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Thread: Theory - generator VoV

  1. #1
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    Theory - generator VoV

    Hello

    This has been on my mind awhile so here goes.

    With a VoV genny the issues are voltage on the earth wire.

    So in the image attached i have made it so that this is sepearted but also has Earth Fault protection as well as overcurrent.

    There is no rod being added either for a TT style of the genny as the ELU takes care of Earth Faults.

    PNB: Protective Neutral Bonding
    ELU: Earth Leakage Unit

    Let me know what i am missing please.

    Yes the Sans says no VoV gennies but lets be real almost all are VoV and if it can be done as added protection and tell the client in writing dont do it but they will at least they safe.

    Note: i attached 2 images of differnt image quality.



    Sent from my SM-P605 using Tapatalk

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    Just one thing I want to comment on, and that is the idea that RCDs don't work in non-TN earth systems (TN, terra-neutral, earth and neutral is at the same potential). RCDs are used even in IT systems (no bonding). If there is an imbalance of 30mA caused by some kind of earth fault (probably through a human being), it trips.

    There are two reasons for bonding the system. One is to limit the maximum voltage. As an example, it is possible for neutral and live to both be thousands of volts above earth while still only being 230V RMS between L and N. By tying one end to earth you know the highest voltage in the system is 230VRMS and it's on the live wire.

    The second reason is to help detect earth faults. With an unbonded system, the first earth fault goes unnoticed, but the second one closes the circuit and causes current to flow. By bonding the system, a single earth fault can be detected.

    There are many ways of referencing a system to earth. There is even something in three-phase systems called corner-earthing, where you have a delta transformer (aka three wires, no neutral), you will tie one of the phases to earth. Again, this is to limit the maximum voltage (some part of the system corresponds to 0V) and aid in detection of earth faults.

    Now note that even in unbonded systems with an undetected existing fault, an RCD still does the job. If 30mA of current passes through a human, the RCD will still see it and trip.

    Now... what's up with the V-0-V configuration? If you earth the central point, you still have some part of the system referenced to earth, the maximum voltage in the system is now 120V, and if there is a low-enough impedance path to make 30mA flow, your RCDs still trip. So on the upside, there is no reason to consider a V-0-V setup as unsafe in principle.

    BUT... South African regulations does not allow you to do this in a residential setting, and your neutral wiring is also not really neutral anymore... it is a second "hot" like it would be in North America.

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    Thanks for the indepth reply! I didn't know why we referenced earth like that till now and the whole 2 faults thing!

    Will read it more later with a more clear head as that is great info thank you.

    I enjoy the topic of earthing and bonding and learn something new every time.

    Sent from my SM-N960F using Tapatalk

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    Quote Originally Posted by Dylboy View Post
    Thanks for the indepth reply! I didn't know why we referenced earth like that till now and the whole 2 faults thing!

    Will read it more later with a more clear head as that is great info thank you.

    I enjoy the topic of earthing and bonding and learn something new every time.

    Sent from my SM-N960F using Tapatalk
    Phase monitor relays provide the same reliable protection on corner grounded delta systems.
    A corner grounded delta system is a common way to establish a reference to safety ground when dealing with an otherwise floating output from a delta secondary transformer.
    In order to apply a phase monitor relay in this type of installation, it is important to realize that despite the connection to ground the three phase delta output remains unchanged. A phase monitor relay should be connected to phases A, B, & C (one of which will be grounded) as you would in any standard application.

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    Justloadit (11-May-22)

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    Drawing of such systems
    Click image for larger version. 

Name:	corner-grounded-delta-system1.jpg 
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Size:	6.6 KB 
ID:	8529

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    That is very cool ans makes a lot of sense!
    Now I am a lot more confident in the grip of earthing as to why we have earthing at source and use Tera. It all has to do with keeping voltages from floating way above tera as well as fault return path with single faults.

    Very cool thank you!

    The delta earthing is very cool and I read it a long time ago and never got it but I believe I do now.

    Sent from my SM-N960F using Tapatalk

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    Don't know about you guys, but I have found that most generators ( or let's call them alternators ) seeing as they are AC, 5Kva and above that are now being sold are NOT vov wound.

    Just checked my records and the last 9 installations that I have done, not one alternator was vov wound.
    All new units recently purchased.

    All I do in that case is bridge out the earth and neutral terminals on the alternator, so the characteristics of the supply to the installation is identical irrespective of whether it is coming from the backup alternator or the supply authority.

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    Andy mentioned in a post that if the generator is of tubular frame they are v0v wound.These generators are actually designed for site use but even then you are supposed to use an earth spike. Like you said there are thousands of v0v used in domestic environment in this country just use a changeover switch (not plug to socket) with indication mains and gen with labelling according to regs

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    Hi there Derlyn.how about posting a couple of pics of how you did those connections on the alternator of those bennies please

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    Quote Originally Posted by BEVIN View Post
    Hi there Derlyn.how about posting a couple of pics of how you did those connections on the alternator of those bennies please
    see https://www.theforumsa.co.za/forums/...ors#post163293

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