Bonding and grounding online (double conversion) UPS with input and output neutrals

A

alej27

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Venezuela
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Electrical Engineer
I have a single-phase two-wire online (double conversion) UPS, with 120-V input and 120-V output.

The UPS has its input neutral isolated from its output neutral internally. It does not contain a maintenance bypass switch nor a static bypass switch.

The input of the UPS is an assembled flexible cord with a L15-30P plug (with a ground prong).

The output of the UPS is not a feeder, it does not contain a main breaker. It simply has multiple 5-20R receptacles, each with a ground slot, to which PDUs (power strips for telecom) are connected, to which IT equipment is connected (switches, routers, servers).

Additionally, the chassis of the UPS has a ground screw, internally tied to the input and output grounds, and the chassis, of course.

The UPS is from the manufacturer CDP (Chicago Digital Power), model UPO11-3RT (3kVA, rack-mount).

1753733617098.png


The UPS classifies as a separately derived AC system, correct? If so, how should the bonding of the output neutral be done? Would opening the UPS to internally install a jumper between the output neutral and its chassis (provided the user manual allows it) comply with sec. 250.30(A)(1) and 250.20 of 2020 NEC, correct?

Finally, how should the output neutral of the UPS be grounded? Technically speaking, since I'll bond the output neutral to the chassis, and the chassis is connected (through the EGC of the UPS' flexible cord) to the main panelboard ground bar, which is already grounded, then the output neutral is already grounded (this of course would apply to any SDS). But I think this would imply using an EGC as a GEC, which is not allowed per 250.121(A), correct? If so, then how should it be done? Would it be acceptable to install a wire between the chassis ground terminal of the UPS and the RGB (rack ground busbar) of the server rack, which is then connected to the MGB (master ground busbar) of the room, which is then connected to a GES (grounding electrode system)? Because I'll actually have two UPSes per rack, and various racks (each with two UPSes).

If you need diagrams to understand my ideas please say so.
 
Any applicable requirements should be included in the owner's manual, but assuming you meant L5-30 rather than L15-30, and going from my memories of the schematics I saw 20 years ago...
- Opening the thing beyond any user-serviceable parts (aka the batteries) will void UL. Do not pass 'GO', do not collect $200.
- Unless UL requirements have changed versus 2005, during online operation the ground and neutral are carried the whole way through as if the UPS were just a sort of fancy-pants line conditioner. When the power goes out and things switch to battery, not only is the hot physically disconnected from the input, BUT SO IS THE NEUTRAL. This is to prevent any sort of objectionable backflow current from getting onto the upstream lines.
- This part is fuzzy, but IIRC there was a second relay to physically bond NG during battery operation only.
 
I would think...
1) The neutral and EGC should be factory bonded inside the unit.
2) The unit should have a grounding electrode conductor (GEC) terminal for grounding the separately derived system.

(Maybe the product standard doesn't actually require (2), I don't actually know.)

Or, perhaps you're wrong about the neutrals being internally isolated.

What does the manual say?
 
Figured I would bust out my skills of an artist (bonus points if you recognize the quote). Obviously things may have changed in the 20 years since I last mucked about supporting these things, but this is how I remember a single-phase double-conversion system looked "back then," input on the left, output on the right, and using euro-style coloration (brown hot, blue neutral, green ground)
power base.png
 
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Yikes! Here's what you do:

1) Plug the UPS into the power source.
2) Plug your power strips into the output receptacles.

I have no idea where you are getting your ideas from. Read the instruction manual and follow it.

-Hal
 
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