As the most basic electrical protection switch component, circuit breaker is widely used and has a large social reserve. It has become the most important, most commonly used and most basic protection device in modern production and life. Therefore, it can ensure the circuit breaker can reliably protect the circuit. And the security of the load is especially important.
For a long time, there has not been an exact safety data and calculation formula for the circuit breaker load current. The national standard GB14048.2 circuit breaker is the inspection standard for the rated current of the circuit breaker. Under what conditions the load current is within the safety protection range of the circuit breaker is a cumbersome calculation problem for the professional technicians, and is a very difficult problem for non-professional technicians and operators. It is also difficult to find corresponding calculation formulas in related technical literatures at home and abroad, and no relevant empirical data is available. Therefore, when designing or selecting and using a circuit breaker, the rated current of the circuit breaker is selected according to the expected maximum load current, or the rated current of the circuit breaker is selected according to the short-circuit breaking capacity, and even some people avoid the frequent tripping phenomenon. The rated current of the circuit breaker is chosen to be very large, or the setting current of the adjustable trip unit is adjusted very high, far higher than the expected maximum load current. Circuit breakers that cause many occasions are not within the scope of safety protection lines and loads from design to use. To be precise, the circuit breaker does not provide any protection for the line and load at all. When the line and load are in an unprotected state, when the line or load is overloaded or short-circuited, accidents and fires will inevitably occur, and even people with heavy property losses will be killed or injured.
Since people choose the rated current of the circuit breaker or set the rated current of the circuit breaker of the adjustable trip unit, there is no exact current range that can be quantified, and there is no formula for accurately calculating the safe load current of the circuit breaker, which causes a lot of People think that as long as the circuit breaker is installed, the safety of the line and the load can be protected. Even professionals in many industries have such knowledge, so that the actual rated load current is not within the safety protection range of the circuit breaker, and the very important protection function is lost. It even caused major technical errors and after-effects.
By summarizing many years of practical work experience, analysis and experiment, with reference to the national circuit breaker standard GB14048.2 and the international standard idt IEC 60947-2:1997 circuit breaker, the author summarizes the calculation of the safe load current for the circuit breaker. Formula, usually only calculate the minimum protection limit load current according to the formula, according to the calculation result of this formula to select or set the rated current of the circuit breaker, you can play the safety protection line and load. The maximum limit current of the circuit breaker generally does not exceed the rated current of the circuit breaker.
1. Inverse time overload protection
According to the national circuit breaker standard GB14048.2, the anti-time limit overload protection disconnection characteristic of the distribution protection type circuit breaker agrees that the non-operating current is 1.05 times of the rated current or the setting current, and the agreed operating current is 1.3 times of the rated current or the setting current.
Reverse time-off characteristics of distribution protection type circuit breakers (see table below)
The following table shows the inverse time-breaking characteristics of the distribution protection type circuit breaker.
Set the current multiplier by the current name.
Ir≤63A Ir>63A
Conventional non-operating current 1.05Ir ≥1 ≥2
The agreed action current is 1.30Ir <1 <2
The derived circuit breaker inverse time limit overload protection safe load current calculation formula is as follows:
The minimum limit load current is greater than: the agreed non-operating current divided by the agreed operating current, and multiplied by the rated current of the circuit breaker, ie
1.3IL>1.05Ir;IL>1.05/1.3Ir; ie IL>0.81Ir
The highest ultimate load current is: the agreed non-operating current of the circuit breaker, ie ≤1.05Ir
Note: Because if it is greater than the agreed current of the circuit breaker, the inverse time protection will start to control and finally complete the tripping action.
According to the inverse time overload protection characteristic of the above circuit breaker, the minimum limit current IL can be calculated as: The agreed non-operating current is divided by the agreed operating current, and then multiplied by the rated current Ir of the circuit breaker. The agreement of the disconnection characteristic does not operate the current than the agreed action current, and reflects the ratio of the agreed non-operating current to the agreed operating current. This ratio is the minimum limit range of the tripping action, and is multiplied by the rated current Ir of the circuit breaker as the scale. That is, the minimum limit tripping current IL of the circuit breaker is obtained, and the rated current of the load device is only greater than the minimum limit tripping current IL of the circuit breaker, and the circuit breaker has the corresponding protection performance for protecting the line and the load. which is
1.3IL>1.05Ir;IL>1.05/1.3Ir; ie IL>0.81Ir
That is to say: when the rated current of the load device is less than 81% of the rated current of the circuit breaker, the circuit breaker has lost the normal overload protection function for the load device. Because even if the actual current of the load device has exceeded 1.3 times its rated current IL, it still has not reached or exceeded the rated current of the circuit breaker or 1.05 times of the setting current, and the circuit breaker is still within the range of the agreed non-tripping current. Therefore, the circuit breaker does not enter the trip protection state.
Where: Ir is the rated current of the circuit breaker, and IL is the rated current of the line or electrical equipment.
For example, a circuit breaker with a rated current of 100A is calculated according to the formula IL>1.05/1.3Ir. The minimum limit current of the load device is 81A. For the circuit breaker trip protection action, the minimum tripping action current is greater than the rated current. The current 100A is 1.05 times, that is, 105A.
The following check whether the minimum limit current 81A of the load device is 1.3 times Ir is greater than 105A, 81A×1.3=105.3A, which is just greater than the minimum limit current 105A of the circuit breaker trip protection action.
2, timing limit short circuit protection characteristics
For the circuit breaker, the definite time short circuit protection disconnection characteristic according to the national standard GB14048.2, 8.3.3.1.2, the short circuit release under the short circuit condition (see 4.7.1) should be short-circuited in the trip unit. Validation was performed at 80% and 120% of the current.
When the test current is equal to 80% of the short-circuit setting current, the trip unit should not operate.
When the test current is equal to 120% of the short-circuit setting current, the trip unit shall operate:
- For a momentary trip, it should operate within 0.2 s;
- For time-delay releases, operate at intervals equal to twice the delay time specified by the manufacturer.
In addition, the time-limited trip unit shall comply with the requirements of 8.3.3.1.4.
The derived circuit breaker timing limit short circuit protection safe load current calculation formula is as follows:
The minimum limit load current is greater than or equal to: the agreed non-operating current divided by the agreed operating current, and multiplied by the rated current of the circuit breaker, ie
IL>0.8n/1.2nIr; IL>0.8/1.2Ir; IL>2/3Ir; IL>67%Ir
The maximum ultimate load current is: less than the agreed operating current of the circuit breaker, ie <1.2Ir
Note: Because if it is equal to or greater than the agreed action current of the circuit breaker, the time limit protection will be immediately controlled to complete the tripping action.
Where: n is a multiple of the setting current, Ir is the rated current of the circuit breaker, nIr is the setting current, usually an integer multiple of Ir, generally takes 2, 4, 6, 8, 10, 12, 20..., IL is The rated current of the line or load device.
For example, a circuit breaker with a rated current of 1,000A calculates the minimum limit current of the load device according to the formula IL>0.8/1.2Ir to be 667A, and the minimum tripping current is greater than the tripping protection action of the circuit breaker. The rated current is 0.8 times 1,000A, which is 800A.
The following check whether the minimum limit current 667A of the load device is 1.2 times Ir is greater than 800A, 667A × 1.2 = 800.4A, which is greater than the minimum limit current of the circuit breaker trip protection action 800A.
That is, when the rated current of the load device is less than 67% of the rated current of the circuit breaker, the circuit breaker has lost the short-circuit protection capability for the load device. Because at this time, even if the actual current of the load device has exceeded the overcurrent multiple of its rated current IL, the circuit breaker is still within the range of the agreed non-operating current, therefore, the circuit breaker will not enter the trip protection state, and the circuit breaker at this time Only the switching function, but no protection function, is extremely dangerous for the safety of transmission lines and electrical equipment!
Whether it is the inverse time overload protection characteristic or the time limit short circuit protection characteristic, between the minimum limit load current and the highest limit load current, it is the safe load current range of the circuit breaker, that is, the safety protection zone.
The minimum limit rated current of the load device is equal to the agreed non-operating current of the circuit breaker to compare the agreed action current, and then multiplied by the rated current of the circuit breaker, that is, the lowest limit rated current of the load device = the agreed non-operating current of the circuit breaker / the agreed operating current × rated current.
The overload protection range consists of the agreed non-operating current to the inverse-time agreed action current segment. The lowest limit rated current can also be calculated from the rated current compared to the actual overload protection range, which is nIL>1.05Ir; IL>Ir/(1.3-1.05) )
The short-circuit protection range is composed of the definite current from the non-operating current to the agreed action current. The lowest limit rated current can also be calculated by comparing the rated current with the actual short-circuit protection range.
IL>0.8n/1.2nIr; IL>0.8/1.2Ir; IL>2/3Ir; IL>67%Ir
If the rated current IL of the load is known, it is only necessary to change the above formula by calculating the maximum rated current of the circuit breaker. For the inverse time overload protection, the maximum rated current of the circuit breaker is: Ir=1/ IL=1 /0.81≈1.23 IL, which is 1.23 times the rated load current of the known load, and the minimum rated rated current of the circuit breaker is equal to the rated current IL of the known load;
For time-limited short-circuit protection, the maximum rated current rating of the circuit breaker is: Ir=1/IL=1/0.67≈1.49 IL, which is 1.49 times the rated load current of the known load. The minimum rated current of the circuit breaker is equal to the known load. Rated current IL.
The calculation formula of the rated current of the circuit breaker is based on the selection of the rated current of the circuit breaker according to the expected maximum load current, and the minimum protection limit current of the circuit breaker is given. The selection principle is that the rated current of the load is at the rated current of the circuit breaker to the minimum protection. At any value between the limiting currents, this is the basic principle and basis for selecting the size of the circuit breaker.
The minimum protection limit current of the circuit breaker calculated by the "circuit breaker rated current selection calculation formula", the rated current of the circuit breaker is selected or set according to the minimum protection limit current, and the minimum load current is also ensured during the safety protection action range of the circuit breaker. Within the circuit, the circuit breaker is truly safe.
Guangzhou Yunge Tianhong Electronic Technology Co., Ltd , https://www.e-cigarettesfactory.com