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Contact Resistance Of Low-voltage Switch Gear Contacts
Oct 11, 2017

Contact resistance of Low-voltage switch gear contacts

The contact of the Low-voltage switch gear is the most important part of the actuator. The contact head of the Low-voltage switch gear is used for connecting and splitting the power off, so the required contact electrical conductivity and thermal conductivity are very good. Usually the contact material is a copper, silver and nickel alloy material, also has on the copper contact surface electroplated silver and nickel composition.

The surface of copper is extremely easy to oxidize. If only copper is used as a contact material, it will increase the contact resistance of the contact, causing the loss and temperature of the contact to increase. Therefore, in the intermediate relays, such as small-capacity Low-voltage switch gear, the contact often uses silver alloy, its oxide film resistance is only one of the more than 10 points of copper contact.

(1) Film resistance

Membrane resistance is an oxide film produced by natural oxidation of contact surfaces in the atmosphere. The resistance of the oxide film is dozens of to a few thousand times more than the resistance of the contact itself, and the conductivity is very poor. This oxide film resistor is called the contact membrane resistor.

(2) Shrinkage resistance

Because of the roughness of the contact surface, the actual contact area of the contact is less than the area of the contact section, which leads to the decrease of the effective conductive section of the contact, and the phenomenon of some conductive islands with current contraction when current flows. This contraction phenomenon increases the resistance known as the contact's contraction resistance.

(3) Contact wear

The wear of low-voltage electrical contacts includes electrical wear and mechanical wear.

The electrical wear of the contact is due to the loss of the contact material caused by arc ablation during the process, and the electrical wear depends on the charge amount and the contact material of the contact gap after the arc is drawn. Electrical wear is the main cause of the loss of the contact material.

The mechanical wear of the contact is due to the loss of the contact material caused by mechanical friction, and the mechanical wear depends on the hardness of the material, the contact pressure and the sliding mode of the contact head.

(4) Contact form of contact

Contact forms of contacts are divided into three types: Point contact, line contact and surface contact.

Point contact because the unit area of the pressure is strong, can reduce the contact surface resistance, so point contact is often used in small current Low-voltage switch gear. For example the contactor's auxiliary contact and relay's contact.

The line contact is accompanied by the rolling friction between the moving and the static contact, which is helpful to remove the oxide film from the contact surface. Line contact is generally used to operate frequent and relatively large current occasions. such as contactors and circuit breakers.

The contact material is generally alloy, it has the characteristics of small contact resistance, welding resistance, abrasion resistance, allowing through larger current and so on. Surface contact is generally used for medium and small capacity contactors.

(5) The state of the contact

The contact head is divided into open (dynamic) contacts according to its original state. This refers to the original state of the Low-voltage switch  electrical equipment, or the electrical power of the switch.

The Low-voltage switch gear is divided into main contacts and auxiliary contacts according to the size of the breaking current of the contactor. The main contact is used for the opening of the main circuit, allowing the passage of a larger current, the auxiliary contact is used for the control loop, and the breaking current is generally 5 a.