Table of Contents
What is electrical breakdown
Charges flowing in electrical cables carry with them the life-giving power needed to run our devices. When you plug in your smartphone battery charger, hair clothes dryer, toaster oven, or any other home appliance, you might see that the plug you keep in your hand as well as the cable is constructed from plastic or rubber. Both of these materials are called insulators (or dielectrics) in the electrical world, as well as their key work is to keep electricity from getting away. Insulation makes sure that electricity flows inside the cable, straight to the device being powered, instead of with your body, for instance, which can end unfortunately for you.
The world of insulators is much larger, obviously, and not simply restricted to our home appliances. Fairly a great insulator is, as an example, air, thanks to which walking near a high voltage line we are not suddenly electrocuted. One more example is the various sorts of oil in which we immerse high-power equipment that requires both insulation as well as excellent cooling.
Insulators secure our lives daily, however unfortunately they additionally have their limitations. As well as when such an insulator fails, after that a phenomenon called flashover takes place. This is when the situation transforms 180 degrees as well as from a material that perfectly blocks the flow of electrical power, the insulator unexpectedly ends up being an incredibly great conductor.
Conditions leading to electrical breakdown
Since an insulator can instantly “quit working”, the concern is when this can happen and also just how to prevent it. Essentially, there are 2 methods leading to the phenomenon of breakdown:
Mechanical damages of the insulation – we have to take care of it almost everywhere, where an electrical wire or a housing of some gadget is harmed. And also it’s uncomplicated, due to the fact that the cables in our homes do not have an easy life. The one from the vacuum we usually knock the door on, the one from the dryer we continuously unfold as well as twist around the device, and also the consistent dropping of smaller sized or larger devices on the flooring. If you discover that there are copper wires sticking out from under the insulation of the cable, or the broken real estate of the device exposes some steel aspects, after that under no circumstances should you remain to make use of such a device. Obviously, the housing or wire can be replaced or repaired, however you ought to refrain from doing it if you do not know how to do it as well as do not have the right tools.
Going beyond electric resistance of an insulator – lots of insulators are resistant to human ideas – it is difficult to harm air or oil mechanically. Nonetheless, any type of insulator capable of hampering the flow of power can do it only somewhat. The already pointed out air shields us from being electrocuted by high voltage lines, however it is helpless against the forces of nature. During a thunderstorm, lightning literally tears apart the atoms of the air and enables a tremendous quantity of current to travel through it. For a fraction of a second, air stops to be an insulator and comes to be an extremely excellent conductor. Of course, lightning in and of itself is extremely stunning and has a fatal energy, but this kind of flashover can also take place on a much smaller sized scale. Any sparks from gas ranges or lighters are nothing more than the result of pierced air.
Insulator breakdown conditions
Electric breakdown may seem strange and challenging, yet it is not at all. To recognize what exactly happens inside an insulator currently of breakdown, we need to take a look at it really close … I imply on the scale of solitary atoms.
Conductors conduct electrical energy due to the fact that inside their framework there is a sea of “homeless” electrons. These are so called “valence electrons” that don’t really feel also strongly connected to their atoms as well as drift openly in the conductor structure. Insulators (or as you like dielectrics) are an entirely various tale. Each atom keeps its electrons in check, so there are no free electrons. Given that there are no electrons, no current can flow through such a material either.
It would seem that since an insulator has no cost-free electrons as well as is unable of conducting electricity, it is additionally completely indifferent to electrical voltage. This is not real! Bear in mind that although atoms are essentially electrically inert, they are still composed of small electrons with a negative charge as well as slightly bigger protons with a favorable charge. Surrounding such an atom with a potential distinction creates the electrons orbiting the atoms to deflect their orbits toward the high potential, and also the nucleus in turn is brought in somewhat by the low potential.
In the image above, you can see that bringing 2 oppositely charged plates close to the atom triggered it to flaw, which is adeptly called polarization. Note that the oppositely charged plates do not also need to touch the atom to deform it. This is possible as a result of the presence of an unnoticeable electric field that can bring in and also fend off charges. Every electron as well as every proton gives off such a field and so does every charged item. This is why the atom shown in the figure over is stretched. Negatively charged electrons as well as favorably billed protons are surrounded by a solid electric field from two oppositely charged plates. Under its impact, they feel a force of destination – the electrons towards the favorable potential, the protons towards the adverse. The entire atom comes to be polarized in a way let’s call it “non-contact”, or as electrical experts state, “by electrostatic induction”.
As you might presume, the stronger the electric field is, the much more highly it will certainly flaw the atom it surrounds. With an adequately huge pressure such an atom can exceed its “extending restriction”, which results in one or two electrons breaking away from it. Exactly this phenomenon I had in mind when discussing tearing of air molecules intoxicated of lightning. In technological terms this impact is called ionization.
Dielectric strength
The trick to getting to the strength restriction of a material is to create a powerful electrical field. Exactly how to do it. The intensity of an electric field has 2 fundamental properties:
- Its value enhances as the voltage increases
- Its value decreases as you move far from the source
The electrical field strength E is gauged in volts per meter [V/m] If you connect the terminals of a 1 V battery to two steel plates and put them 1 metre apart, you get an electric field strength of 1 V/m. By bringing home plates better with each other or enhancing the voltage, the field strength increases.
What is the electric field strength needed to trigger a breakdown? For most insulators … very huge. The dielectric strength of the worst insulators gets on the order of a number of million volts per meter. Consequently, to prevent writing unneeded nos, the regular unit made use of in dielectric strength tables is megavolts per meter [ MV/m] or kilovolts per millimeter [kV/mm] These units are equivalent – 15 MV/m is precisely the like 15 kV/mm. Let’s see after that just how much is the dielectric strength of some materials:
| Substance | Dielectric strength [MV/m] |
|---|---|
| Air | 3 |
| Glass | 9,8 – 13,8 |
| Polycarbonate (PC) | 15 |
| Paper | 16 |
| Polyethylene terephthalate (PET) | 17 |
| Polyethylene (PE) | 18,9 |
| Polytetrafluoroethylene (PTFE) | 19,7 |
| Epoxy resin | 19,7 |
| Polystyrene (PS) | 19,7 |
| Polyvinyl chloride (PVC, PCW) | 13,8 – 19,7 |
| Polypropylene (PP) | 23,6 |
| Polyurethane (PU, PUR) | 25,4 |
| Neoprene (synthetic rubber) | 15,7 – 27,6 |
| Insulating tape | 28 |
| Silicone rubber | 26 – 36 |
| Distilled water | 65-70 |
| Transformer oil | 110 |
| Porcelain | 35 – 160 |
| Water glass | 200 |
| Natural rubber | 100 – 215 |
| Quartz glass | 470 – 670 |
| Diamond | 2000 |
It deserves keeping in mind that among gases the greatest strength has trichloro-trifluoroethane (regarding 19.5 MV/m), for fluids it will certainly be butylbenzene (275 MV/m), and also the crown of insulators in the case of solids is held by ruby, which with the result of 2000 MV/m outclasses its opponents. Much better than maybe just ideal vacuum cleaner, however those really attained in laboratories have dielectric strength reaching “just” 30 MV/m.
What does dielectric strength depend on
You must know that the values given up the table over relate to high pureness substances at room temperature as well as air pressure. Electric strength is a value that is very sensitive to external conditions, not to mention contaminations in the insulator, which substantially aggravate its specifications.
All these points appear somehow natural to us but there is another, a lot less apparent, issue. It happens that the voltage is much lower than the limit and also the insulator is ruined anyway. How does it occur as well as what is accountable for it? Let’s consider 2 examples shown in the graphic below:
You must know that the values given up the table over relate to high pureness substances at room temperature as well as air pressure. Electric strength is a value that is very sensitive to external conditions, not to mention contaminations in the insulator, which substantially aggravate its specifications.
All these points appear somehow natural to us but there is another, a lot less apparent, issue. It happens that the voltage is much lower than the limit and also the insulator is ruined anyway. How does it occur as well as what is accountable for it? Let’s consider 2 examples shown in the graphic below:
Taking a look at 2 items of flat sheet steel, we may assume that they are completely smooth. Under a microscopic lense, however, we would observe a mass of bumps and also bulges. Just how does this impact the strength of the capacitor? Well, the charge accumulating on its plates suches as to gather in all spaces and also crannies. The greater density of charge makes that in these locations the electric field strength can be also a number of times greater than in places that are a little bit smoother.
Furthermore, where two “protrudes” fulfill, the distance between the plates reduces, which additionally reinforces the field. In the case of air capacitors it does not matter so much (too large range in between the plates), yet in capacitors making use of plastic (PP, PET, PTFE) the insulation layer does not surpass several micrometers. Therefore, also the tiniest abnormality can considerably lower the value of the voltage with which we can securely power such a capacitor.
The concern of “inequality” is also partially the response to why lightning, regardless of “only” 10 million volts, is able to travel a range of 8 kilometers. Because as soon as it can and wherever it can, it tries to make its way shorter as well as easier! Do you understand why solitary trees are so preferred targets for lightning? Of all, they are high objects sticking out from the ground. Thus, first of all, they shorten the path of charges intending to run away to the ground a little bit – it’s always a number of meters much less air to cover. Secondly, the top of the tree is a dreamland to focus the charge and thus boost the electrical field strength. Both of these impacts, incorporated with the rainfall and the certain system of lightning, suggest that lightning can quickly tear via the air and eliminate excess adverse charge. The air can not be completely damaged and also the electronless atoms really swiftly discover a substitute among the millions of fragments moving via the air, and also we can proceed to stroll securely near substations and high voltage power lines.
References:
https://teoriaelektryki.pl/czym-jest-przebicie-elektryczne/
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