Why choose a SELV Safety Extra Low Voltage power supply

First, let's start with the definition of SELV: a SELV (Safety Extra Low Voltage) system is an electrical circuit in which the safety voltage is extremely low and therefore cannot under any circumstances generate voltages that are dangerous for people. It is electrically separated from other circuits that carry higher voltages, has no direct connection to the primary power supply and is powered by a transformer or equivalent isolation device.

The SELV system is also separated from the earth in such a way as not to cause electric shock to anyone who comes into contact with the system. Therefore, SELV systems are among the safest electrical systems due to their combination of low voltage, basic insulation and protective separation.

The SELV system is used in the design of electrical systems to guarantee a high level of safety, here are the essential characteristics:

• Low voltage: which should never exceed 120V in DC and 50V in AC

• Basic insulation: which guarantees the minimum distance between the conductors and the insulation barriers

• Protective separation from other circuits of the system using insulation or a metal shield connected to earth for circuits with higher voltage. Furthermore, the active parts must not be connected to earth, nor to active parts or to protective conductors that are part of other circuits.

Before the current 62368-1 standard came into force, SELV circuits were subject to the 60950-1 standard.

IEC/UL/CSA/EN 62368-1 classifies potentially hazardous energy sources that can cause pain or injury to users into 3 levels (ES1, ES2, ES3), recommends appropriate protections to prevent these energy transfers, and provides guidance on the application and requirements for protections. It does not include requirements for performance or functional characteristics of equipment.

In the new standard, SELV systems fall into the ES1 subcategory with the following definition.

When talking about extra-low voltage (SELV) circuits, the reference standard for the industrial market is IEC 62368-1, which adopts a performance-based approach to assess the safety of electrical and electronic equipment, considering both voltage and current.

However, when talking about equipment with use limitations for the end user, it is equally important to also consider the 60335 standard, which does not limit itself to defining the restrictions from a product design point of view, but also goes into specifics by identifying the type of application such as treadmills and fitness equipment, household tools and vending machines (60335-2).

In the EN60335 standard, the SELV concepts are resumed, defining the output voltage values ​​compatible with this standard, although it is not explicitly stated as a mark affixed to the power supply/component.

The concept of SELV is also taken up within lighting management solutions through the EN61347 standard, where the guidelines to follow in order to affix this mark to the product are defined.