When to choose Class I vs Class II AC to DC power supply?

Question

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?

Answer 1

In your case, the Class I will force your device to be Earth referenced. That can be necessary for some signal level compatibility with other devices. Or can be bad for the same reason.

Neither will prevent ground loops! In case of Class II, the loop is AC coupled, which is more benign, though. Note that good design has no issue with ground loops. And ground loop current over the USB cable is usually perfectly fine. If you are struggling with ground loops, it's often more fruitful to improve the circuit than witch hunt for ground isolation.

Safety is not better or worse with either supply. Your device is Class III, so no dangerous voltages are near.

For EMC, it is best to treat your metal chassis as your local RF-Earth, as neither the PC chassis nor the PE will have much relevance being too far away. DC potential will have to be close to the PC for USB to work. This is automatically achieved with either Class.

All in all, I see no bonus in Class I, so Class II it is.

Having said that, I agree with datenheim in that I prefer a solid PE connection to the chassis, makes handling EMC and especially ESD simpler. But this is mainly down to my inexperience; it is surely possible to make this stuff work for Class III devices.