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Thread: 5 KVA inverters in parallel

  1. #1
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    5 KVA inverters in parallel

    Lets say you install 3 X 5 kva inverters in parallel ... The system is used as a backup only (no solar panels) from the meter to the main DB ... from the main DB to the inverters and back to the main DB (no splitting essential/non essential ... everything is connected to the 3 inverters in parallel and ther eis sufficient power to supplt the entire installation so no switching off breakers required) how do you configue the circuit breakers and wire size?

    Do you fit a 63 amp tripple pole mcb on the input side and the 1 on the output side and use 16 mm wire to connect?

    Do you fit a 32 amp triple pole mcb on the input and 1 on the output side and use 4 mm wire?

    Do you fit a 63 and single pole breaker ... then take a 16 mm wire from the busbar to each inverter ... out of each inverter to a busbar and then to a 63 amp breaker ... from the breaker to the main switch

    Then to complicate the setup ... there is a changeover/ bypass switch.

    A 5 kva inverter can carry a max of 22 amps x 3 = 66 amps

    It is interesting to see the way some people are doing it ... is it the correct way to do it ... who knows ... I think a lot of people are just taking a chnace and hoping their method works...

    if you look at the instruction manual supplied with the inverter manufacturer ... you could get yourself into a bit of shyte ... fusing the neutral ... eeeish.

    Looking at anex P ... earht leakage before the inverter ... why would you install and earth leakage before the inverter ... and generally the load is "essential".

    This reminds me of back (many many years ago) when we started doing generator installations ... thanks to load shedding (genrators went from R3K per unit to R12K wihin weeks and the containers were arriving by the the thousands ... then sudddenly the load shedding stopped without notice and people lost a lot of money for all those container which sat for months) ... there was much confusion ... eventually the technical people visted a few of the sites and so we learnt how to do it correctly.
    Comments are based on opinion...not always facts....that's why people use an alias.

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    Diamond Member Justloadit's Avatar
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    If the inverters are hybrid, then the ELU before the inverter is to ensure that if there is any issue in the inverter, that the ELU will disconnect the inverter.
    This off course means that there must be an ELU after the inverter to ensure the load is protected.
    If only using one ELU, and before the inverter, an earth fault would disconnect the supply to the inverter, but the back up would then continue to operate, there by negating the earth fault.
    A minimum would be an ELU on the output of the inverter. However if there is an earth fault in the inverter, i am not sure what the dangers would be, since the fault would create a number of scenarios.

    Placing the inverters in parallel, would require in my opinion, an individual suitably rated breaker per inverter. Anything higher could create a fire hazard if the one inverter fails, and the other two now can not take the load. Yes the inverters have internal overload detection, but the circuit breakers are there for when there is a failure in the inverter not to protect the inverter from overload.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

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    The setup:

    Supply from the pole to the meter ... 230 VAC single phase ... 80 amp.

    From the meter to the main DB 25 mm ... house wire in a conduit.

    From a 60 amp single phase breaker in the main DB to the DB mounted close to the inverter 16 mm wire.

    From the main switch to a bussbar (connecting the 3 breakers in parallel) on top of 3 x 25 amp D curve breakers or 3 x 32 amp std breakers ... 16 mm wire and to the (eishkom) input of the changeover/bypass switch.

    From the bottom of each 25 amp breakers to the input of each inverter ... 6 mm wire ( the same length)

    From each inverter output to the top of 3 x 25 amp D curve breakers ... 6 mm wire (the same length) ... I am told using 4 mm ... the wires get warm ... it could be that the person used different length tails.

    A bussbar at the bottom of the 3 x 25 amp breakers to link them in parallel to the inverter input of the changeover/bypass switch ... 16 mm wires.

    From the output of the changeover/bypass switch to the main switch of level 2 of the inverter DB.

    From the main switch to circuit breakers feeding 3 other Sub DB's and to an earth leakage which supplies plugs and lights from the inverter DB.
    Comments are based on opinion...not always facts....that's why people use an alias.

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    I dont believe an earth leakage is required from the supply to the inverter ( more nuisance tripping to worry about and 2 earth leakage units in series ... bad idea) ... it shows it at the back of the SANS book ... but I do believe that it is only for examples and is a bad example ... I could be wrong.
    Comments are based on opinion...not always facts....that's why people use an alias.

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    Diamond Member Justloadit's Avatar
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    Semiconductor tabs are isolated from the heat sink, which is earthed with a membrane for isolation.
    These membranes are rated at 4KV, depending on their thickness.

    There is no reason that there could be an iron filling trapped between the membrane and the semiconductor, which in time can pierce the membrane and cause a path way to earth.
    There is also no reason that could protect an arc between the mounting screw, or tab to earth due to a surge, which then punctures the membrane and create a path to earth.
    Now the devices are switching a DC source to create AC, but there is nothing to stop the arc from continuing on the DC side and creating havoc.

    Of course what I have explained above is exceptional cases, which I have experienced before, but is protection not there for the exceptional cases for additional protection to users?

    Your call.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

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    Quote Originally Posted by ians View Post
    I dont believe an earth leakage is required from the supply to the inverter ( more nuisance tripping to worry about and 2 earth leakage units in series ... bad idea) ... it shows it at the back of the SANS book ... but I do believe that it is only for examples and is a bad example ... I could be wrong.
    Hi Ians

    Had a look at the back and see there is a note ELU may be used as main switch .
    Also went through the regs and only find 7.12.4.2 that provides insight to the schematic at the back.
    I read the regulations that you install earth leakages as you would normally use an ELU

    There is a reference to the earthing and neutrals on the output side of the inverters and the fact that you cannot rely on the ELU before the inverter to protect the output of the inverter

    7.12.4.2 Where a common neutral and a bypass switch are used, the part of
    the installation supplied by the alternative supply shall be provided with earth
    leakage protection when required in 6.7.5. (See figure P.3.) (See also
    6.7.5.5(a) for exclusion relating to safety supplies.)

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    I see you guys are no longer living in PE ...you now live in GQ

    Would I be correct in stating that earth leakage protection is "not" required on the supply side of an inverter (input) ... however all circuits as per the SANS regulation after the inverter (output) require earth leakage protection.

    It is not advisable to install earth leakage protection on the input side of the inverter as protection for circuits on the output side of the inverter?

    The only time a person would use an earth leakage device on the input side ... would be if the inverter was mounted on a mobile trolley with a flexible cable plugged into a wall socket or used on a construction site as a mobile power supply.
    Comments are based on opinion...not always facts....that's why people use an alias.

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    Diamond Member Justloadit's Avatar
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    In my opinion, if the inverter is of the Hybrid type, or Grid tied inverter, I would insert an ELU before the inverter. The protection of the ELU is not for the loads after the inverter, but rather for protection of a user on the inverter equipment in the case of failure.
    Many of these new Hybrids have batteries, communication cables, remote displays and other monitoring equipment connected to it which is not protected by the load side ELU. All this equipment is handled by a user at some time or other while there is power, so it is logical to put some form of protection, besides a circuit breaker. Maybe use a 30mA ELU to reduce nuisance tripping. Usually at switch on, there is a huge inrush in current, which in some instances generates a momentarily earth fault and could trigger the ELU if too sensitive. Also using a higher mA rating, would prevent both ELUs tripping in the case of an earth fault on the load side of the inverter.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

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    According to the regs it is not requirement to place an ELU before the inverter and I would be hesitant to do so as there will be leakages to earth which could cause nuisance tripping. The minute there is a surge arrestor , or mosfet built into the inverter any spike will be forced to earth creating a problem.
    Have experienced that problem with the surge arrestor type plug tops.

    A circuit breaker will trip on an earth fault but yes at a slightly higher leakage to earth. If your earthing is correct you will not experience a touch potential and the risk of being shocked is basically zero.

    The earth leakage unit was designed and installed mainly due to portable appliances with flexible cords as extra protection in case a flexible cord was damaged and lost an earth. At least the ELU would pick up an imbalance between live and neutral on a person touching the appliance and becoming the path to earth. With CB protection only you would know that you became the path - With an ELU tripping at 30Ma you may not even feel it.

    If you take an extreme case and on the big electrode boilers you are allowed a 10% leakage to earth before having to trip the system ( Reg pasted below) . On a 800KVA boiler you would easily measure 100Amp on the earth wire. You could touch the boiler , you could touch the earth wire and there would be no problem. You use an Adit unit that you can set up to measure the imbalance.
    If your earth came loose then you knew all about it.

    I personally see more problems with installing an ELU before the inverter. If you think of a 30KW or 100KW inverter with an earth leakage device before could be a nightmare of nuisance tripping.


    6.16.7.3 Earth leakage protection shall be provided for the circuit that
    supplies an electrode water heater, steam generator or boiler. This
    protection shall be set to operate in the event of a leakage current exceeding
    10 % of the current consumed by the appliance under normal conditions of
    operation. The characteristics of the circuit and of the earth return path shall
    be such that the earth leakage device will operate before the potential
    between earth and the shell of the steam generator or boiler exceeds 50 V,
    including under short-circuit fault conditions

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    Diamond Member Justloadit's Avatar
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    Quote Originally Posted by GCE View Post

    A circuit breaker will trip on an earth fault but yes at a slightly higher leakage to earth. If your earthing is correct you will not experience a touch potential and the risk of being shocked is basically zero.

    The earth leakage unit was designed and installed mainly due to portable appliances with flexible cords as extra protection in case a flexible cord was damaged and lost an earth. At least the ELU would pick up an imbalance between live and neutral on a person touching the appliance and becoming the path to earth. With CB protection only you would know that you became the path - With an ELU tripping at 30Ma you may not even feel it.
    I agree when a system is set up to regulations, as it should be!
    In today's climate, with the theft of supply cables, Earth and Neutral wires at substations and mains supplies being a norm, then the typical systems installed may not protect the user from these faults occurring.

    If plug tops are tripping the ELU's then they are poorly designed, or are faulty due to previous surges that the plug top has experienced. Once of the characteristics of MOVs is that with each light surge it protects, the device begins to breakdown, and if this is one of thew MOVs connected to earth, then it will cause the ELU to trip. It should be replaced if this happens, as it already has done its job.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

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