Why Does the Contact Resistance Testing Need 100A or Higher?

Contact resistance refers to the resistance to current flow due to surface conditions and other reasons when the contact surfaces are in contact with each other (in the closed state of the device). This can happen between parts such as disconnector, circuit breakers, relays and switches. The contact resistance test can understand the connection quality of the line and its conductive characteristics, and avoid dangerous overheating of the contacts.

How does contact resistance come about?

Looking at the surface of the connector contacts under a microscope, although the gold plating is very smooth, 5-10 microns of bumps can still be observed. It will be seen that the contact of the mated pair of contacts is not the contact of the entire contact surface, but the contact of some points scattered on the contact surface. The actual contact surface can be divided into two parts; one is the real metal-to-metal direct contact part. That is, contact micro-points without transition resistance between metals, also known as contact spots, are formed after the interface film is destroyed by contact pressure or heat. The part accounts for about 5-10% of the actual contact area. The second is the parts that are in contact with each other after contaminating the film through the contact interface. Because any metal has a tendency to return to its original oxide state.

In fact, there is no really clean metal surface in the atmosphere. Even if a very clean metal surface is exposed to the atmosphere, an initial oxide film of several microns will soon be formed. For example, it only takes 2-3 minutes for copper, 30 minutes for nickel, and 2-3 seconds for aluminum to form an oxide film with a thickness of about 2 microns on the surface. Even the particularly stable precious metal gold will form an organic gas adsorption film on its surface due to its high surface energy. In addition, dust and the like in the atmosphere also form a deposited film on the surface of the contact. Therefore, any contact surface is a contaminated surface from a microscopic analysis.

Contact resistance test content
Two common inspections performed on circuit breaker contacts are visual inspection and contact resistance inspection.
1. Visual inspection includes checking the circuit breaker contacts for dents due to arcing and contact wear or deformation.
2. The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 100A, 200A and 300A, through the contacts and measuring the voltage drop across it. This test is done using a special contact resistance measuring instrument. Then, use Ohm's law to calculate the resistance value. The resistance value needs to be compared with the value given by the manufacturer. This value should also be compared with previous records.
These two tests need to be done together. Since there are cases where the contactor has good contact resistance but is still in a damaged state. Therefore, for a contactor to prove healthy, it needs to have good contact resistance and should clear the visual inspection test.

Why does the contact resistance testing need 100A or higher?

The electrical conductivity of the circuit breaker of the switch gear plays an important role in ensuring the safe operation of the switch. The quality of the conductive properties can be reflected by the size of the conductive contact resistance. Therefore, both IEC standards and manufacturers specify that the contact resistance should be measured and that the contact resistance of the various types of switches be clearly defined. In the past, dual-arm bridges were used. Due to the voltage of the bridge, the current is relatively small. When there is impurity or surface oxidation between the moving and stationary contacts of the switch, the data dispersion of the test is large. At the same time, the bridge has poor anti-interference, so the standard stipulation must use more than 100A DC current for testing.

Purpose of Contact Resistance Test

The contacts in the circuit breaker need to be checked regularly to ensure that the circuit breaker is healthy and functioning properly. Poor or poor contact can lead to arcing, loss of phase, and even fire. This test is especially important for contacts that carry large amounts of current, such as switchgear busbars, where higher contact resistance results in lower ampacities and higher losses. Contact resistance tests are usually performed using a micro/milliohm meter or a low ohmmeter. The measurement of contact resistance helps to identify fretting corrosion of contacts and can diagnose and prevent contact corrosion. An increase in contact resistance can cause a high voltage drop in the system and needs to be controlled.

Other Related Articles:
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Hazards and Treatment of Excessive Contact Resistance of Circuit Breakers or HV Switches
How to measure the contact resistance without changing the circuit?
How to Correctly Test the Contact Resistance of HV Switchgear or Circuit Breaker?
Why Does Excessive Contact Resistance Occur In Electrical Secondary Circuits?
What is the Testing Checklist for 110kV/220kV Substation Acceptance and Maintenance Testing?

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