In general, we choose circuit breakers that meet the breaking capacity according to the expected short circuit current of the line, but the calculation of the expected short circuit current of the line is a very tedious work. This article will briefly introduce how to correctly select and use low voltage circuit breakers in low voltage systems.

Key words: low voltage distribution system

Circuit breaker short circuit

Undervoltage releaser

Introduction

Circuit breakers are widely used in low voltage distribution systems, and they are protective devices. In the design of low voltage distribution system, the selectivity of the circuit breaker should be paid attention to, and the rated current of the overcurrent tripping device of the circuit breaker is selected and adjusted to ensure the full play of the overcurrent release. When the ambient temperature is greater than or less than the calibration temperature, the manufacturer should adjust the temperature and current carrying capacity correction coefficient provided by the manufacturer. The rated current value of the low voltage circuit breaker.

1, several current parameters of the circuit breaker

The rated current In of a circuit breaker is the current that the releaser can pass for a long time, that is, the rated current of the release.

The rated current of the circuit breaker shell is Inm, which is expressed in the frame of the same basic geometry and the structure similar to the structure or the maximum tripping current in the plastic shell. It determines the maximum rated current value of the releaser that can be installed. For example, DW15 - 1600 circuit breaker with rated current 800A, 1600

A is the shell frame rating of the circuit breaker, rated current Inm, and the rated current In of the circuit breaker is 800A.

Over current tripping device can be divided into overload release and short circuit (electromagnetic) tripping device, with long delay action current (Ir1), short delay action current (Ir2) and instantaneous action current (Ir3). If CHINT produces DW15 - 1600 Ir1, (0.7 - 1) In, Ir3 is (1~3) In, there is no short delay release; Changshu produces CW2 1600A.

Ir1 is (0.4-1) In, Ir2 is (0.4-15) In + OFF, short delay time is 0.1s-0.4s, a total of 4 stages, Ir3 is 1.6KA-35 kA + OFF.

The rated limit short circuit breaking capacity (Icu) of a circuit breaker: the breaking capacity of the circuit breaker to continue carrying its rated current capacity as specified in the prescribed test procedure, that is, the test voltage specified in the circuit breaker and the limit short circuit breakage value under other specified conditions, and the amount of the circuit breaker continues to carry it. The current is fixed.

The test procedure of ultimate short-circuit breaking capacity Icu is O - t - CO.

The specific test is to adjust the current of the circuit to the expected short circuit current value (such as 380V, 50kA), while the test button is not closed, the test circuit breaker is in the closing position, the test button is pressed, the circuit breaker passes the short circuit current of the 50kA, the circuit breaker immediately breaks (OPEN abbreviation O) and extinguishes the arc, the circuit breaker should be perfect and can reclose. Sluice. T is a batch time, usually 3min, when the line is in a hot state (the test button is still in the press state), and the circuit breaker is then switched on (CLOSE for short, C) and the immediate opening (O). (the connection test is to check the electric and thermal stability of the circuit breaker at the peak current and the wear of the dynamic and static contact with the bounce). This program is CO. Circuit breaker can completely break, extinguish the arc, without exceeding the specified damage, it is determined that its limit breaking capacity test is successful.

The rated operating short circuit breaking capacity Ics refers to a broken current value that is smaller than the rated limit short circuit breaking current under the prescribed test voltage and other specified conditions. The circuit breaker should have the capacity to continue carrying its rated current after the prescribed test program O - t - CO - t - CO action. It's a CO more than the Icu test program. Ics is a percentage of Icu. For the universal and plastic case circuit breaker,

Ics values are slightly different. The minimum allowable Ics for plastic case type is 25% Icu, the minimum allowable Ics for universal type is 50% Icu, and the circuit breakers for Ics = Icu are very few. The Ics of DW45 intelligent universal circuit breaker in China is 62.5% ~ 65%Icu.

2, current setting of circuit breaker

The rated current of low voltage circuit breaker overcurrent release shall not be less than the calculated current of the line, that is, In is equal to Ijs

(Ijs is the calculated current of the protected distribution line).

The long delay action current (Ir1) of the circuit breaker is mainly used to protect the overload. In general, Ir1 takes 1.1 times the calculated current of the circuit, i.e. Ir1 (> 1.1 *Ijs).

(Ijs for the calculated current of a protected distribution line)

Short delay overcurrent release current (Ir2) should avoid the peak current Ipk of the line.

Ir2 > 1.2 x (Ipk+Ijs')

Among them:

Ipk is to protect the starting current of the 1 largest motors in the circuit.

Ijs' calculates current for lines other than 1 motors with the largest starting current.

The setting time of short circuit delay action is usually divided into 0.1s, 0.2S, 0.3s and 0.4s. To ensure the selectivity of the protection device, the difference between the upper and lower 2 stage circuit breakers usually takes 0.1 to 0.2.S, action time should also meet the thermal effect requirements of protected lines.

The instantaneous action current Ir3 of the circuit breaker should meet Ir3 equal to 1.3 x (Ipk+).Ijs "), in order to meet the selectivity between circuit breakers that are protected, it is also required that Ir3 is more than 1.1 times the short circuit current of a circuit protected by the next stage circuit breaker.

3. Selectivity of overcurrent protection action of circuit breaker

In a distribution system protected by a circuit breaker, when an electrical fault occurs, the nearest circuit breaker moves from the fault point to remove the fault, while the other circuit breakers do not move, so that the fault caused by the fault is limited to the minimum range, so that other fault free power supply circuits can still maintain normal power supply, this is the circuit breaker. The required selectivity. The selectivity of the circuit breaker is very important in the design of the low voltage distribution system. It can bring convenience to the user and ensure the continuity of the work of the power supply circuit.

Circuit breakers used in low voltage distribution systems can be classified into two categories: selective and non selective according to their protection performance. The instantaneous and short delay characteristics of selective low voltage circuit breakers are suitable for short circuit operation, while the long delay characteristics are suitable for overload protection.

In the low voltage distribution system, the selective action between the upper and the lower two circuit breakers is guaranteed. The general upper circuit breaker adopts the selective circuit breaker. The next stage circuit breaker adopts the non selective circuit breaker or the selective circuit breaker, mainly using the delay action of the short delay tripping device or the time of the delay action.