This is a 1kV SMPS design and it is considered a PRIMARY CIRCUIT by UL because it connects directly to 120V AC MAINS. It does not expose its 1kV output directly to the user, but uses the voltage internally.
Like many off-line SMPS designs it charges its bulk capacitor through a bridge rectifier connected to AC MAINS. The positive terminal of the capacitor (CIRCUIT POS) is typically around +85VDC with respect to GND and the negative terminal (CIRCUIT COMMON) is typically around -85VDC.
Items such as the fuse, MOV, inrush limiter, rectifier bypass caps, etc. have been omitted because they are not relevant.
The high dv/dt E fields of the SMPS initially created a common-mode EMI problem because the user-accessible metal case was connected directly to GND.
The EMI problem was solved by connecting the metal case to CIRCUIT COMMON through a Y capacitor. The diodes and resistor ensure the voltage level of the case is always within a volt of GND while the Y capacitor conducts high frequency currents back to the circuit common (which is typically 85V below GND).
I have not been able to find answers to these 2 questions:
Under what conditions does UL require that exposed metal on a consumer device be grounded? Switching supplies typically connect a Y cap between primary and secondary and my ETL listed space heater has an un-grounded metal mesh. However, I have read conflicting information about the grounding requirements for metal cases.
Which UL insulation requirements (in terms of B/R/S) and which voltage level should be used to determine the creepage/clearance of the gap between the CIRCUIT COMMON (which connects to MAINS through a diode) and the USER ACCESSIBLE METAL SHIELD, given that the SMPS also boosts AC MAINS to 1kV?