Safety voltages: ELV, SELV, PELV, and FELV

As an ex rock climber, I always liken the system of safety voltages (ELV, SELV, and PELV) to the slightly opaque British Trad grading system for climbing (which I think should have been extended to include Tricky, Jolly Tricky, Bit Spicy That One, Whoops There Goes My Lunch, and Guaranteed Hospital Visit)

One effect of this is non intuitive nature is that people often say “SELV” when they mean “voltages too low to exceed the 50Vac / 75Vdc threshold of the Low Voltage Directive” or “a low voltage supply”.

This isn’t accurate, as one could meet the ELV/SELV voltage limits for a DC supply of 100VDC, which would fall under the LVD.

Importantly, these terms capture not only the voltage levels but the nature of the supply from mains.

  • ELV defines safe(ish) voltage levels for user accessible parts
  • ELV limits correspond to ES1 levels for AC and ES2 for DC from EN 62368-1
  • SELV, PELV and FELV levels do not exceed ELV limits, even under no-load conditions (a useful consideration when using an unregulated power source like a 50/60Hz mains transformer)
  • SELV indicates a double insulated mains supply (Class II mains)
  • PELV indicates a single insulated with earth (Class I mains)

One thing I do like about EN 62368-1 is that it skirts around this non intuitive terminology and makes voltages and insulation much easier to understand.

A breakdown of the terms can be found below:

Intialisation ELV SELV PELV FELV
Meaning Extra Low Voltage Safety Extra Low Voltage Protective Extra Low Voltage Functional Extra Low Voltage
Electropedia Definition 195-05-24 195-06-28 195-06-29 n/a, defined in BS 7671 wiring regulations
Voltage Limit
(Conductor-to-Conductor or Conductor-to-Earth)
50Vac/120Vdc 42Vac
50Vac/120Vdc with no load
42Vac
50Vac/120Vdc with no load
42Vac
50Vac/120Vdc with no load
Insulation to Hazardous Mains n/a Double
Reinforced
Basic + earthed screen
Double
Reinforced
Less than Basic
Earthed n/a No Yes Yes
Can be Accessible? Yes Yes Yes No
Can exceed voltage limits under conditions:
Normal No No No No
Single Fault No No No No
Earth Fault No No No No
Example ELV just defines acceptable voltage levels Class II a.c. mains with un-earthed output e.g. typical USB phone charger Class I earthed a.c. mains suply with low voltage outputs e.g. desktop PC power supply Low voltage that is not separated from a.c. mains e.g. voltage for SMPS controller primary side not accessible to user

.