Extra Low Voltage

Extra-Low Voltage (ELV) systems are broken down into three categories:

  1. Safety Extra Low Voltage (SELV)
  2. Protective Extra Low Voltage (PELV)
  3. Functional Extra Low Voltage (FELV)

These categories describe circuits operating at voltage levels that are generally considered safe (typically ≤ 50 V AC or ≤ 120 V ripple-free DC) but distinguish between how they are isolated from higher voltage systems (like the mains) and whether they are connected to earth.

1. SELV (Safety Extra Low Voltage)

  • Definition: An extra-low voltage system that is electrically separated from earth and other systems in such a way that a single fault cannot give rise to an electric shock risk.
  • Earthing: Not earthed. SELV circuits must have no connection to the ground/earth.
  • Insulation: Requires Double or Reinforced insulation from hazardous mains voltages to ensure high safety levels.
  • Typical Use: Applications where isolation from earth is critical for safety, such as in typical double-insulated devices (e.g., USB phone chargers).

2. PELV (Protective Extra Low Voltage)

  • Definition: An extra-low voltage system that offers similar safety levels to SELV but is connected to earth.
  • Earthing: Earthed. One point of the PELV circuit is usually connected to the protective earth.
  • Insulation: Like SELV, it requires Double or Reinforced insulation from hazardous mains voltages.
  • Typical Use: Systems where an earth reference is needed for functional or protection reasons, such as a desktop PC power supply.

3. FELV (Functional Extra Low Voltage)

  • Definition: An extra-low voltage system used for functional purposes (e.g., machine control) that does not fulfill the strict safety requirements of SELV or PELV regarding separation from higher voltage circuits.
  • Earthing: Earthed.
  • Insulation: Has Less than Basic or simple separation from high voltage mains. Because it does not have the protective separation of SELV/PELV, it is not considered safe to touch.
  • Safety Requirement: Since it effectively shares the risks of the higher voltage system it is derived from, FELV circuits require basic protection (such as barriers or enclosures) to prevent direct contact, unlike SELV and PELV parts which are generally accessible.

References

  1. IEC 61140 & IEC 60364-4-41 these standards provide the global baseline.
  2. Safety voltages: ELV, SELV, PELV, and FELV, Unit 3 Compliance
  3. Difference Between ELV, SELV, PELV and FELV, Voltage Disturbance

See also

Voltage Architecture, Insulation and Isolation