A3700 Series Circuit Breakers
A3700 series circuit breakers are widely used in power systems. They belong to the class of automatic devices for voltages up to 500 V (AC) and 220 V (DC) and are available for rated breaking currents from 50 to 600 A.
The machines are produced in five standard sizes: A3760, A3710, A3720, A3730, A3740. They are equipped with thermal, electromagnetic or combined releases. Execution without releases (non-automatic switch) is possible. The machines are made in single-pole, two-pole and three-pole versions and allow 10,000 switching operations at rated current and rated voltage.
The main technical characteristics of the circuit breakers are shown in table 1.
Section. 1. Main technical characteristics of A3700 automatic switches
Rice. 1. A3700 Series Circuit Breaker
In fig. 2 is a cross-sectional view of the A3700 series circuit breaker in the open state.
Rice. 2. Sectional view of the A3700 series circuit breaker
The machines are attached to the plastic base 1. All parts of the machine are covered with a cover 2, which protects the service personnel from contact with live parts.Fixed contacts 3 and movable contacts 4 (for each phase) are made of metal-ceramic materials based on silver and cadmium oxide to reduce wear.
Arc extinguishing, when the machine is turned off, is carried out using a grid of steel plates 11 mounted on a fiber frame 10 (in machines for currents over 600 A, in addition to the main ones, there are extinguishing contacts). To turn on the machine, the handle 5 must be moved to the down position while the lever 6 engages the release bar 7. When the handle 5 is moved up, the disconnect springs 8 are pulled up. go beyond the dead center, and the contacts 3 and 4 of the machine are closed, since the contact lever rotates around the axis 13 under the action of the levers 9.
With manual shutdown, handle 5 moves down. The springs 8 are stretched again and break the levers 9 in the other direction. Thus, the mechanism of the machine has an instant on and off.
The circuit breaker shown in fig. 2 has a combined version. Overload bimetallic plate 18, rationalized by current, bends and through the adjusting screw acts on the lever 14, which with the help of the toothed segment 15 releases the lower end of the lever 7. The latter rotates clockwise, releases the lever 6 and the machine is automatically switched off.
In the event of a short circuit, an electromagnetic release is triggered, which consists of a stationary magnetic circuit 17 with a current-carrying busbar and an armature passing through it 16. When the short-circuit current flows through the busbar, the armature retracts and by its thrust turns the levers 7 and 14 clockwise, releasing lever 6, then the circuit breaker is actuated.
The response time of the thermal release depends on the overload current: the higher the overload current, the shorter the response time (from 1 — 2 hours to fractions of a second). After releasing the thermal release, the bimetallic plate returns to its original position after 1 — 4 minutes. The electromagnetic release is triggered at short-circuit currents equal to (7 — 10)Az. The total trip time of the A3700 series circuit breaker is in the range of 15 to 30 ms.