A photocoupler is an isolation device. What is dielectric strength, which indicates its insulation capability?

Dielectric strength is a voltage defining the amount of dielectric withstand voltage between input and output. For component standards applicable to photocouplers, the UL standards in the U.S. and the VDE standards in Germany have completely different definitions of test methods for measuring input-output insulation performance and of acceptance criteria.
For details, also refer to Toshiba Application Note "Safety Standards for Photocouplers", Chapter 3. Safety standards.

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