Why choose a SELV Safety Estra Low Voltage power supply

First of all, let's start from 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 generate in any case voltages dangerous for people. It is electrically separated from other circuits carrying higher voltages, has no direct connection to primary power, and is powered by a transformer or equivalent isolation device.

The SELV system is also separated from the ground so 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 main characteristics of the SELV system

The SELV system is used in the design of electrical systems to guarantee a high level of safety. Below 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 insulating protections (insulation barriers)
• Protective separation from other circuits in the system using insulation or grounded metal shielding for higher voltage circuits. Furthermore, the active parts must not be connected to the earth, nor active parts or to protection conductors that are part of other circuits.

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What are the standards for industrial power supplies when it comes to SELV?

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

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

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

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

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

In the EN60335 standard, the SELV concepts are taken up again, defining the output voltage values compatible with this standard, although it is not made explicit 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 to affix this mark are defined on the product.

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