What is transient contact resistance and why it is so important that it was minimal

All of us are well aware that, according to PUE There are several ways to connect the conductors permitted, namely, crimping, welding, soldering or the use of squeezing. And, by and large, no matter what type of connection will be selected in a particular case. The most important thing to removably connect with minimal transition resistance. And why is it so important, and I'll tell you in this article.

What is transient contact resistance

So, let's see, what is a transitional contact resistance, and it is necessary to start with this pin connection.

Contact connection - a constructive unit, which is generated due to mechanical and electrical connection of two or more conductors belonging to the electric circuit.

At the point of contact of the two conductors creates electrical contact - a conductive connection, in which a current flows from one conductor to another.

And if we'll just attach two wires to each other, it does not provide a reliable contact. Since the actual contact wire is carried out not on the entire surface leaning against the plates, but only in few points.

And all due to the fact that the conductor contains microscopic pits and mounds and even meticulous machining (grinding) will not eliminate such irregularities.

It turns out that because of such a small contact surfaces in a given location will be quite a lot of resistance to current flow.

Resistance at the transition of current from one conductor to another, and was named "Contact resistance."

Definition. Transitional contact resistance - a resistance at the transition of current from one part to another.

The magnitude of such resistance can be found by the formula which has been obtained by experiment:

As you may have noticed, if you carefully examine the formula, it becomes obvious that the contact resistance is not directly dependent on the size of the contact surfaces. In this case, the transition resistance is much more important than the strength of the pressure (contact pressure).

Now let's see what the contact force

Contact pressure is called the force with which the pressure is a contact surface on the other.

The number of contact points starts to increase in proportion as the pressing force increases. Moreover, even with a small pressure deformation process of peaks and valleys greatly increases the number of contact points.

And for this reason, to create a strong connection using a variety of compression methods and bonding conductors:

1. The mechanical connection via bolted connections.

2. The use of springs for elastically pressing (Wago terminals).

3. Welding, soldering and crimping.

It turns out that the transient contact resistance, the smaller the more the press force, and therefore more real area of ​​contact between the conductors.

Note. At first glance it seems that the above statement is not submitted by agreement with the experimentally obtained formula, but it is actually perfectly consistent. After all, according to the formula we'll calculate the resistance at a specific point, but with an increase in the amount of pressure contact points increases, creating the point of contact more conditionally parallel resistances. As it is known, when the combined resistance of a parallel connection decreases.

But it should be noted that increasing the pressure (thereby reducing impedance) is possible only to a certain level. Do not allow plastic deformation of connected conductors, as this can lead to their destruction.

Also, this resistance depends on the temperature, because at heating conductor transient increases contact resistance. Yet it should be noted that when the temperature rises considerably faster changes resistivity of the material, whereby contact resistance decreases conversely.

It turns out that little heat is not so terrible thing that the temperature did not exceed certain limits.

The change in contact resistance with time

As they say the classics: "Nothing lasts forever." So things and reliably made contact. The minimum transition resistance it will have at the very beginning, and in the operation time, it may change, and that's why.

Strong influence on this resistance has temperature. Thus, even at a temperature of 20 degrees Celsius copper oxidizes, resulting in surface conductors formed on the oxide film, which is quite easy to destroy.

But if the wire temperature resulting from greatly increased load, or poor contact will increase to +70 degrees or more, the process of forming the oxide repeatedly accelerated film that will further increase the resistance and hence increase the heat that can lead to disastrous consequences.

But even worse things with aluminum, because it is a very active metal and the process of formation of the oxide film is much more intense. A formed film (as opposed to copper) is very stable and refractory. The resistance of aluminum oxide is 10 ^ 12 ohm * cm.

We conclude that the use of aluminum for performing, for example, home wiring, undesirable. After thus establish reliable contact connection with stable low contact resistance will be difficult.

And, summing up all vyshenapisannogo, I want to say that whatever you choose way of connecting wires, the most important thing that the contact was made strictly according to the rules and requirements, and whereas transitional contact resistance will be minimal, and the connection with proper periodic maintenance (if not welding service here is not necessary) will serve you for a long time and without a single accident.

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