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Testing requirements between mains supply and a backup generator.

MS01
Firstly to set the scene, i have an electrical installation with has a mains LV switch panel that is supplied via a DNO supply and has a standby generator. The Generator feeds into a section of the panel which can be isolated via a bus coupler device to ensure only the key services are fed via the Generator. My question is these  circuits will be tested under the characteristics of the mains DNO supply but is there a requirement to do the same exercise with the generator in operation?

I appreciate the difference with the lower fault level contribution with a higher ze from the Generator but …why would you need to undertake the cable tests for the standby Generator scenario as well as the normal mains supply?.   


Thanks
  • Former Community Member
    0 Former Community Member
    You just need to document the two sources of supply characteristics  - you don't need to duplicate the individual circuit tests


    You do need to verify that safety is assured when using either source of supply


    Regards


    OMS
  • 0

    why would you need to undertake the cable tests for the standby Generator scenario as well as the normal mains supply?.   




    In short, You don't .

    If there are bits of the installation that are only live under backup gen operation, yes, do remember they need testing too. Otherwise the main thing is to verify the ADS still works if the genset is a bit short on PSSC compared to the mains, and the other biggy to verify is that Zs  and Ze are going to be OK if the incoming mains is cut off - including loss of all supply side earthing. 

    That can be mostly a paper work exercise, and is normally trivial for things protected by RCBO or a genset incorporating an earth fault relay, and bit more involved if there are parts of the installation where it is protected by fuses and breakers only.

     


  • 0
    So its clear there is no requirement that states you must undertake the installation cable tests for both mains and generator operation?   


    I would only expect to undertake the cable tests under mains operation and not to duplicate the results when under Generator operation.
  • Former Community Member
    0 Former Community Member
    Most of the circuit tests are dead tests for a new installation - so they are relevant regardless of the source of supply


    For the rest, then you should be verifying (for example) phase sequence on both sources of supply, voltage and frequency on both sources of supply and so on and so forth


    Finally, you need to verify safety on both sources of supply - what testing you need to "duplicate" is likely to be those live tests only to verify safety.


    Regards


    OMS

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