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How can you determine by measurement if you have two separate sources if supply?

Former Community Member
Former Community Member
Hi everyone, I am looking for some answers for the following situation:


There are two identical three phase step-down transformers 11kV to 0.4kV, connected to a 11kV ring main, each have an UPS system connected downstream and both are connected to the same Automatic Transfer Switch:

TX1 → UPS1 → ATS SOURCE 1

TX2 → UPS2 → ATS SOURCE 2


I want to plug in a parallel supply ciruit in the ATS for source 1 (considering the new and existing circuit would be live) and I want to double check by measurement that the new circuit is not connected to another source by mistake.


I have measured at the ATS the voltage and frequency on the same source, U=400V / Uo=230V / 50Hz and I wasn't expecting any voltage between the two separate sources but this is what i found:

TX1/ L1 - TX2/ L1 = 2V / 700-800Hz

TX1/ L1 - TX2/ L2 = 400V / 50Hz

TX1/ L1 - TX2/ L3 = 400V / 50Hz


TX1/ L2 - TX2/ L1 = 400V / 50Hz

TX1/ L2 - TX2/ L2 = 2V / 700-800Hz

TX1/ L2 - TX2/ L3 = 400V / 50Hz


TX1/ L3 - TX2/ L1 = 400V / 50Hz

TX1/ L3 - TX2/ L2 = 400V / 50Hz

TX1/ L3 - TX2/ L3 = 2V / 700-800Hz


From the readings above I can see that the different frequency between TX1/ L1 and TX2/ L1 etc is an indicator of a different source of supply.


Is my assumption correct? 

Why is there a voltage between the two sources of supply if there is no physical connection due to the internal windings in the transformer?

Why is there a highly fluctuating frequency between the same lines of the two sources?


Thank you in advance!


Andrei Ionescu
  • 0
    Ask yourself the question: what technology and topology are those UPSes? Are they true-on-line, buck-boost or transfer? Now you might have some insight into how well they are regulating the voltage at the ATS, if they provide any isolation, and why there might be a carrier or intermodulation product appearing that your instrument is registering.


    Force one or the other or both UPS out of standby and into backup and repeat your measurements. What did that just tell you?


    Where are the filters in the system?


    Draw a diagram of what you are thinking of doing. Are you planning to put UPSes in parallel operation without thinking of the impedances of the networks on both sides of those converters? Best of luck with that!
  • 0
    Of course you will get a potential difference between the two sources as the neutrals will be linked which, by the way, must be kept intact with UPS systems to prevent floating neutrals - 3 pole switching only.

    The 700-800Hz seems like the PCM frequency of the UPS invertors and is being imposed on the line albeit at a very low level, I think that is quite normal.

    Not sure about the rationale of paralleling another source alongside the existing source to one leg of the ATS, sounds a bit dodgy to me.

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