UPS system normal and bypass power sources

[joshuagarcia]

Hello,

Can you direct me in the code where it states that the normal power source and the bypass power source shall be fed from the same power source? I remember locating it a few years ago, but am unable to do so now.
To elaborate, the battery charger/inverters in all of the plants that I have worked in, all have a source that feeds the unit and a bypass power source which are always fed from the same MCC.
Also, can you shed some light on why this is?

 

[iwire]

I don't know of NEC rule like that however it would not surprise me that a manufacturer would require the static bypass to be supplied from the same source or there could be a voltage potential between the two sources which would mean it could not go into bypass.

 

[don_resqcapt19]

I have installed UPS systems where the bypass source was a separate source. It had a sync check and would not permit a closed transition switch to the second source if the two were not in sync.

 

[ron]

There is no national code requirement for the bypass input and the rectifier input to be from the same source. I am also not aware of many manufacturers that have this requirement. I recently did a Schneider Symmetra MW and since they are line interactive (not double conversion) they wanted the static bypass input and UPS rectifier input to be the same source.

The transfer from UPS output to maintenance wrap around bypass often is done closed transition, so a sync check relay makes sure it is synchronized (maybe not the actual same source, like utility / gen).

The transfer from UPS output to static bypass often is done quickly but via open transition, but the UPS output is created with the static bypass as a reference, so they are typically synchronized anyway.

 

[jmargolis79]

Some UPS systems use double conversion. They are always taking ac to dc and then dc to ac all the time. So during the main to bypass power switch they could be using the last have of the conversion for the few cycles of the transfer.