Primary contact of American standard medium voltage circuit breaker

IEEE C37.20.2 standard, primary contact of withdrawable circuit breaker for medium voltage metal armored switchgear.

Unlike domestic vacuum circuit breakers that use standard plum blossom contacts, American standard circuit breakers have various types of contacts. Due to the fact that the current transformers in American standard switchgear are directly fitted into the contact box, it is required that the outer diameter of the primary contact box be small, and the inner diameter of the compressed contact box be small. Therefore, the outer diameter of the primary contact should be small, especially for high current ones.

For example, the inner diameter of the current transformer is 168mm, the outer diameter of the contact box is 166mm, and the epoxy resin cast contact box has a maximum inner diameter of only 146mm (with a contact box wall thickness of 10mm).

A general 36 inch (914mm) wide 15kV switchgear with a phase spacing of 10 inches (254mm), regardless of the current size, the width size is the same. For small currents such as 1200A, a static contact with a diameter of 49 plum blossom contacts and an outer diameter of over 80 can be used, while a static contact with a diameter of 109 plum blossom contacts and an outer diameter of 160mm cannot be used.

Therefore, different manufacturers have their own unique technological designs. There are mainly differences such as contact fingers and contact springs.

There are two types of tactile fingers: sheet tactile fingers and block tactile fingers. The primary contact of the 3000A 63kA circuit breaker is a sheet-like contact finger with a wall thickness of 5mm and a total of 32 pieces.

The contact spring adopts a plate spring. When the leaf spring is under stress, the maximum stress usually occurs at the root of the spring (fixed end or maximum bending moment). The 1mm thick spring steel sheet can achieve a contact pressure of 100N per sheet under maximum deformation.

For contacts, pressure is very important, and each contact finger requires a contact pressure greater than 40N.

IEC version 3150A high current contacts are also fixed with threads and contact arms, unlike low current contacts which are tightened through four holes. 22 contact fingers, the diameter of the static contact is the same as that of a regular plum blossom contact, both being 109mm.

The conductive cross-sectional area is 1760mm2, which reduces the amount of copper used compared to 82 plum blossom contacts with a cross-sectional area of 3460mm2.

IEEE version 3000A has a static contact diameter of 2.75 inches and 69.8mm, with a reduced contact box diameter that can accommodate current transformers. Thickened touch arms for reducing heat generation and increasing heat dissipation.

There is also a type of block shaped contact finger in China, which uses leaf springs to provide contact pressure. The thickness of the contact finger is 4mm, and 24 pieces are used for 1250A. The distance between the moving contact and the contact arm is the same as that of the plum blossom contact. Due to the small movement of the contact, if the dynamic and static contacts are not well matched, the contact is prone to falling apart. Therefore, a clamp is generally used to fix it at the back, which can resist the force of poor alignment of the static contact.

Insulated clamps are used to support the contacts, and metal shielding covers are also used for the contacts. Although the static distance is reduced, it is beneficial for uniform electric field and reduces the maximum electric field strength. The insulation performance has been improved.

For American standard switchgear with small contact box diameters and large contact diameters that cannot be accessed, small plum blossom contacts are required. In addition, the contact arms and static contacts also need special heat dissipation area treatment. At the same time, due to the relatively low upper limit of temperature rise, the heating of circuit breaker contacts needs to be dissipated as much as possible to reduce temperature rise.

The miniaturized switchgear is 26 inches (660mm) wide, with a maximum current of 2000A, a short-time withstand current of 40kA, and a phase spacing of 7.28 inches 185mm. The inner diameter of the current transformer is 110mm, and the inner diameter of the contact box is only about 90mm. The extended contact finger of the plum blossom contact has an outer diameter of 65mm, a static contact diameter of 38mm, a contact finger wall thickness of 4.76mm, a length of 110mm, and 16 contact fingers. By extending the touch finger to increase the heat dissipation area, the heat generation and heat dissipation are balanced.

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