Want to connect the neutrals from 2 small UPSs.

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sfscott

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I am being asked to build a control panel that will be fed from 2 UPSs. The UPSs will be supplied with separate 120VAC circuits not necessarily from the same source. I can keep the UPS load side power separated in my panel except for some shared 120VAC loads and this is my problem. Can I install isolation transformers on the load side of the UPSs and then connect the neutrals? If I can I will then just use a contactor to switch from one UPS hot to the other. I do not care is I lose power to these shared loads during the contact switching.
 
The 120V UPS sources are likely separately derived sources upstream, so the neutrals need to stay isolated from each other. Make the contactor a 2 pole so the neutral transfers with their associated hots
 
Hi Ron
Sorry for the late reply i didn't refresh my browser and i didn't realize you replied. Thanks for the reply. i have my code book out and i am looking at where it is permissible to break the neutral and i did find in 430 where it is acceptable as long as all circuit conductors are interrupted at the same time. Thanks again for the input.

steve
 
Hi Ron
If you're still listening i have a follow up question; These small UPSs when they go on battery will have a floating neutral. Can i install an isolation transformer on the load side of the UPS and eliminate the floating neutral?
 
sfscott said:
These small UPSs when they go on battery will have a floating neutral. Can i install an isolation transformer on the load side of the UPS and eliminate the floating neutral?
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There is nothing stopping you ever from installing an isolation transformer.
In any of the cases, you will still want to open both the energized and neutral conductor when transferring.
 
If the UPS supplies "premises wiring" as defined in Article 100...a very broad definition, you will not be permitted to have a "floating neutral" per 250.20.
 
Gentlemen, looks like i have some homework. The UPS are off the shelf APC #SMT1000 desktop units. I believe to get UL certification the UPS manufacturers have to isolate their outputs while on battery only to avoid backfeeding i presume. My idea behind the isolation transformer is this would eliminate the floating neutral but i mistakenly thought this would also allow me to connect the neutrals together and only break the hots when transferring. i will plan on having a 4 pole contactor with 2 NO and 2 NC contacts. I can deal with loss of power while transferring. As Ron pointed out above i have to assume the 2 feeds to my UPS are separately derived sources.
 
For background these panels will control ventilation fans and dampers in a railway tunnel.
 
My opinion is this panel and UPS will fall under 250.21.3 and it meets all of the conditions except d. I do not have a ground detector and would have to research this subject. The UPS while operating on AC power is not isolated from the incoming power source so no floating neutral, it will only be floating for the 5 minutes of battery life it has. I will do some more code book reading tomorrow, i have an old handbook here in my office but i have the new one at home and i think there might be more examples in the new handbook. Thanks to Don and Ron for the help so far.
 
These units don't have floating neutrals when on battery. Neutral and ground go straight through. They are just line-interactive UPS systems and believe it or not the inverter is always running.
 
OK, thanks Bjenks and Ron sounds like i misread the documentation. I am redfaced....i will watch the video but I think i will still have to follow Ron's advice regarding the fact that i need to treat them as separately derived systems. I have the complete set of 'E' drawings from the project but i cannot find the 120/208 panel(s) that feeds the UPSs so until i do i must assume they are separately derived. Thanks to all fo helping me out.
 
ron said:
BJenks - Well done
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That video is pretty interesting, but he found out later that his one-line for the line-interactive was wrong or based on the real cheap versions. Typically higher quality/larger line interactive units connect the inverter directly to the output and use the fly back diode of the inverter to charge the batteries. Many times the inverter keeps running and connected directly to the output so as to have some line regulation (see Tesla powerwall with inverter that markets voltage regulation). Then when he loaded up the google trade mark site, I noticed it said Best Power who got bought by Exide, who got bought by Eaton and now is gone. Best Power was basically an obsolete technology that used the standby UPS method and connected a special ferro-resonant transformer/capacitor on the output. The AC output was in resonance and was recreated a 60 hz signal with isolation. When the power went out the UPS switched over to inverter, but the ferro -resonant system stored energy in it and kept things going while it switched over.
 
sfscott said:
OK, thanks Bjenks and Ron sounds like i misread the documentation. I am redfaced....i will watch the video but I think i will still have to follow Ron's advice regarding the fact that i need to treat them as separately derived systems. I have the complete set of 'E' drawings from the project but i cannot find the 120/208 panel(s) that feeds the UPSs so until i do i must assume they are separately derived. Thanks to all fo helping me out.
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Unless there is an actual isolation transformer, which APC does sell as a separate item, you don't have isolation. Trust me (I am an ex-APC employee)
 
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