1. Transformerless UPS Neutral handling and grounding considerations
Ed Spears
May 2013

2. Installation practices, especially in the U.S.
What do I need to know about neutral handling in Eaton transformer-less UPS applications?
Installation practices, especially in the U.S.
Considerations when designing systems with transformer-less UPS
How to answer engineers’, installers’, and inspectors’ questions
What we recommend (and why), and what we will accept
How the transformerless UPS handles faults downstream of the UPS A C B N
3W + Gnd Source
To the UPS

3. Installation practices
FUSE A B C N G
SAFETY
GND TO FRAME
PDU
GND
SSW
SOURCE
X FMR
North American power systems at 480V are almost exclusively fed through a “3-wire + ground” wiring system from “Wye” (neutral grounded) facility transformers. This is commonly known as a “3-wire solidly grounded system.”
The neutral from the source transformer is not run to load-side equipment, AND it may not be necessary to run a neutral to the UPS.
Why not? Because we can provide an internal center point reference, as long as we know that the source is 3W+G.

4. Installation practices
208V data center distribution establishes a neutral at the output of the distribution transformer (PDU), so in this case we do not need to provide a neutral from the UPS (PDU transformer primary is delta, and its wye secondary makes a new neutral).
As systems get larger and more complex, design practices call for ground fault protection on feeders 1000A and above, on 480V and higher sources of supply. This can be an issue if the neutral is bonded to ground before the PDU transformer. It may cause nuisance tripping of the feeder due to ground current.
So what do we do about this? If this is an issue, it can be solved by not bonding E12 bus in the UPS. Note that the UPS is NOT a separately derived system per the definition in the NEC, and therefore it is not required to be bonded.
FUSE A B C
E12 N G
SAFETY
GND TO FRAME
PDU
GND
SSW
SOURCE
X FMR

5. Neutral and Ground: What does the site require?
Safety: protect personnel from hazardous voltages.
Performance: The datacenter equipment needs a common reference point to work reliably and safely.
Fault Handling: When an electrical fault occurs, the operation of circuit breakers and fuses function to “isolate” the fault; that is, to limit the potential for electrical shock, and avoid a fire. So their operation must be “predictable”. That includes the response of the UPS and power distribution network. 5

6. Neutral and Ground: What does the UPS require?
Safety: protect personnel from hazardous voltages.
Performance: The UPS requires a “center point reference” in order to create a 3-phase output, and keep it aligned with the bypass source. That “CP reference” could become a neutral under certain circumstances, but that is not always the case. The UPS only requires that the CP reference be stationary, not “moving around.”
Fault Handling: When an electrical fault occurs, the operation of circuit breakers and fuses function to limit the potential for electrical shock, and avoid a fire in the UPS. So their operation must be “predictable”.
3-Phase UPS Output A C B
Center
Point
Reference 6

7. What if the CP Reference moves?
The UPS center point reference should not “drift”! If it does drift, and we need to transfer to bypass, the phase voltages may not be in sync or have the same magnitude as shown at right…….
How do we keep the reference stable? Three choices:
Tie it to ground (bond E12-G)
Use our internal center point reference
Tie it back to source neutral A C B
UPS Output A C B
Bypass
Center
Point
References 7

8. So, what’s the Process when specifying the 9390 or 9395 UPS?
Step One: Find out the configuration of the Utility source:
3-wire plus ground, wye source, neutral solidly bonded: no neutral available to UPS
4-wire plus ground, solidly bonded: neutral is available to be connected to the UPS.
Delta Source: no neutral available to be connected to the UPS
Delta source is NOT supported for at this time
Corner grounded delta, or “open jaw” delta, or “high leg” sources are never allowed. In these cases a transformer with a solidly grounded secondary must be placed ahead of the UPS.
High Resistance Gnd Source (HRG): This is a wye source with a resistance connected between the secondary neutral terminal and ground, often seen in factory environments, but appearing in datacenter applications more often lately.

9. So, what’s the Process when specifying the 9390 or 9395 UPS?
Step Two: Find out if the load that is directly connected to the UPS, will require a “current carrying neutral.” That is, are 277V or 120V loads connected to the UPS output terminals?
If yes, then a source neutral must ALWAYS be connected to the E12 terminal in the UPS, then that neutral wire will be passed on to the critical load distribution. Do not bond E12 to Gnd.
If no, then we have some choices; the Installation Manual states our preferred choice, but other choices are safe and Code compliant.
1) Bond E12 to ground inside the UPS. The UPS is NOT a separately derived source, but NEC does not prohibit a N-G bond at this point.
Some inspectors may disagree, and are unlikely to “change”
Ground noise may be a problem, when unbalanced utility exists
2) If #1 above is not desirable, connect our internal reference to E12, and do not bond E12 to ground in this case.
3) Pull in a minimal code rated neutral from the source. Do not bond in UPS.
Step Three: Communicate this info to the consultants, engineers, and UPS installers. The next slides will help answer typical questions…….

10. Lots of Questions….. What if I have a delta source feeding the UPS?
NOT ALLOWED on 9395; get a delta-wye transformer and place it ahead of the UPS
What if I want to feed a 9395 with a “corner-grounded delta”? Use NRK? NRP? NNRK? NFG! (No).
This configuration is not allowed
What if I want to feed a UPS with an “ungrounded wye” source? That is, where the center point of the wye source is connected to …….nothing.

11. Lots of Questions…..
What if I have a High Resistance Ground (HRG) source?
OK for 9390 and 9395, for single source applications.
No phase to neutral loads allowed with HRG, per NEC
Internal center point reference is required (CTO option in 9390)
FUSE A B C N G
SAFETY
GND TO FRAME
PDU
GND
SSW
HRG
SOURCE

12. Lots of Questions…..
What if I have a /208 UPS which supports phase to neutral loads?
Note there is no PDU or IDC cabinet. The UPS has phase to neutral loads directly connected at its output.
Source neutral passes right thru the UPS, but connects to E12, so the output neutral will be there when the UPS is on battery or on bypass.
FUSE A B C N G
SAFETY
GND TO FRAME
GND
SSW
208 OR 400V
LOAD PANEL
W/ P-N LOADS
SOURCE
X FMR

13. Battery Operation with ungrounded output
What does it mean that my UPS output is an ungrounded system? Is it safe? Will it work? What about faults?
The UPS output Wye is ungrounded when on battery without a ground referenced center point.
Transformer-less UPS pose no threat to the critical load during battery operation.
The same safety requirements apply to both grounded and ungrounded systems. The UPS chassis still has a safety ground. The UPS frame will not rise above ground potential under any scenario
Downstream distribution will locate the 3 phase conductors with respect to ground. This is the case with 480v to 208v PDUs.

14. Battery Operation - continued
Is my load at risk?
The NEC describes grounded systems as generating fault current upon the first phase-to-ground fault. Ungrounded systems generate current on the second fault.
Two concerns with ungrounded systems include not detecting output faults, and ground fault detectors tripping “unexpectedly” upon the return of utility voltage.
Phase to ground faults while on battery are rare, but
We can set the UPS to shutdown on a detected fault while on battery
BTW, Load faults are ALWAYS “unexpected”.
Phase to phase faults will ALWAYS generate fault current.
Phase to ground faults downstream of step-down transformers (PDU) will ALWAYS generate fault current upon the first output-ground fault.
Phase to ground faults before downstream distribution ALWAYS generate fault current upon the second fault.
Bottom line? The UPS will operate safely in all modes, on utility or on battery.

15. What about faults?
Is the transformerless UPS safe? And does it operate predictably when a fault occurs?
What if a fault occurs between the UPS and the PDU?
What is the “return path” for fault current in a transformerless UPS?
What if a fault occurs on the load side of the PDU?
FUSE A B C N G
SAFETY
GND TO FRAME
PDU
GND
SSW
SOURCE
X FMR
Phase to Gnd
Fault
Fuse will not clear. UPS will enter current limit, and transfer to bypass. Bypass fuse will clear.

16. FUSE A B C N G
SAFETY
GND TO FRAME
GND
ON BATTERY 208/120V OR 415/240V
SSW
208 OR 400V
LOAD PANEL
W/ P-N LOADS
SOURCE
X FMR
Phase to Gnd
Fault
Return current can NOT flow; IGBTs are off when utility has failed
Fuse will not clear. UPS will enter current limit at 300% nominal for 20 cycles
What about faults?
What if I have a fault between the UPS and the PDU while on battery??
There is NOT a return path for fault current! (so no current will flow)
What happens to the load? (hint: nothing)
What happens when Utility returns or Generator comes on line?
Note: the UPS can be set to shutdown instantly on this condition (ref moves away from gnd.)
FUSE A B C N G
SAFETY
GND TO FRAME
PDU
GND
SP5W
SOURCE
X FMR
Phase to Gnd
Fault
Return current can NOT flow; IGBTs are off when utility has failed

17. What about faults?
OK, wise guy, what happens if I have a fault while on battery in a V UPS with no PDU?
In this case, there IS a return path, so fault currents will cause UPS to enter 300% current limit for 20 cycles, then trip off.
FUSE A B C N G
SAFETY
GND TO FRAME
GND
ON BATTERY 208/120V OR 415/240V
SSW
208 OR 400V
LOAD PANEL
W/ P-N LOADS
SOURCE
X FMR
Phase to Gnd
Fault
Return current can NOT flow; IGBTs are off when utility has failed
Fuse will not clear. UPS will enter current limit at 300% nominal for 20 cycles

18. What about faults? What if I have an HRG source? HRG SOURCE
FUSE A B C N G
SAFETY
GND TO FRAME
PDU
GND
SSW
HRG
SOURCE
Phase to Gnd
Fault
Resistance limits return current, and triggers gnd fault alarm

19. What if I have a parallel system?
If there are phase to neutral loads connected at the output of the tie cabinet or SBM, the same rule applies: bring in a source neutral to each unit’s E12 terminal, and pass that neutral on to the SBM or tie cabinet. Do not bond E12-G in the UPS or at the tie point.

20. Parallel Systems—Distributed Bypass
If there are NO phase to neutral loads (more typical), we still need a full-rated 4th wire from each UPS E12 to a common tie point in the tie cabinet.
This may confuse inspectors/installers, but the reason is that we like to have a SINGLE center point reference for the whole parallel system. The impedance from each UPS to its common CP reference needs to be the same. That’s why we require a “neutral” wire from each UPS.
The Neutral bar in the tie cabinet can be bonded to ground. If the inspector objects, we can use NRK kits in each UPS, and remove the N-G bond in the tie cabinet.
Pull an E12 wire from each UPS

21. Parallel Systems—with SBM
If there are no phase to neutral loads, a full-rated 4th wire should be connected from E12 in each UPS to a common tie point in the SBM.
This may confuse inspectors/installers, but the reason is that we like to have a SINGLE center point reference for the whole parallel system. The impedance from each UPS to its CP reference needs to be the same. That’s why we require an E12 wire from each UPS.
The E12 bar in the tie cabinet “can” be bonded to ground. If the inspector objects, we can use NRK kits in each UPS, and remove the E12-G bond in the tie cabinet.
Pull an E12 wire from each UPS

22. Key Take-Aways
The Installation manual is your best (first) reference for our grounding recommendations.
If the engineer/inspector balks, then we do have options.
There are many ways to handle grounding, that are safe and code-compliant…..it’s not just ONE answer. We do have our “preferences”, for best operation of the UPS, and that’s what shows in the Manual.
If there are phase to neutral loads, we MUST ALWAYS bring in a source neutral, so no delta or HRG sources allowed in that case.
The absence of transformers in the UPS does not impair the safety or code-compliant nature of the UPS, but it can cause confusion…..for the real grounding gurus, refer them to the white paper. It should help answer the really deep questions.
Refer to the “Q&A” section of the paper to answer most installers and engineers’ concerns.