When does UL allow a user to access the a metal component connected to AC MAINS through a Y capacitor?

Question

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:

1. 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.

2. 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?

Answer 1

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

I am not sure what the exact wording is in the IEC requirements (probably you will be subject to IEC61010 or equivalent. You should consult a saftey consultant or NRTL. But I'll summarize my understanding based on the product testing I have done. Anything that has a dangerous voltage (more than 63V) must not be user accessible or have a grounded chassis. The grounded chassis is important during a fault because any currents that fault will not expose the user to a dangerous voltage. In addition current limiting devices such as fuses and breakers also prevent dangerous currents in the event of a fault because the fault will trigger these devices. You can have a power supply without a frame in a product if the product has a chassis and it isn't user accessible, in that event, you must print "no user accessible parts" or something like that on the chassis and instructions.

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?

That would also fall under the IEC listing your product will fall under, typically most products file under IEC61010 (there is a new version of it) which is a consumer product filing (there are others for industrial and commercial products).

Another thing that affects creepage and clearance is what kind of environment the product will be in, if the product is exposed to dust or other contaminants, the pollution rating is higher because arcing occurs more readily and the distance must increase to prevent arcing.

See this question: What is the general rule of thumb regarding UL's creepage clearance regulation? And this one: How to define working voltage class net to calculte the creepage?