The best contact connection is the one with which the contact resistance forms a small value for a long time. Connecting contacts are an integral part of any electrical circuit, and since the stable operation of electrical devices and wiring depends on them, then it is necessary to understand what the contact resistance is, what it depends on and what standards of value exist today.
Content:
- The causes of the phenomenon
- What does the resistance depend on?
- Measurement technique
The causes of the phenomenon
Connecting contacts connect two or more conductors in an electrical circuit. A conductive contact is formed at the junction, as a result of which current flows from one area of the circuit to another.
Overlapping contacts will not make a good connection. This is due to the fact that the surface of the connecting elements is uneven and the contact is not carried out over their entire surface, but only at some points. Even if the surface is thoroughly sanded, minor depressions and bumps will still remain on it.
Some books on electrical devices provide a photo where the contact area is visible under a microscope and it is much smaller than the total contact area.
Due to the fact that the contacts have a small area, this gives a significant contact resistance for the passage of electric current. Transient contact resistance is such a value that occurs at the moment the current passes from one surface to another.
In order to connect the contacts, various methods of pressing and fastening the conductors are used. Pressing is the force by which surfaces interact with each other. Mounting methods are:
- Mechanical connection. Various bolts and terminal blocks.
- The contact occurs due to the elastic pressure of the springs.
- Soldering, welding and crimping.
What does the resistance depend on?
When two conductors touch, the total area and number of pads depends both on the level of pressing force and on the strength of the material itself. That is, the transient contact resistance depends on the pressing force: the greater the force, the less it will be. Only the pressure should be increased to a certain figure, since at high mechanical loads, the contact resistance practically does not change. And such strong pressure can lead to deformation, as a result of which the contacts can break.
Also, the contact resistance of the contacts significantly depends on the temperature. When an electrical voltage passes through the conductors and their surfaces, the contacts heat up and the temperature rises, as a result of which the transfer resistance increases. Only this increase occurs more slowly than the increase in the resistivity of the material of the structure, since, when heated, the material loses its hardness.
The more the device heats up, the more intensive is the oxidation process, which in turn also affects the increase in the transient resistance. So, for example, copper wire is actively oxidized at temperatures from 70 ° C. At normal room temperature (about 20 ° C), copper oxidizes slightly and the forming oxidizing film is easily destroyed by compression.
The picture shows the dependence of the value on pressing (A) and temperature (B):
Aluminum oxidizes much faster at room temperature and the oxidizing film that forms is more stable and has a high resistance. Based on this, we can conclude that it is difficult to achieve normal contact with stable values during use of the device. Therefore, the use of aluminum conductors in electrics is dangerous.
In order to obtain stable and durable connecting contacts, it is necessary to properly clean and process the surface of the cable itself. Also create enough pressure. If everything is done correctly (regardless of which method was used to connect), the meter will indicate a stable value.
Measurement technique
It is necessary to measure the contact resistance at the specified values of current and voltage. How to determine this value? Conventional devices in the form of an ohmmeter or a tester will not work, since they pass through an electrical circuit at a voltage of up to 2 V currents of 0.5-1 mA. With such small loads, most powerful devices cannot provide the passport data for this phenomenon. Its definition is possible if you collect the usual measurement scheme. It is provided below:
Ballast resistance (R) suspends the current through the contacts, and decreasing the voltage across them at a certain current makes it possible to determine the contact resistance by the formula. When selecting elements in the circuit, it is necessary to enter the currents during testing, which are provided by the table below (the data are indicated taking into account the norm, PUE and GOST):
| Working current of relay contacts, A | Contact resistance test current, mA |
| 0,01 – 0,1 | 10 |
| 0,1 – 1 | 100 |
| > 1 | 1000 |
Instead of the above measurement scheme, you can use special devices, for example Microohmmeter F4104-M1 or imported analogue C.A.10. How to measure this value is shown in the video:
It is important to note that the test results depend on how dirty the contacts are and what their temperature is. Therefore, when making measurements, it is necessary to choose such a current and voltage that will correspond to certain conditions for using the relay in the specified circuit.
What should be the contact resistance? The maximum allowable value of this value is normalized and equals 0.05 Ohm.
When establishing large loads, do not forget about the initial high contact resistance. After switching, it is significantly reduced by electrical cleaning. If the device is used in signal circuits, this value can be neglected.
That's all that I wanted to tell you about what the contact resistance of the contacts is, what is its permissible value and how the measurements of the value are performed. We hope the information was useful and interesting for you!
It will be useful to know:
- How to measure the insulation resistance of a cable
- Ways to connect electrical wires
- How to identify a short circuit in the network
PTEEP obliges to make measurements: 1. Measurement of transient resistances of connections of ground electrodes with grounded elements (Appendix 3, p. 26.1). 2. Transitional resistance of the contact between the grounded installation and its element (Appendix 3, p. 28.6). In both cases, the resistance should be no more than 0.05 ohms. How measurements can be taken in practice. Thanks in advance
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