Design options for fuses and circuit breakers in the design

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Development of distribution line protection and protection appliances

Advantages and disadvantages of fuses and circuit breakers

Distribution line characteristics and protection appliance selection

Fuse, circuit breaker selection scheme

Advice on the proper application of fuses

1. Proposal of the problem

"What ages, and also use fuses!" "The fuses are out of date!" This seems to make sense, but it is a technical problem that is actually faced. Really, in the design of low-voltage distribution systems for industrial buildings, civil buildings and outdoor installations, there have been fewer and fewer low-voltage fuses in most of the past decade, and most even use low-voltage circuit breakers in a uniform manner; The installation of fuses in the low-voltage distribution box is also greatly reduced. In the application of protective equipment for low-voltage power distribution systems, the author believes that this is an incorrect or incomplete understanding. Therefore, it is necessary to make some comparisons and analysis between the fuses and the circuit breakers (hereinafter referred to as the low voltage), so that the two protective appliances can be selected more correctly and reasonably.

2. Distribution line protection and protection of electrical appliances

2.1 Distribution line protection requirements

Low-voltage distribution lines, in order to protect against personal safety (electric shock caused by indirect contact) in the event of a fault (such as overload, short-circuit and ground fault), or if the line is overheated and cause damage or even cause an electrical fire, the distribution line should be necessary. Protective measures to protect the safety of the line and the safety of electricity. This protection is particularly important because low-voltage distribution lines are spread over a wide range of buildings, and there is a high probability of failure, and there are a large number of non-professionals who may be in contact. The most important protection measure is to install protective appliances at all levels of distribution lines to ensure that the faulty circuit can be effectively disconnected in the event of a circuit failure. These protections shall comply with the relevant provisions of GB50054-95 "Design Specification for Low Voltage Distribution". To this end, the lines at all levels must not only set up protective appliances, but also must correctly set their parameters to ensure that the faults are reliably cut off within the specified time. It is also required to selectively cut off the circuit, that is, to protect the protective device that is closest to the fault point. And the protection device of its superior does not operate, so that the range of the cut-off circuit is minimized.

2.2 Types and development of protective appliances

There are two main types of protective appliances: one is a circuit breaker and the other is a fuse. There are many types of circuit breakers. From the protection characteristics related to this article, there are two types of non-selective and selective circuit breakers; in addition, there are circuit breakers with leakage protection. These protective appliances have their own characteristics, and naturally have their inadequacies. They should be selected according to the specific conditions and requirements of the power distribution system. It is not easy to use advanced or backward evaluation.

In today's world, especially in some developed countries, circuit breaker products and technologies are developing very rapidly, and circuit breakers with newer and more perfect protection functions are being developed. In the past ten years, it has been updated almost every ten years, until the introduction of intelligent circuit breakers with perfect functions and communication modules, providing excellent protection appliances for distribution line protection. In the past 20 years, China's electrical industry has developed very rapidly. Circuit breaker products have followed the trend of international advanced technology and developed a variety of intelligent circuit breakers to provide a more complete protection function for distribution lines. However, in some developed countries in Europe and America, fuses have not been eliminated due to the rapid development of circuit breakers, and fuses have not been regarded as backward or outdated products. It is known that in Germany, France and other countries such as Siemens, Gaomei and other electrical companies, not only continue to produce fuses, but also continue to develop new products, technology continues to improve. From these aspects, the circuit breaker is an advanced protection device, and the fuse is not an outdated or backward product. It should be said that the two complement each other and each has its own application range.

3. Comparison of fuses and circuit breakers

The protection performance and other characteristics of the fuse and the circuit breaker are compared. The circuit breaker is described in two categories: non-selective type and selective type.

3.1 fuse

(1) The main advantages and features of the fuse

1 selectivity is good. As long as the rated current of the fuse of the upper and lower fuses meets the requirements of 1.6:1 for the overcurrent selection ratio specified by the national standard and the IEC standard, that is, the rated current of the upper fuse is not less than 1.6 times of the value of the lower stage, it is regarded as the upper and lower levels. Selectively cut off the fault current;

2 good current limiting characteristics and high breaking capacity;

3 relatively small size;

4 the price is cheaper.

(2) The main shortcomings and weaknesses of fuses

1 The fuse must be replaced after the fuse is blown;

2 The protection function is single, only one period of overcurrent reverse time characteristic, such as overload, short circuit and ground fault;

3 When a phase fuse occurs, the three-phase motor will lead to the adverse consequences of the two-phase operation. Of course, it can be compensated by a fuse with an alarm signal, and one phase can be disconnected to disconnect the three phases;

4 can not achieve remote control, it is possible to combine with electric knife switch and switch.

3.2 non-selective circuit breaker

(1) Main advantages and features

1 After the fault is disconnected, it can be reset manually, without having to replace the components, unless it needs to be repaired after cutting off the large short-circuit current;

2 Two-stage protection function of long-delay release and instantaneous current release with inverse time characteristic, which are used for overload and short-circuit protection respectively, each performing their duties;

3 Remote control is possible with the electric operating mechanism.

(2) Major shortcomings and weaknesses

1 It is difficult to achieve selective disconnection between the upper and lower non-selective circuit breakers. When the fault current is large, it is easy to cause the upper and lower circuit breakers to be instantaneously disconnected;

2 the relative price is slightly higher;

The breaking capacity of the 3 parts of the circuit breaker is small. If the circuit breaker with a small rated current is installed close to the position of the large-capacity transformer, the breaking capacity will be insufficient. Products with high breaking capacity can now be met, but at a higher price.

3.3 Selective Circuit Breaker

(1) Main advantages and features

1 has the advantages of the above non-selective circuit breaker;

2 With a variety of protection functions, long delay, transient, short delay and ground fault (including zero-sequence current and residual current protection) protection, respectively, to achieve overload, open circuit delay, large short-circuit current instantaneous action and ground fault protection, The protection sensitivity is extremely high, the adjustment of various parameters is convenient, and it is easy to meet various protection requirements of the distribution line. In addition, it can have cascade protection function and has better selective action performance;

3 Today's products have many intelligent features. In addition to the protection function, there are power measurement, fault record, and communication excuses to realize centralized monitoring and management of power distribution devices and systems.

(2) Main issues

1 The price is very high, so it should only be used at the head line of the distribution line and the sub-main line of the special important place;

2 size is large.

4. Characteristics of distribution lines and selection of protective appliances

4.1 Characteristics of distribution lines and requirements for protective appliances

(1) Power distribution systems usually have two types of trunk type and radial type, and a hybrid system of the two. Generally, the trunk line system has a long trunk line and requires high protection electric appliances, and often requires high-grade protection appliances, that is, a selection type circuit breaker.

(2) Distribution lines can be divided into three types: main line, sub-main line and end line. The main line is the feeder from the low-voltage power distribution panel of the substation. When it is a trunk line, when the trunk capacity is large, the main line is usually used.

(3) The end line is directly connected to the electrical equipment. When the short circuit or ground fault occurs, it is required to cut off the circuit as soon as possible or even instantaneously, and there is no selectivity requirement.

4.2 Distribution line fault characteristics

(1) Short circuit and ground faults occur in the end loops, accounting for more than 90%, especially in the socket circuit, because plugs, sockets and mobile appliances and their wires and connectors are more likely to fail;

(2) In terms of the type of fault, there are many ground faults and fewer phase-to-phase shorts, the former accounting for 80%~90%;

(3) The end circuit of the motor and other equipment is usually overloaded, the short-circuit fault is less, the overload of the motor accounts for more than 80%, and the overload is protected by the thermal relay, which will not make the fuse and circuit breaker operate.

4.3 Protection equipment selection plan

According to the characteristics of the circuit faults described above and the comparison of the performance of several protective appliances, the recommendations for the protection of electrical appliances are proposed. This article only discusses the selection of fuses and circuit breakers, and does not involve the setting of protective electrical parameters.

(1) The following positions should be selected

2 The mother trunk line drawn from the low-voltage power distribution panel of the substation, or the protection of the trunk line that has a large current capacity (such as 500A or more);

3 The low-voltage power distribution panel of the important place has a large current capacity (such as 300A or more) for radial line protection.

(2) Non-selectable circuit breakers can be selected at the following positions

1 end loop protection;

2 The protection of the upper-level sub-mains near the end loop, when there are not many power supply devices, and the impact of accidental power failure is not too large.

(3) Fuse should be used in the following locations

1 Protection of the main lines in the middle of the distribution line;

2 The protection of the main line of the substation low-voltage power distribution panel with a small current capacity (such as below 300A);

3 It can also be used as the protection of the motor end circuit when it is used, but it is not suitable to use gG type fuse (ie full-range breaking, general-purpose fuse), but should use aM type fuse (ie partial range breaking, motor) Protective fuse). Because the rated current of the fuse used in the aM type fuse is much smaller than that of the gG type, it is beneficial to improve the protection sensitivity and avoid excessive selection of the upper protection device.

5. Suggestions on the rational application of fuses

(1) Correctly understand the role and status of fuses in distribution line protection. Fuses and circuit breakers have their own characteristics and function under different conditions.

(2) Revise the fuse product standard. The current national standards for fuses GB13539.1-92 and GB13539.2-92 were promulgated and implemented in 1992, which is equivalent to the IEC269 standard. The IEC has revised the standard in 1998 and 1999, and it is recommended to revise the national standard in time. The fuse standard keeps up with the international advanced level.

(3) Strive to improve the level of fuse products. Due to some incorrect understanding of fuse applications and other reasons, the product market has been sluggish in recent years, and some companies have made less technological advances. It is hoped that according to the new IEC standards and the newly revised national standard, if necessary, foreign advanced technology will be introduced to produce higher-level and more varieties of products, such as aM series fuses.

(4) Low-voltage power distribution complete sets and distribution boxes, there should be a certain number of fuse solutions for power distribution designers and users to choose.

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