Quickly distinguish the power distribution system
There are various ways to classify the power distribution system. The following are the core methods for **quickly
distinguishing common power distribution systems**, compared from three dimensions of **voltage level, grounding
method, and structural form**:
I. Distinguish by Voltage Level
| Type | Voltage Range | Application Scenarios | Characteristics |
| High-voltage Power Distribution | Above 1kV | Substations, large-scale industrial power consumption | Long transmission distance, low loss |
| Medium-voltage Power Distribution | 1kV to 35kV | Urban power distribution networks, industrial parks | Balancing transmission efficiency and safety |
| Low-voltage Power Distribution | Below 1kV | Homes, office buildings, small equipment | Direct connection to terminal loads |
II. Distinguish by Grounding Method (International Standard IEC 60364)
**Meaning of Core Symbols**:
- **T**: Power source end grounded (French "Terre")
- **N**: Load end grounded (French "Neutre")
- **S**: Neutral wire (N) and protective wire (PE) **separated**
- **C**: Neutral wire (N) and protective wire (PE) **combined** (PEN wire)
1. TN System (Power Source End and Load End Share the Same Ground)
YouTube：https://youtube.com/shorts/acQkw21v-vI
| Type | Wiring Method | Application Scenarios | Safety |
| TN-S | N wire and PE wire completely separated | Residences, hospitals, data centers | Strong anti-interference, no electric shock risk |
| TN-C | N wire and PE wire combined into PEN | Early rural areas, simple industries | Low cost, but there is electromagnetic interference |
| TN-C-S | The first half is PEN, and the second half is separated into N/PE | Urban residential areas, commercial buildings | Balancing cost and safety |
2. TT System (Power Source End and Load End **Independently Grounded**)
- **Wiring**: The power source end is grounded (T), and the load end is grounded (T) through a **separate grounding body**.
- **Application**: Remote rural areas, street lighting systems.
- **Characteristics**: Small current during faults, and it needs to be used in conjunction with a **residual current device (RCD)**.
3. IT System (Power Source End **Ungrounded or High-impedance Grounded**)
- **Wiring**: The power source end is ungrounded, and the load end can be grounded or ungrounded.
- **Application**: Hospital operating rooms, mines, chemical plants (requiring **continuous power supply**).
- **Characteristics**: It can still operate when there is a single-phase ground fault, but it requires an **insulation monitoring device**.
III. Distinguish by Structural Form
| Type | Topological Diagram | Advantages | Disadvantages | Application Scenarios |
| Radial Type | Power source → Branches → Loads | Small impact of faults, simple protection | Many lines, high cost | Power supply for important equipment |
| Tree-trunk Type | Power source → Main trunk line → Branches | Few lines, low cost | Power outage of the whole section due to one fault | Lighting, small loads |
| Ring Type | Power source → Ring network → Loads | High reliability, bidirectional power supply possible | Complex protection, need for sectional switches | Urban power distribution networks |
IV. Quick Distinguishing Mnemonics
1. **Voltage Level**:
"High is above 1k, medium is from 1k to 35, and low voltage is below 1k."
2. **Grounding System**:
"TN shares the ground, TT is independent, IT is unconnected and relies on insulation; TN-S separates the neutral
and ground, TN-C combines the neutral and ground."
3. **Structural Form**:
"The radial type is like branches, the tree-trunk type is a single line, and the ring type can go in a circle."
V. Typical Application Scenarios
- **Household Electricity**: **TN-C-S** (PEN before entering the house, N/PE separated after entering the house).
- **Factory Equipment**: **TN-S** (anti-interference) or **IT** (requiring continuous power supply).
- **Rural Power Grid**: **TT** (independent grounding to reduce the risk of leakage).
Through the above dimensions, the type and characteristics of the power distribution system can be quickly located
to meet the needs of design, maintenance, or fault troubleshooting.
