Arc Extinguishing Method of Low Voltage Switchgear Contact

Release time：

2022-10-31 09:47

Arc Extinguishing Method of Low Voltage Switchgear Contact
1. Contact resistance of low-voltage switchgear contacts
The contact of low-voltage switchgear is an important part of the actuator. The contacts of low-voltage switchgear are used to make and break the circuit, so the required conductivity and thermal conductivity of the contacts are very good. Generally, the contact material is an alloy of copper, silver and nickel, and it is also formed by electroplating silver and nickel on the surface of copper contact.
The surface of copper is easy to oxidize. If only copper is used as the contact material, it will increase the contact resistance of the contact, and the loss and temperature of the contact will also increase. Therefore, in low voltage switchgear with small capacity such as intermediate relay, the contact is usually made of silver alloy, and its oxide film resistance is only one tenth of that of copper contact.
(1) Film resistance
Thin film resistance is an oxide film formed by natural oxidation of contact surfaces in the atmosphere. The resistance of the oxide film is tens to thousands of times higher than the resistance of the contact itself, and the conductivity is very poor. This oxide film resistance is called contact film resistance.
(2) Shrinkage resistance
Due to the roughness of the contact surface, the actual contact area of the contact is smaller than the contact cross-sectional area, which leads to the reduction of the effective conductive cross-section of the contact. When the current flows, there will be several conductive islands of current contraction. The resistance increased by this shrinkage phenomenon is called the shrinkage resistance of the contact.
(3) Contact wear
The wear of low-voltage electrical contacts includes electrical wear and mechanical wear.
The electric wear of contact is the loss of contact material caused by arc ablation during switching. The electric wear loss depends on the amount of electric charge passing through the contact gap and the contact material after the arc traction. Electrical wear is the main cause of contact material loss.
Mechanical wear in contact is the loss of contact materials caused by mechanical friction, which depends on the hardness of materials, contact pressure and contact sliding mode.
(4) Contact form of contact
There are three types of contact: point contact, line contact and surface contact.
Point contact can reduce the surface resistance of the contact due to its strong voltage per unit area, so point contact is often used in low current low-voltage switchgear. For example, auxiliary contact of contactor and contact of relay.
Line contact is accompanied by rolling friction between moving contact and static contact, which is conducive to removing the oxide film on the contact surface. Wire contact is usually used for frequent operation and high current. Examples are contactors and circuit breakers.
Generally, the contact material used for surface contact is alloy, which has the characteristics of low contact resistance, welding resistance, wear resistance and allowing large current to pass through. Surface contact is generally used for small and medium capacity contactors.
(5) Contact status
The contacts are divided into normally open (closed) contacts according to their original state. The initial state here refers to that the coil of low-voltage switchgear is not energized, or the switchgear is not under force. Low voltage switchgear is divided into main contact and auxiliary contact according to the breaking current of its contact. The main contact is used to disconnect the main circuit, allowing large current to pass through; The control circuit adopts auxiliary contact, and its breaking current is generally 5A.