What is transition contact resistance and how to deal with it

For reliable and long-term use of switching contacts of electrical devices, you can use the method of artificial aging of contacts (mechanical destruction of oxide films that were formed if the contacts were open for a long time, this reduces their contact resistance). For this, it is convenient to use fritting (though only for powerful contacts of high-voltage devices). Contacts in a closed state or closed after a long time in the open state by connecting them through a resistance to a power source, emf which is enough to start fritting. When the electric field in the film reaches a value of about 10 to 6 degrees V / cm, the current through the contacts increases sharply, and the voltage at the contacts drops to 0.3 - 0.5 V. Fitting allows to significantly reduce the transition contact resistance. The state of fritting is determined by the voltage at the contact, approximately about 0.3 V.

Perfect contact with minimal contact resistance can only be obtained in a vacuum. Therefore, the presence of oxide films in any contact parts and wires suggests that the quality of contact compounds depends primarily on the professionalism of this contact making. The choice of contact creation tools is secondary here. It’s just that someone loves the terminal blocks, understands their features and knows how to work with them well, while someone cannot live without a soldering iron. So they swear to infinity. Although in essence you can learn to make good and trouble-free contacts in any civilized way.

If welding is used to connect the wires, then all the difficulties of combating transient contact resistance disappear by themselves. A normally made welded contact has no transition resistance! If there is, it is very insignificant.

As I understand it, this article can be considered as the third part of a series of articles about terminal blocks VAGO))
Briefly, the essence of the problem is the following, as in the VAGO terminal blocks they manage to connect 2 wires, for example, with a section of 4 mm2, through a contact surface with an area of ​​less than 4 mm2, for example 3 mm2))?))
In this article, the emphasis is made in bold on the fact that the area of ​​the transition contact is not important !!!:

Take a normal 4-pole contactor and measure the resistance through 1 pole (contact pair), we get the transition resistance R
If we parallel all 4 poles, then we get the resistance R / 4, WHY?!?! because AREA !! contact surface increased 4 times.
Although judging by the highlighted text, we should have the same resistance with one pole as with 4 .... = R
this is for the IMPORTANCE of the AREA of the contact surface.

I agree with this and from this we can conclude
so that the contact contact resistance has a minimal effect on the overall circuit resistance, the area of ​​the contact surface must be MORE !! sections of the connected cable !!!

One can argue with the independence of resistance from the contact area. There are big doubts, let the afftor prove his point.

This is not what I came up with. The above formula is derived from the results of more experiments and measurements and is described in any textbook on electrical apparatus. From the theory of electrical apparatuses: “The contact transition resistance does not depend much on the size of the conventional contacting pad. However, with an increase in the rated current, the external surface of the contact parts must also be increased, as losses increase with the current, and a larger surface is required for their dissipation," t. e. the need for a large contact area arises not to reduce the transition resistance, but to increase the heat sink from the contacts. Although indirectly, the dimensions of the contact surfaces affect the transition resistance, since the less heat is removed from the material, the greater the transition resistance, but this is the influence of the heating temperature and the oxidation process.

I absolutely agree withYura Yakovlev. Moreover, when welding, the integrity of the conductor is practically restored. If at any mechanical connection there is a maximum surface diffusion, then during welding - an intermolecular bond. And as stated in the article, the resistance of an integral conductor (that is, welded) will be anyway less than the resistance of any contact resistance!

I agree with the author on almost all points. The (relative) surprise is only related to the area of ​​contact. A high school course, it would seem. The contact surface area, strictly speaking, can be considered as an element (resistor) included in the circuit. However, in the course of school physics there are formulas for calculating the resistance value, where the cross-sectional area of ​​the conductor has its place. "Do not knock out the ax." Those. To argue about the "unimportance" of the contact area, I consider it below my dignity. Terminal blocks "Vago", and like any other company, are probably thought of for assembling garlands on LEDs, bulbs from flashlights, etc. Installation of network wiring on them is simply dangerous !!! Those who prove their expediency, simply work out the MZDU from a trading company. I fully and fully support the idea of ​​brazing twists if brazing is performed by copper. Soldering with ordinary solder is quite risky. In my practice, the usual copper twisting, performed competently, in conditions of constantly high humidity (Latvia), has been working for more than 25 years. At the established maximum loads, there is no heating! I wrote earlier, but I repeat, - terminal blocks, only for hoses and suckers. Had more than once, redoing such a "creativity", throw out sockets with dozens of terminal blocks.

Let’s explain my reasons again. When I say that the transition resistance is practically independent of the contact area, I mean pure contact (stripped, without oxide films). This mathematically confirms the formula given in the article. Naturally, during oxidation, the contact temperature increases and its resistance increases, so the contact area must be increased in order to remove as much heat as possible from it and slow down the oxidation process.

And then, if someone is very worried that I like the WAGO terminal blocks, then I confess, I love things and technologies that greatly facilitate the performance of certain work and in some situations they can and should be used.

with the same success, I proved the opposite in the example with a 4-pole contactor ...
I can assume that the above article and formulas refer to point contact ..., i.e. A POINT with an ultra-small area ... but you should probably consider some kind of surface contact that has an area ...
but I repeat ...
if we put a contact with a contact with a surface area of ​​10 mm2 on a cable with a cross section of 185 mm2, then no matter how small the contact resistance is ..., it will burn up with us .., because in this place there will be the bottleneck (as in direct and figuratively)

Nobody sports, that in this case, such contact can burn out. It all depends on the current flow and how this contact is made.

And as for the point contact, so the size of the apparent and actual contact area coincides, since the contacting is carried out at only one point, i.e. all of the above applies to surface contact (physical contact occurs along a number of points on the surface of the contacts). By the way, point contacting is used in low-power relays, since there, due to their small size, it is not possible to create normal pressing forces. And now everyone will be horrified: the resistance of point contact is less than surface! I can imagine how now, after this phrase, everyone will start to resent. Just electrical contacting is a complex phenomenon and, by the way, is still not fully understood and it is not entirely correct to approach it with only one Ohm's law.

I rummaged through my computer. Look at one interesting little book (fifty pages in total): https://env.electricianexp.com/kontakty.zip There, about electrical contacts, a lot of interesting things have been written.

And so, I do not convince myself that terminal blocks with flat spring clips are a panacea for all ills. It’s just that there is nothing criminal in their design and it’s clearly not worthwhile to focus on a small area of ​​touching the contact in such terminal blocks, since if you do not allow oxidation and, accordingly, overheating of the contact (and the design of such terminal blocks ensures proper installation), then there is a small contact area does not play a big role in this case.

nuuuu ... and why the contact burns .. ??, suppose that the current flows 90% of the permissible cable current, and the contact is "perfectly" made))), silver-plated surface ..., ideal pressing force ...., yes even if it is welded by welding ...,
anyway .. this contact will burn, the cross-section of the contact pad should be LARGE than the cross-section of the cable.

Directly some kind of mantra turns out. In your example, with a cross section difference of 18.5 times, the contact will surely someday burn out. I agree with that. But this does not mean anything. How much less is the contact area of ​​the same WAGO than the cross-sectional area of ​​the connected conductors? Factor of? And if there is a difference, then maybe it is compensated by the terminal block design (tin-lead layer and high contact pressing) and in this way a stable contact resistance is ensured? This is taking into account what is written in the article, i.e.with a clean and non-oxidized contact, the contact area practically does not affect the transition resistance, and if the contact is not allowed to oxidize, it will not affect it during operation (the transition resistance will remain the minimum possible).

the area should be LARGE but not equal or less .., tk. contact resistance is greater than the resistance of a solid conductor ...., and no conditions (force, temperature, oxidized contacts) can compensate for the insufficient transition area .....
ehhh forced books to read)))
quote from your bookhttps://env.electricianexp.com/kontakty.zip

as we understand it (as I said)))) PERFECT point contact is present only in theory, (contact at a point whose area tends to zero ...), but in practice we have a SURFACE type of contact (even in low-current relays, it contacts not a point, but a surface, although small enough) ...
A surface contact consists of a set of point contacts, the number of which increases in proportion to the compression force ...., i.e. if an ordinary point contact has a resistance R, then a surface contact that has at least three points of contact already has a resistance R / 3, and if you press harder, the number of such points will increase and the resistance will decrease .., and the larger the surface area, the more such points appears other things being equal ......
ps the quote refers to the APPEARING SURFACE OF THE CONTACT (this is not quite what you think))))), if we have a contact area of ​​at least 100 m2 and DO NOT press it, then the transition resistance will be great .., but if you put a little pressure on such contacts, .., due to the LARGE area, we will have MORE number of contact points than in a contact with an area of ​​1 mm2 at the same pressure

I once mentioned that one and the same theory can be interpreted in completely different ways ....

The apparent contact surface is the common surface of the bodies on which the contact is made. It differs from the actual contact surface (a platform of deformed microprotrusions that perceive the forces of contact pressing). This is what I wrote in the article. What am I wrong here and how can I interpret it differently?

Then, applying sufficient force to the contact area of ​​10 mm is much easier than to the area of ​​100 m. Therefore, even under equal conditions, in the second case we will get contact with a large transition resistance.

And where in which document, in which book is there an instruction not to use contacts in which the contact area is less than or equal to the cross-sectional area of ​​the connected conductors?

to be honest ... I don’t know such a document .., maybe it doesn’t exist ..., just like there is no document .. obliging you to fasten your car to the ground so that it does not fly up and fly into space at night, on the full moon. ..))))
In principle, both in the case of contacts and in the case of a car, it is clear that this is nowhere to be prescribed. and so everything is clear))))
take a WHOLE conductor with a cross section of 4 mm2, draw a transverse secant plane (mentally) .., and divide it into 2 pieces left and right .., in this case, two pieces of wire are connected to each other through an imaginary secant plane through a contact surface of 4 mm2, pay attention to that it is an IDEAL contact surface, i.e. they are connected at the molecular level over the entire contact area of ​​4mm2 .....
Now we cut this conductor and connect it through a relay whose contact surface is 2mm2
in view of the IDEA of our physical world ..., the contacts in the relay do not lie adjacent to each other, but only with some contact points (in accordance with the book)))), but even if we PERFECTLY press the contact to the contacts ... after polishing it and silvering))), we will EVERYTHING get the contact area (2mm2) less than the cross-section of the conductor (4mm2), which means that more heat will be released in this place than on the wire itself in proportion to the square of the current ... and when the cable is fully loaded in terms of power. .., in this place the contact will simply burn off ...
therefore, in order to equalize the contact transition resistance with the cable resistance, in our REAL world, the contact transition area should be LARGER than the cable section ... because in reality, even when using a 4 mm2 contact pad, the transition area will be slightly smaller ...

this is understandable as a white day)))))

This dispute can only be resolved by real testing. It is necessary to take the Vago terminal block and the CO block, you can solder the twist. It is better not to take welding, since it is clear and hard to compete with any other contact connection with welded contacts. The wires must be of the same cross section and pass the same currents, i.e. contacts should be in the same conditions. It is necessary to measure the voltage drop across the contact at the time of installation and after half a year (year). By the voltage drop, one can judge the transition resistance of the contact and its change in time. Otherwise, all the numerous disputes on the sites and forums around the Vago terminal blocks are all a transfusion from empty to empty. Only real tests are needed.

By applying sufficient contact pressure to the contact point to the quality-prepared stripped wires, a stably low transition resistance can be achieved even with the cross-sectional area of ​​the contacts equal to the cross-sectional area of ​​the conductors.

I agree with Pavel Baranov on the need for testing. And then, no matter how much I asked, no one can even send a dozen photos of melted terminal blocks with a flat spring clip, and there are a lot of discussions about how scary such terminal blocks are to use. Those who are not afraid to use for a long time and everything works fine for them. I also support that welding is an ideal way to create electrical contact with a minimum transient resistance, but it is not always convenient to use welding, you need special equipment, and you need to be able to do everything correctly. Terminal blocks with a flat spring clamp are an order of magnitude easier in both installation and operation. Naturally, they are not always worth applying. In particularly difficult and critical cases, you can think about welding. But there are options when you can not complicate everything, and while in advertising, "connected and forgot."

ehhh

so that the contact does not heat up .... it is necessary not to have a "sufficiently low" resistance, but a resistance lower than or equal to the specific resistance of the conductor, and if the contact area is equal to the cross section of the conductor this cannot be achieved, it is written in your book))))))) I already quoted)))
and in view of the fact that it is difficult to ensure ideal conditions for reliable contacting over a long period of time ... , temperature, environment), the resistance remains lower than the cable resistivity ...

the fact that the transition resistance depends on the area, and testing is not necessary .., I brought dofig arguments ..,)))))) even one example with a contactor puts all the points on i)))
but the debate about the reliability of the VAGO terminal blocks ...., then of course the verification would not hurt)))
it is possible to take a wire in the apartment panel from the introductory machine, cut into pieces and garland several VAGO terminal blocks, and other types of connections ..., everything will be in the same conditions))), under the same load .., the infrared thermometer wasn’t disturbed to remove the temperature of the contacts ....,)))

If you take the WAGO terminal block (I recommend using such terminal blocks only for connecting copper conductors), then its design allows you to stably keep the transition resistance at a low level without increasing the contact surface due to the force of pressing the spring and tin-lead coating of the contact point.

It is only necessary to increase the contact area in those cases when it is not possible to stop the oxidation process in time, therefore, oxidation causes local overheating, and already an increase in temperature leads to an increase in transient resistance. That is, I still hold the opinion that in the case of terminal blocks with a spring-loaded clamp, there is no need to increase the contact area beyond what the terminal block design provides, since in the absence of overheating at the contact point, the contact resistance of the contact does not depend on its size (this the formula from the article and the theory according to which the contact is considered as two planes with microprotrushes in the form of pyramids and tubercles) proves.

The other day, I’ll somehow get together and write an article in continuation of the thoughts presented here. You just need to think a little and systematize.

the fourth part of the epic about the transition resistance of the contact is coming)))

I think in the article, it is necessary to confirm or refute the example with the contactor in which the contact resistance of the contacts decreases depending on the number of contacts i.e. total contact area .. which contradicts the theory from the book
(you can even call this subsection, the errors of some users)))))

In addition to the terminal blocks discussed here, their advantages and disadvantages, there is also one-piece electrical connections in accordance with GOST 17441-82. They also have transitional contact resistance, and a struggle is also underway to reduce the transitional resistance. GOST is rigid, clearly defines the requirements for indicators that will ensure safe operation for the overhaul period.
We tried everything. They did mathematical calculations using the above formulas.Used spraying, copper-aluminum adapter plates and gaskets, gallium-indium liquid gaskets, lubricants such as lithol, cyatim, petroleum jelly. The ideal method was not found. How many ways, so many opinions. In 1989, specialized lubricants appeared on the market. The principle of operation, which boils down to filling micro- and macro-voids with metal powders. The transition resistance can be reduced by a factor of 2 or more. The problems are different. There is such a concept in Russian practice - overload. And this is a sharp heating to temperatures at which the melting and destruction of the contacts occurs. Many greases do not withstand such heating, burn out, create an additional source of heating. An avalanche-like process begins.

There is no clear and unified understanding of these points, as practice shows, now. For use, low grade greases are purchased. The purchase of lubricants was left at the mercy of financial institutions with little understanding of the purpose of procurement. The main role begins to play the price. The lower, the more likely to sell. For the consequences of these structures are not responsible. T.ch. and these points can be discussed

Good day to all!
I carefully read this discussion and decided to express my thoughts.
In my opinion, the above example with a contactor is not entirely correct, since with an increase in the number of contacts, the number of CONTACT POINTS increases, but not their area. After all, the contact of the starter, relay (etc. of similar devices) is, by virtue of its design, PRECISE in essence, this should be the basis. In general, the contact surface area in the case of movable contacts (i.e., when it is impossible to ensure forced pressing) is a very, very conditional value, and the quality of the contact material and the quality of surface treatment come to the fore here.
Further, to make any comparisons between the twist connection (with subsequent welding) and any terminal strip, it is the same if you compare a healthy person with a legless one. Which has a prosthesis instead of its leg (even if it was ideally made using modern nanotechnology). It is clear that the best contact is the missing contact :), but if it is impossible to do without it, then a good high-quality terminal block (for example from WEIDMULLER) is far from the worst solution. Therefore, attacks on WAGO are completely incomprehensible to me - spring terminals have long won their place in the sun for certain applications. The aforementioned WM also does not neglect them for completely industrial applications, and not “hoses with suckers” work there at all :))
According to the connection methods, it is clear that twisting with welding "drives" here (subject to the technology of this procedure). But about soldering or tinning, alas. Not so clear. Firstly, at least two contact transitions are added. Secondly, a lot depends on the composition of the solder (lead, tin, silver, etc.), flux, compliance with temperature conditions, etc. It is not accidental that in many applications for high current contacts the use of soldering (and even tinning! ) - only a high-quality crimp tip under the screw clamp.
In general, not everything is as clear as it seems, it all depends on specific applications.

THEORY IS GOOD. School, factory, army, factory, institute ... A lot of theory and, at the same time, a lot of practice, which for exactly half a century now confirms that a correctly performed lay-up (twisting) + responsibility (conscience) of an electrician are a reliable connection. I feel the stones in my garden, but believe me - for 50 years there have been no complaints about me. You just need to correctly and accurately calculate the cross-sections of the conductors for a given load, check for heating, if necessary, and for a voltage drop. Of course, we are talking about the laydown only during installation in residential buildings and public buildings. Electrical installation of machines and other industrial.equipment is performed without twisting. )))

In your formula, the coefficient itself may also depend on the area, since it depends on the shape of the contact. The fact that it depends on the form of contact is mentioned in the textbook from which you most likely took the information. The textbook can be found in the “single window of access to educational resources” by typing in the search for the catalog “Electrical and Electronic Devices: A Training Manual” by E. Telmanova .. By the way, this textbook says the following: “the size of the total area will be equal to the sum sizes of individual sites ”- refers to contact sites. And further, “With the growth of the compression force, the growth in the size of the contact areas slows down,” talk about areas of contact, not about the area of ​​contact.

You can’t give links in the comments, so type in yandex "Science and Education: Assessing the Quality of Contact in a Cone Pair via Electrical Parameters". Go to the first link, look at the graph of the dependence of the transition resistance on the contact area. The larger the area, the less resistance.

How does contact resistance behave at low temperatures (approximately 77 K)? Are there any features?

I completely disagree with the arguments about the resistance of the oxide film of the aluminum compound (

Aluminum contacts in the air oxidize more intensely than copper. They are quickly broken by an alumina film, which is very stable and refractory and possesses such a film with a rather high resistance - of the order of 1012 ohm x cm.) It seems that the author does not really understand what a huge resistance it is and is not friends with elementary arithmetic

Aluminum contacts in the air oxidize more intensely than copper. They are quickly broken by a film of aluminum oxide, which is very stable and refractory and has such a film with a rather high resistance - of the order of 1012 ohm x cm. ????? I completely disagree with this ... it seems the author is not friends with arithmetic .... this is a huge resistance! It is not clear what he means.

In the case that interests me, the formula given in the article hung in the air. After all, where to get those parameters that are included in it? It is advisable to give a link to "numerous studies" or books on electrical appliances. And if the contact is not point? Or "not quite spotted"? - That is, the entire length of the conductor.

Actually, I have a practical question: if you parallelize two nichrome wires with a diameter of, say, 0.4 mm and a length of up to 10 cm (diameters and lengths may be different), twisting them into a "pigtail", then how will their equivalent resistance change - first " cold ", and then - after heating with a current of 10 A? I do not mean the school formula R || R = R / 2, but I am trying to rigorously justify that there is no point in taking into account the transition resistance in such a twist, especially after passing the current and, accordingly, oxidizing. In short, where to read that the equivalent resistance of such a twist will differ from R || R somewhere in the second or third digit? About this shows experience.