One question we see periodically from users is a request for the UL file number for PI parts.
The UL (or other safety agency) file number for a component indicates that it has met the safety requirements for that class of component. Components where this is a requirement are opto-couplers, Y and X class capacitors, fuses and (materials used in the construction of) transformers. All of these components either bridge the isolation barrier or provide a critical safety function.
Like most components on the primary, high voltage, side of a power supply UL certification is not required for a component that does not bridge the isolation barrier from primary to secondary. Examples include power MOSFETs, rectifier diodes and input electrolytic capacitors. However when parts are used that do not have certification and do not meet the operational spacing specified in UL60950 (or similar international variants) the power supply with be tested for single point faults.
Here single adjacent pins or PCB traces that do not have a sufficient creepage or clearance distance will be short circuited. The power supply under this single fault must not fail in a way that damages the isolation barrier or causes fire (emission of incandescent material). Short circuiting adjacent pins on PI parts generally either causes no damage or causes the input fuse to open - considered a safe failure. In addition the plastic compound used to mold PI parts is UL 94V0 rated meeting the safety flammability requirement.