Should you ground a fancy, floating power supply or not?

[bottledwater]

Hi all, long time lurker here, first post:

I've been reading in the user manuals of those fancy Rhode & Schwarz DC power supplies [1]. Specifically, the HMP series that Dave shows and tears down in videos #1173 and #1174 [2].

The HMP20xx and HMP40xx supplies are IEC class II compliant meaning the DC outputs are doubly insulated from the AC input. As far as I understand it, this eliminates the need for grounding because everything is floating [3]. Manufacturers even appear to be required not to give you a 3-prong power connector with it [4, 5].

But the ones Dave tears down come with a 3-pronged power cable. The user manual also says that "The instrument must only be connected to an outlet that has a ground contact". 

So what is the deal? When a power supply is doubly insulated, and thus floating, do you need to earth it or not? Is it good to do even if it's not required? Is it ok not to do it if circumstances don't allow (like if you don't have earth in any of your outlets)?

Thank you!

Bonus info: In another video, someone opens the box of a HMC series supply (not HMP but very similar). This one appears to come with a 2-pronged cable, but the HMC series is class I compliant (not class II like the HMP), and class I requires the earth connector.

References:
[1] https://www.rohde-schwarz.com/us/manual/r-s-hmp-series-power-supplies-user-manual-manuals-gb1_78701-524912.html
[2] and
[3] https://www.keysight.com/dk/en/assets/7018-01897/technical-overviews/5989-9200.pdf
[4] http://machineryequipmentonline.com/electric-equipment/noise-and-groundingclass-i-and-class-ii/
[5] https://slpower.com/archives/8238

[bob91343]

Having the output of a power supply isolated means it has no galvanic connection to the power line.  However, for safety reasons the frame/chassis of the supply should be grounded to the power line grounding circuit.  The output remains isolated.

One reason for connecting the chassis to the utility ground is to avoid random drifting of the chassis to almost any voltage, which can cause some surprises, tantamount to walking across a carpet on a dry wintry day and touching a doorknob.

The user of the equipment has the option of connecting either polarity of the output to ground or anything else, or not connecting it at all.

[Doctorandus_P]

Not everything is floating.

At around 25min in the first vid, you can see a mains power connector on the back. Looks like an earthed C13, but there is a yellow sticker over it. Dave clearly has not plugged it in yet.
Probably the whole outer shell is earthed.

The front connectors are usually floating.
This means you easily create a single reference "GND" point when using multiple power supplies on your test bench without creating ground loops.
You can also use several power supplies in series to get a higher voltage, or create a "+" and a "-" symmetrical voltage, depending on what you use as reference.

[TexasRanger]

You don't need to ground Floating Outputs, but like stated in [4] adding a electrostatic ground screen  in the mains transformer is very useful as it eliminates common mode currents through the parasitic capacitance between primary and secondary windings.

[Benta]

 

Of course you should ground the input as specified.

The only floating supply here is the output, and knowing Rohde & Schwarz, the company knows exactly what it's doing.

[TimFox]

A general safety rule is that any metal that can be touched by the operator must be connected to PE = safety ground.
Depending on your application, the floating (galvanically-isolated) guts can be connected to anything so long as you do not exceed the manufacturer's rating for the maximum voltage between any connection and the chassis.  Typically, if the load is not inherently grounded, you should usually connect either the positive or negative output of the supply to the chassis on the front panel.
Note the metal ground connection at the lower left of the panels to allow a convenient connection with stackable banana plugs.