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# How to measure the contact resistance without changing the circuit?

A new approach will solve it. This method is very useful for measuring contact resistance in complex mechanical assemblies. Contact resistance is defined as the ratio of the voltage across a contact to the current flowing through a pair of closed contacts. It conforms to Ohm's law. There is an interface between Metal 1 and Metal 2. Current I from the current source flows through this interface and can be read from the ammeter. The voltage drop on the interface can then be read from the voltmeter to U. The contact resistance value Rx can then be calculated.

Rx=U/I

Since the contact resistance varies with the environment and the passage of current, the measurement conditions should be close to the conditions of use. Accurate measurements must use four-terminal measurement techniques and thermoelectromotive force elimination techniques. This indirect measurement method can be used to measure contact resistance or loop resistance. It requires three test points, three steps, and three formulas. This method has proven to be correct and can also be used to calibrate loop resistance standards.

A typical method of contact resistance testing

The four-wire (Kelvin) DC voltage drop is a typical method of contact resistance testing with microohmmeters, which ensures more accurate measurements by eliminating its own contact resistance and the resistance of the test leads.

The contact resistance test uses two current connections for injection and two potential leads for voltage drop measurements; The voltage cable must be as close as possible to the connection to be tested and always be located within the circuit formed by the connected current leads.

Based on the measurement of the voltage drop, the microprocessor-controlled microohmmeter calculates the contact resistance while eliminating the errors that may arise from the thermal EMF  effect in the connection (thermal EMF is the small thermocouple voltage produced when two different metals are connected together) They will be added to the total voltage drop measured, if not by different methods (polarity reversal and averaging, direct measurement of thermal EMF). amplitude, etc.) subtract them from the measurement, which introduces error in the contact resistance test.

If a low resistance reading is obtained when testing the circuit breaker contact resistance using a low current, it is recommended to retest the contact at higher currents. Why would we benefit from using higher currents? Higher currents will be able to overcome connection problems and oxidation on the terminals, and under these conditions, lower currents may produce erroneous (higher) readings.

It is important to maintain consistent measurement conditions in contact resistance testing so that it can be compared with previous and future results for trend analysis. Therefore, when making periodic measurements, contact resistance tests must be performed in the same location, using the same test leads (always using the calibration cable provided by the manufacturer) and under the same conditions, so that it is possible to know when the connection, connection, soldering or equipment will become unsafe.

Handheld Contact Resistance Tester JYL: for high voltage switchgear with output current 100A and 50A optional.can be used in 1000kV substations, with accuracy 0.5%.