitor connected across the transformer in SMPSs, at least if the secondary side is grounded), so with ten devices plugged in, total capacitance would be up to 95,500 pF, or 6 joules of energy, which is considered to be dangerous (5 joules or more; 350 mJ is also set as a limit for consumer devices for accidental contact).
Reduce capacitance by removing insulation. The copper will not store charge. Lets go back to K and T wiring,btw, utility conductors are not insulated for that reason. The surrounding air is dissipating the charge induced in moisture/contaminants, etc.,CORONA, it’s more than just beer!!!
Without looking it up*, I know for a fact this is false, earth no doubt has worse conductivity than any metal, much less any metallic object (excluding semi-metals/semiconductors). Even salt water is a much poorer conductor than metal.
*Wikipedia says that the resistance can be as low as 1 ohm-meter (but as high as 10,000 ohm-meters). For comparison, copper has a resistance of only 0.0000000168 ohm-meters - a factor of at least 59 million times lower (silver is actually only slightly better at 0.0000000159 ohm-meters, or 94.6% of the resistance; for comparison, aluminum is 0.0000000282, 67% higher, so copper wire is clearly preferable to aluminum but silver doesn’t seem worth it, especially with the cost). Even sea water is listed as 0.2 ohm-meters, in the same range as germanium (only a semiconductor).
Also, I doubt the insulation on wire introduces significant capacitance when it is surrounded by air, since the capacitance associated with the insulation is in series with the capacitance from the air. Shielded cable is a different matter but most house wiring is unshielded.
While dielectric materials certainly help when you make a capacitor, they aren’t absolutely necessary. Copper alone can easily store a charge. Capacitors would just be a lot larger if that was all that was inside of them.
Yick. Let’s not. That stuff gives me the willies.
I was under the impression it was mostly a cost issue. Some distribution lines (mostly older ones, in my experience) are insulated. Transmission lines could be insulated but it would take a LOT of insulation, which would add a huge cost, plus would add a lot of extra weight, requiring beefier support towers, etc. which also adds cost. And, since transmission lines are generally located as to be difficult to get to, all that cost would provide very little benefit.
Charge is stored potential, its kinetic energy looming in insulators ready to discharge through conductors. Insulation stores charge. Utilities avoid insulated transmission conductors to prevent discharge. Air is an insulator. HF capacitors use air between plates to store charge. A copper wire without a source attached and no insulation will not accumulate charge as the copper atoms will remain fully intact no charge can possible accumulate ON THE CONDUCTOR.
The earth is the single least resistive object known. Use a million tons of silver, submerge it in liquid nitrogen, whatever, and it will have higher resistance than the earth simply because the earth is the single largest mass available and even at hundreds of ohms or thousands, current will flow 8000 miles or more. To understand the truth one must look at the question. Earth conducts electricity, it is the largest object which contains minerals, water, silver, gold, carbon in many shapes, etc., its simply the best, least resistant object because its the largest.
Well, neither of these is strictly correct.
Capacitors store charge in an electric field. Since you can make a capacitor with two plates separated by vacuum, it’s pretty clear that the charge is being stored on the plates, not in the vacuum…
Also, as for as the Earth goes, the critical parameter is resistance per unit volume (ohms per cubic cm), and Earth is pretty poor in that respect.
First, is “ohms per cubic cm” even a valid unit? I have heard of Ω⋅m (bulk resistivity), Ω/square (surface resistivity), and Ω (resistance), but have never heard of resistance/volume.
Second, the bulk resistivity for “earth material” (dirt and whatnot) would undoubtedly be pretty high for a sample you test in the lab. But edge effects would be very significant. If you were to have an *extremely *large sample, where edge effects would be less significant (and thus lots of volume for the field lines to exist), the *resistance * between two points would be pretty low. How low? I have no idea. Like Kengine7, I have also heard that the earth can have extremely low resistance simply due to its very large size. And I am too lazy to do any informational research on this.
You’re right. It’s almost entirely a cost issue, at least for distribution lines. Transmission lines also dissipate a fair amount of heat, enough that buried transmission lines used in urban areas are run through oil-filled pipes for cooling.
Yep, it is. With most materials it ends up being a relatively low number, so in my experience it’s usually microohms per cubic centimeter. Ohms per cubic inch is also common for all of us non-metric folk. You may also see it expressed as the inverse, megmho per cubic inch (or its metric equivalent).
As for the conductivity of the earth, I wouldn’t call it the best conductor out there. There were however several early electrical systems that relied on the earth as one of its conductors so that they could basically cut their wire costs in half. These were simple two conductor systems, with one conductor literally being the earth. In general they worked, but they had a lot of problems with inconsistent earth resistance, and resistance that varied depending on the seasons (due to things like moisture content of the soil and the height of the water table). Systems that worked perfectly most of the time would suddenly get unstable when there was a drought, for example. While there were some working systems, they had enough trouble with it that eventually pretty much everyone went to running another physical wire instead of trying to rely on the earth for one of the conductors, in spite of the extra wire cost involved.
I did some googling, and I did find a couple of old papers that used a unit of resistance/volume for volume resistivity. But I am still not convinced it is a useful or standard unit, simply because it does not make sense to me.
Let’s say something has a resistivity of 5 Ω/cm[sup]3[/sup]. What does that mean, exactly? If I have a volume of 20 cm[sup]3[/sup] of the stuff, does it mean the resistance will be 100 Ω? And if I have a volume of 40 cm[sup]3[/sup], does it mean the resistance will be 200 Ω? :dubious: What if I take 1 cm[sup]3[/sup] of the stuff and make a wire that is a mile long? Are saying the wire resistance will be 5 Ω? Resistance is not dependent on volume… it is dependent on geometry.
Again, I’m not saying resistance/volume is a bogus unit; I simply do not understand how it is derived, or how it can be used. So I’m looking for enlightenment. Perhaps it’s a specialized unit for a very specific application; I dunno.
