4-wire secondary feeds 3- wire UPS

[RUWired]

I came across a 480 to 208/120 secondary transformer feeding a 3-wire 208 volt UPS that was not bonded at the transformer nor any GEC.The neutral was ran with the cable to the ups, but was not able to open up the ups to see what they did with it yet.Would i be right to think the xo needs to be bonded and a gec to the steel, even though its a 3-wire ups.The xfrmr only feeds the ups.
Rick

 

[brian john]

RU;

there are so many different configurations of UPS with one, two and three input and output transformers and bypass configurations. The best thing I could recommend is to get the serial no. and model no. and call the manufacture for drawings and clarifications.

In my experience the UPS field tech seldom know or understand I/O (Input/Output)they just do the box (UPS). Unless you are fortunate enough to get an experienced tech.

 

[Smart $]

...Would i be right to think the xo needs to be bonded...

Yes. Ref: 250.20(B)(1)

 

[RUWired]

Thanks for the replies, I was'nt sure with it not feeding a panelboard and feeding a piece of equipment not needing the fourth wire.
Rick

 

[brian john]

As stated it depends on the type of transformers, bypass configuration. Randomly grounding the XO without knowing the above (AND MORE) can result in net currents, PQ issues and make you look foolish.

Is this a single source UPS? or does it have seperate input and bypass? It is possible that a seperate source is all ready grounded.

 

[don_resqcapt19]

Brian,
If the secondary is 120/208 3 phase 4 wire, what section would permit XO to be ungrounded?
Don

 

[brian john]

Don If I am reading the post correctly. The OP ask about a 480 to 120/208 (Assuming delta wye). If this is a dual source UPS separate bypass and input so there are two transformers Neutrals are common, Grounding both neutrals could result in a net current issue. Now the 208 input would mean no neutral current but still multiple grounds should not be installed.

I asked the OP to verify this. With UPS systems I have sooooo many different configurations, that IMO one has to be careful before assuming (there's that word) anything.

What I always do is draw out the complete system from utility, generator and UPS and loads.

 

[ghostbuster]

Don If I am reading the post correctly. The OP ask about a 480 to 120/208 (Assuming delta wye). If this is a dual source UPS separate bypass and input so there are two transformers Neutrals are common, Grounding both neutrals could result in a net current issue. Now the 208 input would mean no neutral current but still multiple grounds should not be installed.

I asked the OP to verify this. With UPS systems I have sooooo many different configurations, that IMO one has to be careful before assuming (there's that word) anything.

What I always do is draw out the complete system from utility, generator and UPS and loads.

Don
I would have to agree with Brian on this.Many times we have found the UPS techs have inadvertently grounded the same neutral at 2 different locations.This sets up net current imbalances etc.At 1 location, we found over 80 amps flowing on the ground to an unenergized UPS. Reason: The system neutral was tied to ground (2nd time) within this UPS.

 

[don_resqcapt19]

Brian,

Don If I am reading the post correctly. The OP ask about a 480 to 120/208 (Assuming delta wye). If this is a dual source UPS separate bypass and input so there are two transformers Neutrals are common, Grounding both neutrals could result in a net current issue. Now the 208 input would mean no neutral current but still multiple grounds should not be installed.

It appears that we have 480 to 208/120 wye transformer that supplies the normal power to the UPS. The code requires that the secondary of this transformer be grounded. There is no choice.
Don

 

[brian john]

Don:

I am not saying he shouldn't have a grounded secondary. BUT if the transformer is UNGROUNDED, I would check to verify that there was not a second transformer for bypass. I have seen this exact installation before. Be complete in your investigation, that's all I am saying.

 

[don_resqcapt19]

Brian,

BUT if the transformer is UNGROUNDED, I would check to verify that there was not a second transformer for bypass. I have seen this exact installation before.

How would the existance of a second transformer change anything? There is no code compliant way to install a transformer with a 208/120Y secondary without grounding XO.
Don

 

[brian john]

Don: In addition the code states that the objectional should be avoided. If you have two transformers and the load/secondary neutrals common in the UPS I/O cabinet as it would/could be in a dual source UPS (seperate input and bypass) and the neutrals are grounded in one of the transformers or the I/O cabinet. Then the neutral is grounded in the other transformer as well.

Much the same situation as if you install a generator with a 3-pole ATS, do you ground the neutral in bothlocations or one?

 

[brian john]

Saw this today

Saw this today

Double Post

 

[brian john]

Saw this today

Saw this today

Not quite the same as the original posters question but still wrong.


Dual source UPS 3-phase 3-wire, 480 VAC input, the contractor pulled neutrals and connected them in the I/O-bypass cabinet. Note the neutral to ground bond. Both source input neutrals originated at the main service.

 

[Smart $]

...do you ground the neutral in bothlocations or one?

The neutral has to be bonded to ground at only one point and that bond serves both transformers. However, switching can not isolate the bond to one transformer if the other transformer remains energized.

 

[don_resqcapt19]

Brian,

Don: In addition the code states that the objectional should be avoided. If you have two transformers and the load/secondary neutrals common in the UPS I/O cabinet as it would/could be in a dual source UPS (seperate input and bypass) and the neutrals are grounded in one of the transformers or the I/O cabinet. Then the neutral is grounded in the other transformer as well.
Much the same situation as if you install a generator with a 3-pole ATS, do you ground the neutral in bothlocations or one?

I guess I understand this type of installation now, but it doesn't make a lot of sense to me. If the load is so critical that you need the dual input and the bypass, this arrangement seems to make it impossible to replace one of the transformers or the UPS without dropping the load as there is only a single bonding point.
Don

 

[Pierre C Belarge]

As to the OP, the transformer that supplies the power to the UPS needs to be grounded on the secondary side, regardless of the type of UPS supplied. If there is concern of objectionable current, than let it be grounded at the transformer itself, and not at the UPS. (as Don has stated). And yes I can see the issue at the UPS itself, but that is a different issue than being discussed.

 

[brian john]

Don:

Some single UPS's have three sources, UPS input, UPS Bypass, wrap around bypass. Very common installation, and very common for there to be grounding issues with these systems.

 

[ghostbuster]

Don:

Some single UPS's have three sources, UPS input, UPS Bypass, wrap around bypass. Very common installation, and very common for there to be grounding issues with these systems.

Brian

As usual ,you are right on.

 

[don_resqcapt19]

Brian,

Some single UPS's have three sources, UPS input, UPS Bypass, wrap around bypass. Very common installation, and very common for there to be grounding issues with these systems.

That would be a design problem...I'm sure that the installation can be made to fuction properly and still be in compliance with the code.
Don