SELV stands for Safety Extra Low Voltage. Some AC-DC power provide set up manuals include warnings regarding SELV. For instance, there could also be a warning about connecting two outputs in collection as a result of the ensuing increased voltage could exceed the outlined SELV secure degree, which is lower than or equal to 60VDC. As well as, there could also be warnings about defending the output terminals and different accessible conductors within the power provide with covers to stop them from being touched by working personnel or accidently shorted by a dropped instrument, and so forth.
UL 60950-1 states SELV circuit is a “secondary circuit which is so designed and guarded that below regular and single fault circumstances, its voltages don't exceed a secure worth.” A “secondary circuit” has no direct connection to the primary power (AC mains) and derives its power through a transformer, converter or equal isolation device.
Most switchmode low voltage AC-DC power provides with outputs as much as 48VDC meet the SELV necessities. With a 48V output the OVP setting could be as much as 120% of nominal, which might enable the output to achieve 57.6V earlier than the power provide shuts down; this might nonetheless conform to the utmost 60VDC for SELV power.
As well as, an SELV output is achieved by means of electrical isolation with double or bolstered insulation between the primary and secondary aspect of the transformers. Furthermore, to fulfill SELV specifications, the voltage between any two accessible components/conductors or between a single accessible half/conductor and earth should not exceed a secure worth, which is outlined as 42.4 VAC peak or 60VDC for not than 200 ms throughout regular operation. Underneath a single fault situation, these limits are allowed to go increased to 71VAC peak or 120VDC for not than 20 ms.
Don’t be stunned when you discover different electrical specs that outline SELV in a different way. The above definitions/descriptions discuss with SELV as outlined by UL 60950-1 and different related specs relating to low voltage power provides.