# Requirements for the Insulation Performance of the Secondary Circuit of Low-Voltage Switchgear Assemblies
## Abstract
The insulation performance of the secondary circuit in low-voltage switchgear assemblies is crucial for ensuring the safe and reliable operation of electrical systems. This article delves into the specific requirements for the insulation performance of the secondary circuit, covering aspects such as insulation resistance, dielectric strength, and environmental adaptability, based on relevant national and international standards and industry practices.
## 1. Introduction
Low-voltage switchgear assemblies are essential components in electrical power distribution systems, responsible for controlling, protecting, and distributing electrical energy. The secondary circuit, which includes control, signal, and measurement circuits, plays a vital role in the proper functioning of the switchgear. Adequate insulation performance of the secondary circuit is necessary to prevent electrical faults, ensure personnel safety, and maintain the overall reliability of the electrical system.
## 2. Insulation Resistance Requirements
### 2.1 General Standard
According to the national standard GB 7251.1 - 2023 "Low-voltage switchgear and controlgear assemblies - Part 1: General rules", the insulation resistance of the secondary circuit in low-voltage switchgear assemblies shall be no less than 1 MΩ, regardless of the rated voltage level. This requirement applies uniformly to all types of secondary circuits, such as control circuits, signal circuits, and measurement circuits.
### 2.2 Testing Conditions
- **Measurement Instrument**: When measuring the insulation resistance, a 1000 V megohmmeter should be used for circuits with a rated voltage of 690 V or less, and a 2500 V megohmmeter for circuits with a rated voltage of 1000 V or more.
- **Reading Time**: The stable value of the insulation resistance should be read after 1 minute of measurement.
- **Environmental Conditions**: The ambient temperature during the test should be between 10 °C and 40 °C, and the relative humidity should not exceed 75%. These conditions are set to minimize the impact of environmental factors on the measurement results.
- **Pre - test Preparation**: Before the test, the power supply should be disconnected, the equipment should be discharged, and all external connections should be removed to ensure that there is no interference from external circuits.
### 2.3 Comparison with Main Circuit
The insulation resistance requirement for the main circuit is calculated based on the rated voltage, with a minimum value of 1 MΩ per kV. For example, a 400 V main circuit requires an insulation resistance of at least 0.4 MΩ. In contrast, the secondary circuit has a unified requirement of 1 MΩ, which does not vary with the rated voltage. This difference reflects the different characteristics and functions of the main and secondary circuits in the switchgear assemblies.
## 3. Dielectric Strength Requirements
### 3.1 Air - insulated Dielectric Strength
- **Rated Impulse Withstand Voltage**: The rated impulse withstand voltage value is selected by the manufacturer based on the rated working voltage of the switchgear, the power system voltage, and the over - voltage category of the installation site, in accordance with the relevant provisions in GB 7251.1 - 2005. The test voltage value may need to be adjusted according to the altitude of the test site.
- **Test Positions**: The impulse withstand voltage test is applied between live parts and grounded parts, and between poles. For electrical clearances with solid insulation, the test voltage value is specified in the relevant tables of GB 7251.1 - 2005.
### 3.2 Solid - insulated Dielectric Strength
- **Power Frequency Withstand Voltage Test**: For solid - insulated materials, a power frequency withstand voltage test is required. The test voltage values for the main circuit and auxiliary circuits directly supplied by the main circuit, as well as for auxiliary circuits not directly supplied by the main circuit, are specified in relevant tables.
## 4. Environmental Adaptability Requirements
### 4.1 Pollution Level
The insulation performance requirements of the secondary circuit are also related to the pollution level of the operating environment. The standard divides the pollution level into four categories:
- **Level 1**: No pollution or only dry non - conductive pollution.
- **Level 2**: Non - conductive pollution, with possible temporary conductivity due to condensation.
- **Level 3**: Conductive pollution or dry non - conductive pollution becoming conductive due to condensation.
- **Level 4**: Persistent conductive pollution, such as pollution caused by conductive dust or rain and snow.
Different pollution levels require different minimum electrical clearances and creepage distances to ensure the insulation performance of the secondary circuit.
### 4.2 Over - voltage Category
The over - voltage category is divided into four levels (I - IV) based on the installation position of the equipment in the power supply system. Different over - voltage categories have different requirements for the insulation coordination of the secondary circuit, including the selection of rated impulse withstand voltage values.
## 5. Conclusion
The insulation performance requirements for the secondary circuit of low - voltage switchgear assemblies are comprehensive and strict, covering insulation resistance, dielectric strength, and environmental adaptability. Adhering to these requirements is essential for ensuring the safe and reliable operation of electrical systems, protecting personnel safety, and preventing electrical faults. Manufacturers and users of low - voltage switchgear assemblies should strictly follow relevant standards and regulations during design, installation, and maintenance to ensure that the insulation performance of the secondary circuit meets the specified requirements.
