Feels like I'm missing something obvious here, but when is it appropriate to use a Class I vs a Class II AC to DC power supply for a commercial electronic device? Should Class II only be used when the device itself is double-insulated? Should any metal-chassis device get a Class I by default?
Furthermore, do all Class I supplies connect the DC-side negative output to the earth ground pin? The ones I have on my desk do, but I don't know if that should be expected (standard? typical?) for all of them.
Specific to my case, the supplies we are using are self-contained desktop "brick" supplies which are (presumably) well-engineered CE-approved devices, e.g. the Class II SDI65-24-UD-P5 that I have on my desk right now. We build devices that use these (and similar) brick supplies at 24 or 48 VDC. The devices are intended to be connected to a PC via USB, which also has a shielded cable (presumably connected to PC chassis ground, which may be a laptop and/or may not have an earth-grounded power supply). We also can recommend (but obviously cannot guarantee) that the end user powers the device from the same mains circuit as their PC.
Overall, I'm trying to find the best solution for:
- connecting an external device to a PC with or without its own chassis-to-earth ground
- preventing ground potential differences from causing currents in either the USB shield or USB signal ground
- ensuring a metal-chassis device is safe and meets all applicable regulations
and specifically for this question, whether the Class I or Class II supply is better suited to the task.
What criteria should I use to decide between them? Is there any guidance in the safety regulations?