2 UPS's in daisy-chain

[Anonymous user]

Hi,

All we know that we will get know that our UPS battery has worn out during power failure incident.

What if reduce the negative risk of this failure while connecting 2 UPS's in daisy-chain ?

APC FAQ gives some comments on that

http://www.apc.com/solutions/display.cfm?id=59BFAC22-5056-AE36-FE722E8CC1311295&ISOCountryCode=gr

The argumentation that it will not double the capacity is acceptable, but please give your comment if it will work in the situation one of UPS battery is worn out.

Thank you for your comments in advance.

[BillP]

Hello:

While your scenario may theoretically work in some cases, it is not the way a UPS is designed to work and is not something we can recommend.  As the knowledge base you linked states, we don't recommend daisy chaining UPS's in any situation.  As you mentioned, the UPS should notify you in time if the battery has gone bad.  

If your load is critical enough that you think redundant UPS's are necessary, then perhaps you might want to look into an Automatic Transfer Switch.  With two compatible UPS's, you can plug each input of the transfer switch into a different UPS and the device will switch over in the event that one powers off.  

You can find information on these devices here:  http://www.apc.com/shop/us/en/categories/power-distribution/transfer-switches/rack-mount-transfer-sw....  Just make sure you confirm compatibility with the UPS's you are using.

Thanks

[Anonymous user]

Dear Caleb,

re

>> As you mentioned, the UPS should notify you in time if the battery has gone bad.

It seems that I have failed to express myself, I wanted to say that "we can not know in advance that the battery is bad, UPS will never tell this in advance". I used to learn not single time, UPS pass testing and calibration and after a month, during the power supply incident, UPS does not help because of battery is dead.

However your suggestion to look at ATS (which is new for me) answers to my question how to manage this scenario.

Thank you indeed.

Arvidas

[Anonymous user]

I wonder if ATS can be fed with two separate 230V phases of 380 V AC power supply, if it is designed to do so ?

[Brad_C · Accepted solution]

If you are talking about a common neutral, and separate phases into each input, then yes most will allow that. It's not remotely ideal as it means the ATS must make a slower transfer process to avoid arcing and consequential relay welding, but yes.

I think the thing that has been missed about a series UPS configuration is the glitch that causes the UPS to switch over will switch both units simultaneously. If your downstream UPS has a fault, you'll lose your load anyway before it has a chance to switch back as its input is supplied by the upstream UPS. It's just not practical for a whole host of reasons. An ATS is a really nice solution as it will sit and drain the primary UPS while the secondary one idles. It'll then switch over and go until the secondary dies. Not as efficient as properly sizing a UPS, but it'll work.

I use an AP7721 here. I have an SUA3000 on the primary input, and either a spare UPS or commonly just a straight plug to the wall on the secondary input. This allows me to take the UPS out of service for maintenance without losing the load. A second hand AP7721 was much cheaper (and far more versatile) than an external bypass switch.