WHAT IS CONTACT RESISTANCE
Contact resistance is the resistance to current flow due to surface condition and other causes, when contact are touching one another (in the closed condition of the devices)this can occur between contacts of :
- breaker
- contactors
- relays
- switches
- connectors
- other switching devices
contact resistance testing also known as ductor testing, measure the resistance of electrical connections,busbar sections or cable connections and so on.these can be connections between any two conductors for example cable connections or busbar sections.the instrumnet which is used to perform the dutor test is called an ohmmeter and since is function is to perform the ohmmeter is also known as ductor tester.
the ductor tester can be found in many variation such as micro mega and milli ohmmeters static resister tester or DLRO which stands for digital low resistance ohmmeter. is used to measure resistance in different application in electrical testing. this tester consist of a dc ammeter and few other components.is used to primarily to verify tat electrical connections are made properly and can detect the following problems.
- loose connectiions
- adequate tension on bolted joints
- eroded contact surface
- Contaminated or corroded contacts
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance, which is an inherent property, independent of the measurement method. This effect is often described by the term Electrical Contact Resistance or ECR and may vary with time, most often decreasing, in a process known as resistance creep. The idea of potential drop on the injection electrode was introduced by William Shockley to explain the difference between the experimental results and the model of gradual channel approximation. In addition to the term ECR, “Interface resistance”, “transitional resistance”, or just simply “correction term” are also used. The term “parasitic resistance” has been used as a more general term, where it is usually still assumed that the contact resistance has a major contribution.
why you need contact resistance test?
The contacts in the circuit breaker needs to checked periodically to ensure that the breaker is healthy and functional. Poorly maintained or damaged contacts can cause arcing, loosing phase, and even fire.
This test is especially important for contacts that carry large amounts of current (e.g. switchgear busbars) because higher contact resistance can lead to lower current carrying capacity and higher losses. Ductor testing is usually performed using a micro/milli-ohmmeter or low ohmmeter.
Measurement of the contact resistance helps in identification of fretting corrosion of contacts, and allows contact corrosion to be diagnosed and prevented. Increase in contact resistance can cause a high-voltage drop in the system, and that needs to be controlled.
What is Done During Contact Resistance Testing?
The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.
The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 100A, 200A and 300 A through the contacts and measuring the voltage drop across it. This test is done with a special contact resistance measuring instrument. Then, using Ohm’s law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records.
Typical Method for Contact Resistance Test
The four-wire (Kelvin) DC voltage drop is the typical method used by micro-ohmmeters for the contact resistance test, which ensures more accurate measurements by eliminating the own contact resistance and resistance of test leads.
It is very important in the contact resistance test to maintain consistent measurement conditions, to be able to compare with previous and future results for trending analysis. Therefore, when taking periodic measurements, the contact resistance test must be performed in the same position, with the same test leads (always with the calibrated cables supplied by the manufacturer), and in the same conditions, to be able to know when a joint, connection, weld or device will become unsafe
Conclusion
measurements of thermal conductivity are also subject to contact resistance, with particular significance in heat transport through granular media. Similarly, a drop in hydrostatic pressure (analogous to electrical voltage) occurs when fluid flow transitions from one channel to another.
Contact resistance tests provide information about how healthy the contacts are and their ability to handle their rated current.
The maximum contact resistance should be verified against manufacturers’ specifications. Rated current should not be exceeded and testing at 10% of the rated current is recommended.
The minimum DC test current should be used according to manufactures specification; however, the IEC and ANSI recommended levels are: 50 A IEC 60694 100 A ANSI.
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