SELV (Safety Extra Low Voltage) refers to a voltage system that does not exceed a safe contact voltage under both normal and single-fault conditions, ensuring safety for personnel and equipment. SELV systems are commonly used in LED drivers, industrial control equipment, and medical devices to reduce the risk of electric shock and fire.
1. Definition of SELV
SELV is defined in IEC 61140 and other relevant safety standards (such as IEC 60364, IEC 61558, IEC 60950-1, and IEC 62368-1). Key characteristics include:
- A rated voltage typically not exceeding 50V AC or 120V DC (lower limits may apply in humid environments).
- Electrical isolation from the mains supply through isolation transformers, DC-DC converters, or other means.
- Additional protective measures (such as reinforced insulation and grounding) to prevent voltage rise to hazardous levels.
2. Classification of SELV
(1) By Power Supply Type
- AC SELV (Alternating Current Safety Extra Low Voltage)
Supplied by transformers or other isolation devices, commonly used in industrial control and lighting applications.
Example: A 24V AC control circuit powered by an isolation transformer.
- DC SELV (Direct Current Safety Extra Low Voltage)
Supplied by DC-DC converters, isolated power sources, or batteries, widely used in LED drivers, communication equipment, and medical devices.
Example: LED driver outputting 12V DC or 24V DC.
(2) By Protection Method
- Class I SELV (with grounding)
The circuit includes protective grounding but still uses an isolated power supply to prevent high voltage caused by grounding faults. Used in certain industrial and medical applications.
- Class II SELV (double or reinforced insulation)
Does not rely on grounding but instead ensures protection through double or reinforced insulation. Common in LED drivers and consumer electronics.
- Class III SELV (exclusively powered by SELV sources)
Powered only by SELV-compliant power supplies, without additional protective measures. Examples include battery-powered devices and solar power systems.
3. Application of SELV in LED Drivers
SELV compliance is essential in LED driver design, particularly in indoor lighting, stage lighting, and medical lighting applications. Features of SELV-compliant LED drivers include:
- Safe output voltage below 60V DC (commonly 12V or 24V DC) to prevent electric shock risks.
- Electrical isolation via transformers or optocouplers to separate high-voltage and low-voltage sides.
- Compliance with Class 2 (UL1310 for North America) or SELV (IEC/EN 61347-2-13 for Europe) safety standards.
4. Differences Between SELV, PELV, and FELV
| Type | Definition | Electrically Isolated | Grounding Required | Application Scenarios |
| SELV | Safety Extra Low Voltage | Yes | Optional | LED drivers, consumer electronics, medical devices |
| PELV | Protective Extra Low Voltage | Yes | Yes | Industrial control, emergency lighting |
| FELV | Functional Extra Low Voltage | No | Maybe present | General-prupose applications, some household appliances |