Let's look at the picture below:

This image shows a residential area with multiple homes. As a result, there are total distribution rooms and primary distribution equipment in the community, along with intermediate secondary distribution equipment and electricity meter boxes in each home. The electrical energy passes through these components before reaching our homes and connecting to the distribution box inside, completing the home power distribution.

The following diagram illustrates a residential power distribution system:

This image is quite large, so let's take a closer look:

On the left side of the figure is the primary distribution equipment. We see a transformer T that converts the medium voltage of 10kV to a low voltage of 400V/230V. This voltage on the low-voltage side of the transformer is called the system voltage, which is the maximum value of the low-voltage distribution system. 400V is the line voltage between three phases, while 230V is the phase voltage relative to the neutral line, differing by a factor of 1.732.

The primary distribution equipment is also known as first-level low-voltage switchgear. It includes an incoming circuit, a system bus, and multiple feeder circuits. Three-pole circuit breakers are typically used for both incoming and outgoing lines.

Please pay special attention:

We observe that the neutral point of the low-voltage side of the transformer is grounded and connected to the busbar of the primary distribution equipment. This grounding is referred to as working ground or system grounding, and its purpose is to establish a reference ground or zero potential.

The wire drawn from this grounding point is called the PEN line, which serves as the neutral line. The three-phase lines are often referred to as live lines, labeled L1, L2, and L3.

This type of grounding system is called TN-C according to IEC and national standards, specifically IEC60364-1 and GB16895.1.

Let’s continue looking at the diagram:

A four-core cable is drawn from a feeder circuit of the primary distribution equipment to deliver electrical energy to the intermediate secondary distribution equipment.

Why is secondary distribution equipment needed? Because there are many homes, using only the first-level distribution equipment would make it too large and complex. Secondary distribution equipment is used to handle the second stage of power distribution.

Secondary distribution equipment includes an incoming line switch, a busbar, and a feeder circuit. The incoming switch may use a knife-melt switch, and the feed circuits can use fuses or circuit breakers. The diagram shows fuses being used.

We see that on the right side of the figure, three fuses form a feed loop that delivers power to the home distribution cable 2.

Note:

After the primary and secondary distribution equipment, the voltage output of the secondary equipment is reduced to 380V/220V due to voltage drop along the distribution line. This is called the nominal voltage, which is the operating voltage for all household electrical devices.

According to the GB156-2001 standard (standard voltage), there are regulations regarding system voltage and nominal voltage.

Continuing, let’s look at the image below:

We see that the electrical energy from the home distribution cable 2 is sent to the entrance of the metering meter box located at the lower left.

Note that according to national standards, the color of the three-phase lines is yellow-green and red, and the PEN line is blue.

Assuming there are six floors in the residential building, with two units per floor, the L1 phase (A phase) is used for the 1st and 2nd floors, the L2 phase (B phase) for the 3rd and 4th floors, and the L3 phase (C phase) for the 5th and 6th floors. As shown in the diagram, the three phases are delivered to 12 watt-hour meters and then taken to the households.

Please pay special attention:

We observe that the PEN neutral line drawn from the home distribution cable 2 is sent to the right side of the figure, connected to the ground flat steel MEB, and then separated into N lines and PE lines. The N line is called the neutral line, and the PE line is the protective line. From this point onward, the neutral line no longer exists—only the N and PE lines remain.

This type of grounding is called protective grounding, and its purpose is to ensure personal safety.

It is defined as the TN-CS grounding system in IEC60364 and GB16895-1.

Note that the PE line is yellow-green in color. The yellow-green line can only be used for grounding purposes in all electrical appliances and distribution lines, as mandated by IEC60364 and GB16895.

We saw that the N-line was led to the meter and finally sent to the home along with the phase line and the PE line, as shown in the bottom right of the figure.

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From the above description, we have learned several important points:

First: We saw how to convert three-phase voltage to single-phase voltage, understanding the relationship between 380V and 220V. This answers the question: How does three-phase voltage get converted to household voltage?

I believe the subject has now received a clear answer from this explanation.

Second: We observed the system grounding method, including both system grounding and protective grounding, and clarified the meaning of the TN-C and TN-CS grounding systems.

Third: We saw the MEB grounded flat steel. In reality, the MEB is connected to the building's steel mesh, ensuring that the metal casing of household appliances remains at ground potential.

Fourth: We gained insight into the classification and definition of power distribution systems at different levels.

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Finally, let's take a look at the home distribution system.

The image below is a diagram of the home distribution system I have created. Since this image has been explained multiple times, I will skip any doubts:

Finally, let's discuss how the home distribution system is built.

For the community distribution system, it is designed by engineers from the architectural design institute and constructed by the construction company and the electric company. This includes the installation of power transformers, the medium voltage 10kV power distribution system and power distribution cabinets, the low voltage 400V level distribution system and power distribution cabinets, the secondary power distribution system and power distribution cabinets, laying of various cables, installation and wiring of electricity meter boxes in each residential building, and the installation of the main distribution box inside the home.

In other words, the construction company and the electric company complete the installation of all distribution lines from the transformer to our homes.

The internal wiring of the home is completed by the decoration company. Sometimes, the home distribution box may be replaced based on the requirements of the home distribution system.

The design and construction specifications for the residential power distribution system are outlined in GB50054-2011 "Design Specification for Low Voltage Distribution." This specification is written and regulated by the Ministry of Housing and Urban-Rural Development.

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Let us look at the TN-CS diagram provided in IEC60364-1:

At the top left of the figure, we see the three low-voltage windings of the power transformer and their common line. We see that the common line is directly grounded. This is the system grounding, or working ground. After the neutral point is grounded, a line is drawn, defined as the PEN line. This PEN line is the zero line. The three-phase lines L1/L2/L3 and the PEN line are always drawn to the right. We see that the PEN line is grounded again, marked as repeated grounding in the figure. Repeated grounding points are typically placed in the incoming loop of the primary distribution equipment.

In actual use, if the transformer is close to the primary distribution equipment, the system grounding and repeated grounding can be combined, meaning that the system grounding is removed and the repeated grounding is retained.

In the middle of the figure, we see that the PEN line begins to separate, and the point of separation is where the meter chart I drew earlier. Before the separation, the PEN line must be grounded again, i.e., at the MEB grounding point.

In fact, the PEN line allows multiple grounding and requires multiple grounding. The advantage of multi-point grounding is to prevent the PEN line from taking a higher voltage after it breaks. This is essential because the outer casing of each appliance in the home is connected to the PE line. If the grounding is not done before entering the home, once the PEN line is broken at the household level, the outer casing of all electrical appliances may become charged.

When the PEN line is split into N and PE lines, the system now has five lines, which is why the T-S and TN-CS -S parts are referred to as the three-phase five-wire system.

Of course, I don’t recommend using the three-phase five-wire system. Because the term “line” refers to a wire that carries current during normal operation, the PE line does not carry current under normal conditions. Therefore, TN-S is considered a three-phase four-wire system, and TN-C and TN-CS are also three-phase four-wire systems.

The answer to the subject’s question is actually a combination of a phase line, a PE line, and an N line in the right half of the figure.

Additionally, please note the N-line connection of the two loads in the figure:

In the middle of the load, we see that the PEN line is first connected to the shell terminal of the load, and then to the N-line terminal, indicating that the PEN line also serves the function of the zero line with protection priority. The load on the right clearly connects the three-phase lines, N line, and PE line to the load. Of course, for single-phase loads such as refrigerators and air conditioners in the home, only one of the three-phase lines is used.

Another explanation from User A:

First of all: after high-voltage three-phase electricity reaches the residential transformer, it becomes the household (single-phase) electricity we use.

Typically, the voltage on the high-voltage side is 10kV. After stepping down through the transformer, we obtain a low-voltage three-phase power supply of 220V abc.

(As shown below)

The image above shows a three-phase four-wire power supply system.

A three-phase four-wire system, as shown in the image, consists of a, b, c as the fire lines, and any one of them together with the neutral line N forms the fire and neutral lines that residents need.

To avoid waste, the three fire lines are often used, and the neutral line N is generally split with the fire lines b and c. That is, after the above lead, three sets of "fire line + neutral line" can be directly led out.

Therefore, simply put, after passing through the transformer, we obtained three sets of household electricity: “fire line a + neutral line,” “fire line b + neutral line,” and “fire line c + neutral line.”

2. After obtaining the three sets of electricity, we still need to allocate them:

Assume there are only three buildings in the community, and each can have a group of “fire line + neutral line” led out in the past.

If we think of each family as a resistor, we can visualize the following:

3. On the issue of power balance: the teacher mentioned in class. It mainly emphasizes even distribution when allocating load.

Just like the example above, if one building consumes several times more than the others, the load on phase a could be several times that of phases b and c, causing imbalance in load and current.

Thus, in allocation, we first estimate the overall load situation and then evenly distribute it.

4. About leakage:

We often refer to “leakage,” which means direct contact between the metal casing of your household appliance and the live wire, causing the metal casing to become charged. (Most commonly due to aged insulation inside the appliance allowing the wire to touch the casing.)

When you manually touch the metal casing of the electrical appliance, it's equivalent to touching the live wire directly, meaning you're in close contact with 220V voltage. What happens next?

When we ground the casing, the leakage current flows through the ground wire directly into the ground, like this:

And how is the appliance's casing connected to the ground?

Plug wiring as above;

Socket wiring as above;

Most parts of China use the TN-S system. The TN-S system looks like this:

This type of power supply is called a three-phase five-wire system, meaning the line connecting to your home is a combination of "hot line + neutral line + ground line". Compared to the three-phase four-wire system, it has an additional ground line to prevent leakage accidents. Thus, the third grounding wire PE is also known as the protective line.

The metal casing of the appliance is grounded through the socket and then connected to the PE (grounding wire).

Extrapolating a short story: Last year, I encountered a “leakage” problem during the renovation of an old block in Guangzhou.

An old man brought his rice cooker, which he estimated had been used for seven or eight years, and said, “After the birth, help me kneel down, often leaking electricity.”

I plugged it in for the late life test, and it did leak. Okay, repair. Disassembled the rice cooker: Inside, two small wires were exposed! (Don't worry, you don't have to buy meat at home.)

Later, it was discovered that the wire from the rice cooker's metal casing to the plug's ground wire was broken, and the three pieces were removed.

Tested with an electric pen first, no leakage. At this point, I deeply suspected something was wrong and asked my brother to touch it with his hand.

At that time, thousands of grass mud horses rushed through my mind!

But I believed in my own technology. I touched it, nothing happened! Hahaha.

This story is intended to tell everyone:

Home appliances that are too old should be replaced. Don’t keep using them just because they seem okay. Before an electric shock accident occurs, it might feel lucky, but a serious electric shock is regrettable!

5. Finally, the expansion of protection measures for low-voltage systems (for friends in the electrical profession):

Protection measures for low-voltage power systems:

TT system: Requires extra investment; the grid usually doesn’t install it for you.

IT system: The live part is not directly connected to the earth (grounded or ungrounded), and the exposed part of the electrical device is grounded.

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