What Is a Short Circuit?

A short circuit is a circuit or a portion of a circuit that is shorted. If the two ends of the load and the power supply are connected by wires, it is called a short circuit. The current provided by the power supply during the short circuit will be much larger than the current provided by the path. Generally, short circuits are not allowed. Or equipment.

Short circuit

That is, the current flows directly from the positive electrode to the negative electrode through the wire without passing through any consumer. Especially easy to burn out the power supply.

due to

Short-circuit capacity is a characteristic quantity that reflects the electrical performance of a power supply point in a power system. The short-circuit capacity is for a certain power supply point of the power system, and reflects some important properties of the point, such as the load capacity and voltage stability of the point, the strength of the connection between the point and the power system's power supply, When a short circuit occurs at this point, the level of the short circuit current, and the like. Secondly, the short-circuit capacity is also related to the capacity of the entire system. With the expansion of power system capacity,

Component damage

Short circuits are often caused by damaged insulation or careless wiring. For example, equipment insulation materials are aging, equipment defects caused by poor design, manufacturing, installation, maintenance, etc. develop into short circuits ^[2]

Generate large current

Sometimes large currents of tens of thousands or even tens of thousands of amperes are generated. As a result, a large amount of heat is generated, the equipment is damaged, and the arc will melt many components for a short time. At the same time, the generated current will also bring certain electromagnetic force, which will also damage the equipment. Can also cause significant

Checking the insulation of electrical equipment and wiring frequently is an important safety measure. In addition, in order to prevent the consequences caused by short-circuit accidents,

Not all short circuits are bad, and sometimes they need to be used for protection or work. Specific applications are as follows.

1 Application of short circuit in protective zero connection system (TN system )

In Figure 3, the equipment case is electrically and reliably connected to the N line in the line through a protective wire.

image 3

When the charged bodies collide (leakage), the protective wire plays the role of creating a short circuit. Current will flow from the device case back to the neutral point via the neutral wire. Because the neutral resistance is very small, this short-circuit current is very large, and the path through the human body to the neutral point has a large resistance, and the current is almost zero. The resulting large current forces the protective elements in the line (such as a fuse or circuit breaker to trip), quickly disconnecting the device from the power source, thereby avoiding electric shocks. And I always hope that the larger the short-circuit current at this time, the better, because the larger the short-circuit current, the faster the protection element will act. It can be seen that in this case, the purpose of protection is to rely on short-circuit current.

2 Application of short circuit in current transformer operation

The current transformer is mainly used to expand the measuring AC current range.

Figure 4

The voltage circuit is separated to ensure the safety of people and equipment. Its wiring diagram is shown in Figure 4.

Never open the secondary side of the current transformer during operation. Because its primary winding is in series with the load, where the current

The size depends on the size of the load. If the secondary winding circuit is disconnected, the current and magnetomotive force of the secondary winding disappear immediately, but the current of the primary winding

Unchanged. At this time, the magnetic flux in the core is entirely composed of the magnetomotive force of the primary winding.

Produced, resulting in a large magnetic flux in the core (because the magnetic flux potential of the secondary winding is zero at this time, the magnetic flux potential of the primary winding cannot be demagnetized). On the one hand, the core loss increases sharply, causing the core to overheat and burn the winding; on the other hand, a high electromotive force is induced on the secondary winding, which may cause insulation breakdown and endanger equipment and personal safety. Therefore, the secondary winding cannot be connected to a fuse. If the secondary side instruments such as ammeters and energy meters are required to be removed during operation, the secondary side must be short-circuited. Not only is a short circuit allowed at this time, but it is also necessary.

3 Application of short circuit in power transformer load test

Transformer is a common electrical equipment and is widely used in power systems and electronic circuits. Before the power transformer is put into operation or during operation, some tests need to be performed to ensure the safe operation of the power transformer. Test items include: simplification of transformer oil

Figure 5

Test, insulation absorption ratio test, withstand voltage test, no-load test and load test. The purpose of the load test is to measure the load loss P _K and the impedance voltage U _{k of the} transformer to check whether the actual structure of the windings meets the technical requirements.

During the experiment, the secondary winding is short-circuited (see Figure 5), and the voltage is adjusted by the voltage regulator so that the current on the primary side is equal to the rated current. At this time, the reading of the voltmeter V is the impedance voltage U _{k of the} transformer, and the power meter W measures The primary input power is the load loss P _{K of the} transformer. During the load test, because the secondary side is in a short-circuit state, the load impedance is equal to zero, the output power is also equal to zero, and the current flowing in the secondary winding is exactly equal to the rated value, so the input power is lost by the transformer itself. At the same time, the voltage U _k applied to the primary winding during the test is very low, generally only between 5% and 10% of the rated voltage U _N. Because the voltage is low, the main magnetic flux in the core is also very small, only for rated operation. A few percent of the magnetic flux, so the iron loss P _{Fe is} small, so the load loss of the transformer is almost the copper loss P _{cu of the} transformer winding ^_[2] .