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Thread: Neutral earth bonding on backup systems.

  1. #111
    Platinum Member Derlyn's Avatar
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    When the inverter goes into islanding mode, does the point of supply change from the installation main breaker to the inverter output breaker ?

    If this is not the case, then irrespective of whether the bridge is permanent or through a relay, it still contradicts the regulation that says the neutral earth bridge must not be downstream of the point of supply.

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    Diamond Member Justloadit's Avatar
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    When the inverter goes into "Island" mode, a double pole internal relay contacts disconnect the input and output of the inverter.
    So the point of supply now becomes the inverter. If there is no Neutral/Earth bond placed on the output of the inverter, then the ELU will not be able to detect an earth fault as it has now lost the 'Earth' path, placing the user in a dangerous environment.

    Another reason for this Neutral/Earth connection, is also to ensure that the Inverter Live & Neutral lines to not rise above earth, which is usually the case because the inverter is isolated from the grid supply. This can become a few thousand Volts charging up, and in some cases when the Mains returns, and the inverter then connects the load to the mains, is when there is a discharge damaging the inverter.

    It is the same principle, in which the secondary of a supply transformer, Neutral connection is made to Earth. It serves 2 purposes, 1 is used to detect faults, but the second reason which is little understood, is to ensure that the transformer secondary is tied to earth, and not allow the secondary voltage to rise way above earth and causing a discharge to earth during operation.
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    Platinum Member Derlyn's Avatar
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    Thank you, brother.
    Question answered. The point of supply does change.

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    I take it they have stopped threatening people with court action?

    I was threatened with court action by a certain person from the AIA, I am still waiting.

    There are too sheep, he said, they said, we said, I said in the electrical industry and not enough skilled electricians, people who can stand up, answer questions and understand why and how they came up with the answer, not just copy and paste information they find in social media groups.

    That is why I enjoy this platform, we may not all agree with the answers, but at least we are prepared to thrash it out and offer some form of reasoning for the answer create a bit of banter.


    Quote Originally Posted by GCE View Post
    The AIA's ,as a company, have been quite on the issue which is the reason ECA was tasked with putting it together
    Comments are my opinion, unless regulations are attached to support the comment. This is social media, not a court room.

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    What has surprised me most when looking for answers is that the sunsynk practice note and training manual both recommend a relay

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    Platinum Member Derlyn's Avatar
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    Question.

    There are inverters that have an output intended to be used for a relay or contactor for neutral earth bridging purposes.

    On these same inverters there's a time delay after the resumption of Eskom power before the inverter switches out of islanding mode. Sometimes about a minute.

    My question is ......... at what point in time does the internal relay operate ? Is it on the resumption of mains power, or, is it when the inverter switches out of islanding mode ?

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    The 60sec time delay switching back to grid is a Utility requirement and also gives the inverter time to sync with grid before switching - some inverters can be set to change back with a longer time delay .
    We set all of ours above 10min to allow the voltage to settle - If I look at graphs from the inverters and depending on the time , 16h00 to 18h00 and 18h00 to 20h00 , being the worst slots the voltage comes back at 198v and takes around 10 minutes to settle at 220 to 230v

    As far as I am aware the neutral earth bond stays for 10sec after connecting back to grid , coming out of islanding

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    Derlyn (01-Feb-24)

  9. #118
    Platinum Member Derlyn's Avatar
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    Thank you my brother.

    As an aside, on my house, all plug circuits are fed through a contactor.

    The contactor is energised by means of an Ellies Fridge Safe unit that is plugged into a dedicated plug.

    On resumption of power after loadshedding, that unit monitors the voltage for 3 minutes before switching on if the voltage is within standards.

    So I am protecting all the plugs with one unit, the boere way.

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    Platinum Member Derlyn's Avatar
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    I am still learning.

    Recently, our brother Justloadit, explained that one of the reasons the one end of the secondary of a supply transformer is connected to earth, is to prevent the secondary from drifting up to dangerously high voltages that can be in the thousands.

    This is also one of the reasons that the neutral of an inverter output should be grounded.

    My question is. ..... what prevents the output of a safety transformer from drifting upwards ?

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    Quote Originally Posted by Derlyn View Post
    I am still learning.

    Recently, our brother Justloadit, explained that one of the reasons the one end of the secondary of a supply transformer is connected to earth, is to prevent the secondary from drifting up to dangerously high voltages that can be in the thousands.

    This is also one of the reasons that the neutral of an inverter output should be grounded.

    My question is. ..... what prevents the output of a safety transformer from drifting upwards ?
    IMHO, there is no perfect solution for all possible situations. For example, if the earth connection at the transformer is stolen and having your inverter's neutral bonded to earth at all times would mean that you could then be providing the e/n bonding for the entire neighborhood, So, any current that passes from live to earth in the neighborhood would be passing through your bond. There is no guarantee that everyone has proper earth leakage protection. Furthermore, your earthing may not be perfect either. Passing a current through the earth connection which is, let's say, connected to your household's plumbing, may result in a voltage difference between the water that comes out at your shower and the water on the shower floor. I know two families who were experiencing a tingling sensation in the shower after having installed an inverter. In both cases, the installer had a permanent e/n bond on the inverter output. The tingling wasn't there during load-shedding which was a clue. After much discussion, the installers reluctantly agreed to install bonding relays which solved the problem.

    Another consideration if you should have e/n bonding at your location as well as at the local transformer is that some of the current to your home will always be passing through the earth wire from the transformer since it will be in parallel with the neutral wire from the transformer. The earth wire and things that it may be connected to are not insulated, so any voltage difference (no matter how small) between the wire and 'real' earth (moist soil etc.) will result in corrosion. Remember that for current to pass through a wire, there has to be a voltage difference across it unless the wire's resistance is zero which is not possible. (Ohm's law)

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