SELV in power supplies - what does it mean?

SELV in power supplies - what does it mean?

The SELV (Safety Extra Low Voltage) designation is often found on power supplies for LED lighting, but it can also be found on the housings of some voltage power supplies. This symbol, in accordance with UL60950-1, indicates that the voltage at the output of the power supply will never exceed a human-safe value of 60VDC and is galvanically isolated from the mains voltage by means of double or reinforced insulation (Class II protection). As a result, such a power supply does not require protective earthing.

This limitation applies to both normal operation and abnormal conditions, such as voltages that may occur when the power supply is overloaded or damaged. Most low-voltage AC/DC power supplies with outputs up to 48VDC meet SELV requirements because they have an overvoltage protection (OVP) of approximately 120% of nominal value. To meet SELV specifications, the voltage between any output terminal of the power supply and earth must not exceed a safe value, defined as 42.4 VAC or 60 VDC for more than 200 ms during normal operation.

Under fault conditions, these limits can exceed a peak of 71 VAC or 120 VDC for more than 20 ms. SELV-labelled power supplies are therefore designed to provide maximum safety for users, minimising the risk of electric shock even in emergency situations. This makes them an ideal choice for applications requiring a high level of safety, such as LED lighting or other electronic devices.