Composition and importance of contact resistance of HV switches

The contact resistance of the high-voltage switch is mainly composed of three parts: concentrated resistance, film resistance and conductor resistance:

1. Concentrated resistance
The resistance shown by the contraction (or concentration) of the current line when the current passes through the actual contact surface. Call it concentrated resistance or shrink resistance.

2. Film resistance
Film resistance due to contact surface film and other contaminants. From the analysis of the contact surface state; the surface fouling film can be divided into a relatively solid film layer and a looser impurity contamination layer. Therefore, to be precise, the film resistance can also be called the interface resistance.

3. Conductor resistance

When actually measuring the contact resistance of the contacts of the electrical connector, it is all carried out at the lead-out end of the contact, so the actual measured contact resistance also includes the conductor resistance of the contacts outside the contact surface and the lead wire itself. The conductor resistance mainly depends on the electrical conductivity of the metal material itself, and its relationship with the ambient temperature can be characterized by a temperature coefficient.

For the convenience of distinction, the concentrated resistance plus the film resistance is called the real contact resistance. The actual measured resistance including the conductor resistance is called the total contact resistance.

In the actual measurement of contact resistance, a Contact Resistance Tester (milliohm meter) designed according to the principle of the Kelvin bridge four-terminal method is often used. The resistance R consists of the following three parts, which can be represented by the following formula:

R= RC + Rf + Rp, where: RC-concentrated resistance; Rf-film resistance; Rp-conductor resistance.

The purpose of the contact resistance test is to determine the resistance that occurs when current flows through the electrical contacts of the contact surfaces of the contacts. Excessive energy consumption and dangerous overheating of the contacts can occur when large currents flow through the high-resistance contacts. Low and stable contact resistance is required in many applications so that the voltage drop across the contacts does not affect the accuracy of circuit conditions.

As one of the most important characteristic indicators in the electrical parameters of switchgear, contact resistance is the key information for early warning of abnormal power supply and the most reliable indicator of power system performance and decay. In assessing reliability and predicting failure of most types of power connections, including circuit breakers, disconnectors, load switches, etc., the stability test of contact resistance is the most effective. The magnitude of the contact resistance directly affects the through-load rating of the circuit breaker. Temperature rise at operating current and dynamic and thermal stability under short-circuit conditions. The change of the contact resistance value will also directly affect the reliability of the circuit breaker and the safety of the circuit breaker operation. Specifically, if the circuit resistance of the circuit breaker increases for some reason, it will break the original thermal balance and increase the temperature rise of the contact part, thereby increasing the resistivity of the conductor and further increasing the heat generation. The operation of the circuit breaker under the condition of high current will cause oxidation of the contact surface, softening or aging of the supporting insulator, resulting in the failure of the circuit breaker, making it unable to work normally in the short-circuit state.