Interpretation of the Secondary Diagrams of the Box-Type Substation Power Distribution Cabinet

  The following is a professional technical analysis of the **interpretation of the secondary diagrams of the box-type substation 

power distribution cabinet**. Combining with the Electrical Drafting Standard (GB/T 6988) and engineering practical experience, 

the methods of diagram interpretation will be described step by step:

、 I. Basic Knowledge of the Secondary Circuit

**1.1 Definition of the Secondary Circuit**

The secondary circuit refers to the low-voltage circuit used for control, protection, measurement, and signal indication, which is 

different from the primary circuit (main circuit, carrying large current).

**Core Components**:

- Control Components: Contactor (KM), Relay (KA, KT), Auxiliary Contacts of Circuit Breaker (QF)

- Protection Components: Fuse (FU), Thermal Relay (FR)

- Indication Components: Indicator Light (HL), Ammeter (PA), Voltmeter (PV)

- Power Supply Modules: Control Transformer (TC), Switching Power Supply (PS)

II. Core Steps of Secondary Diagram Interpretation

2.1 Identification of Diagram Types

 2.2 Standardized Identification of Electrical Symbols

**Based on the Standard**: GB/T 4728 "Graphical Symbols for Electrical Diagrams"

**Examples of Key Symbols**:

 2.3 Analysis of Circuit Classification

**2.3.1 Power Supply Circuit**

- **Typical Structure**:

  Primary Power Supply → Control Transformer TC → Fuse FU → Power Bus (L+, M-)  

- **Key Points of Diagram Interpretation**:

  - Power supply voltage level (AC 220V/380V or DC 24V)

  - Rated current of the fuse (e.g., FU1: 6A)

  - Grounding symbol

**2.3.2 Control Circuit**

Take the **closing control of the circuit breaker** as an example:

1. **Starting Signal**: Closing Button SB1 (Normally Open Contact)

2. **Logical Conditions**:

   - Normally Closed Auxiliary Contact QF1 of the circuit breaker (indicating that the circuit breaker is in the open state)

   - Normally Closed Contact KA1 of the protection relay (indicating no fault signal)

3. **Actuating Component**: Closing Contactor Coil KM1

- **Circuit Expression**:

  L+ → SB1 → QF1 → KA1 → KM1 → M-  

**2.3.3 Protection Circuit**

Take the **overload protection** as an example:

- **Signal Acquisition**: Thermal Relay FR (connected in series to the secondary side of the current transformer in the primary circuit)

- **Action Logic**:

  When there is an overload, the normally closed contact of FR opens → the control circuit loses power → the circuit breaker opens

- **Key Points of Diagram Interpretation**:

  - Transformation ratio of the current transformer (e.g., CT: 100/5A)

  - Setting value of the thermal relay (e.g., FR: 1.2In)

2.4 Interpretation of the Terminal Block

**Numbering Rules of the Terminal Block**:

- **Vertical Arrangement**: Grouped according to the function of the circuit from top to bottom (e.g., 1-10: power supply

circuit; 11-20: control circuit)

- **Examples of Marking**:

3.1 Example of Diagram Layout

┌───────────────┐       ┌───────────────┐  

│      Primary Main Switch       │       │     Secondary Control Area      │  

│       QF1 630A        │       │    1. Power Supply Circuit      │  

└───────────────┘       │    2. Closing Control Circuit    │  

                          │    3. Opening Indication Circuit    │  

                          └───────────────┘  

3.2 Fault Troubleshooting Logic

If the circuit breaker cannot be closed, the analysis steps through the secondary diagram are as follows:

1. **Power Supply Check**: Measure whether the voltage between terminals X1-X2 is normal (e.g., AC 220V)

2. **On-off of the Control Circuit**:

   - Press SB1 and measure the resistance across the KM1 coil with the resistance gear of a multimeter (it should be 

several hundred ohms normally)

   - Check whether the auxiliary contact QF1 has not been reset due to mechanical jamming

3. **Status of the Protection Circuit**:

   - Check whether the thermal relay FR has tripped due to overload (the red reset button pops out)

 IV. Notes

1. **Safety Specifications**:

   - Confirm the diagram version (such as "2025-03 Rev.2") and the requirements for electrical clearances (such as creepage

 distance ≥ 10mm) before interpreting the diagram

   - The high-voltage area (primary side) and the low-voltage area (secondary side) need to be isolated by a grounded 

metal partition (GB 7251.1)

2. **Principle of Signal Flow Direction**:

   - From top to bottom and from left to right (power supply → control → protection → indication)

   - The coil and contacts of the same electrical component use the same text symbol (for example, both the coil and

 contacts of KM1 are marked as KM1)

3. **Comparison with the Primary Diagram**:

   - The current transformers (CT) and voltage transformers (PT) in the secondary circuit need to correspond to the positions 

of the primary main circuit

V. Commonly Used Standards and Tools

Through the above methods, one can systematically master the skills of interpreting the secondary diagrams of the box-type 

substation power distribution cabinet and achieve the transformation from theory to engineering practice. In actual operation, 

comprehensive analysis should be carried out in combination with the specific details of the diagrams and on-site debugging 

experience.