How can an MSW inverter be protected from hot to ground shorts?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Mar 11, 2015.

  1. sdowney717
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    sdowney717 Senior Member

    An MSW inverter which does not tie neutral to ground is vulnerable to being destroyed if either hot ( both white and black carry about 60 vac) shorts to ground.

    For example from this manual

    http://gpelectric.com/files/gpelectric/Docs/Manuals/Go_Power_MAN_Inverter_GP-1750HD-2500_vA.pdf
    Caution
    So we read even within it's own design usage, if you do this, it is toasted.
    The MSW inverter is also grounded by the DC negative power wire, at least I think it is.
    It seems like an easy way to a destroyed inverter, say you plug in a cord with a short to ground, MSW inverter burns.
    Or plug in a defective device with either neutral or hot shorted internally, like a loose wire, the MSW inverter burns.

    So how can you prevent it from self immolation?

    A dual pole GFCI?
    GFCI for single hot wire, I don't think they will work on both neutral to ground and hot to ground shorts.
    And I am not willing to risk destroying an inverter to test, does anyone have any experience?
     
  2. groper
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    groper Senior Member

    Cut and Paste from online source you might find interesting...

    In some inverters designed for portable use, the two current carrying conductors connected to the "Line/Live/Hot" slot and the "Neutral / Return / Cold" slot of the receptacle ( for example, 15 A NEMA5-15R) are isolated from the metal chassis of the inverter. In these inverters, none of the two poles can be called Neutral as both these poles are isolated from the chassis of the inverter. Both the Line and Neutral slots of the receptacle will be at an elevated voltage with respect to the chassis - normally around 60 VAC (Half of the voltage between the two current carrying conductors). Hence, do not touch the neutral slot of the receptacle!

    These types of inverters are designed to be connected directly to the AC loads. These are not designed to be permanently installed into household or recreational vehicle AC distribution wiring. As this type of connection / installation can not be classified as a permanent installation, the NEC requirement of grounded distribution system doesn't strictly apply.

    The UL standard for this type of inverters- UL458 does not have a requirement for a bonded neutral on the output of inverters. As long as the installation requirement of grounding the chassis of the inverter has been accomplished, loads that are plugged in will have their chassis held at the same ground potential as the chassis of the inverter and the house or RV. The only difference is that the neutral slot of the receptacle has approximately 60V on it instead of the usual 0V. The impact of this is minimal, since parts of wiring and equipment that are connected to the neutral side of the circuit are required by safety standards to be treated as if they were at 120VAC, since there are many receptacles that are wired backwards or 2-prong plugs that are not polarized. Therefore, a voltage of approximately 60VAC of the Neutral slot is not accessible to the user, and any shock hazard presented is mitigated by lack of access. The main safety agencies, CSA, UL, and ETL, have all approved inverters with this half-voltage on the neutral scheme.
     
  3. groper
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    groper Senior Member

    In other words, what you are doing is not how the manufacturer of your inverter intended the product to be used. You can't use an inverter without neutral ground bond, to feed a distribution system in a boat. That type of inverter, should only be used to directly power a single load. As soon as you have a situation where there is more than 1 path for current to flow, treating an inverter like an isolation transformer is not a good idea. It is not as safe as when you have earthed distribution wiring with a neutral earth bond and Gfci protected circuits...

    For what it's worth, I don't know how the inverter could possible be damaged by bonding the ground and neutral if the installation is completely isolated from other current sources... I think they are concerned with fault currents entering the inverter from out of phase grid sources if the inverter is grounded to a common ground with grid wiring. If there is no other current sources connected to any active , neutral or earth wiring of the installation, then bonding the earth and neutral of the inverter should be harmless to the inverter.
     
  4. sdowney717
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    sdowney717 Senior Member

    I am sorry but people do not read the warning.

    If the MSW inverter is sitting there all by itself, not hooked up to boat, house, whatever. And you take either white or black AC output wires (both are hot since it has a live neutral) and ground it to the case or the ground lug, the inverter will short out and burn up.

    The DC ground, the negative 12vdc wire, and the inverter chassis and the AC ground terminal lug are all internally connected inside the inverter. (I will verify check with meter and eventually post back on that, but I have had these things apart before)

    the ac ground wire internally could be disconnected from the AC outlet, that would not really help though since on a boat, there is that AC-DC ground bond, so then 12vdc negative and AC ground is all connected.
    UNLESS, you put a relay for the AC-DC bond point. Which will only work is there is only one bond point between AC and DC ground.


    So if I plug in a defective wire with a ground short, or defective appliance with a ground short, the inverter will self immolate.

    How do you protect the MSW inverter from such a situation?
    I suppose a GFCI is the best I can do. I am not entirely certain if even a fast fuse would be fast enough.

    I did get a good answer from another forum (the hulltruth) which said you can use an isolation transformer. That is a very expensive solution, would be cheaper maybe to buy an inverter where neutral and ground can be bonded together.

    I email the inverter manufacturer and got the same response you gave about house wiring, which does not answer my question. I reminded them about my question and have not heard from them again. So I think all they do is cut and paste from the manual.

    And you can use an MSW with live neutral in a boat, since normally there is no neutral to ground connection onboard the boat. And I have been using an MSW on the boat for several years.
    And I have an onboard gen with the neutral-ground bonded, but that bond is broken by the transfer switch relay when gen is switched out.

    I just want to improve the MSW protection to a short to ground.

    Adding this.
    I am thinking of just using a GFCI and breaking the ground bond between AC and DC using a relay. Relay I think will be NC for that bond all the time but when Inverter powers up, pull the relay to open circuit, breaking the bond. This limits the possibility of hot to ground shorts for the MSW inverter yet with the GFCI limits the electrocution risk to people.
    I suppose there may exist anyway a high ohm link back to the DC negative regardless maybe by way of the bilge water-ocean-bonding system. But a high ohm link will limit current flow on a short and will trip the GFCI. Whereas a low ohm short will also trip the GFCI but also pass a lot of current, if the AC ground wire is intact back to the inverter which could damage it.

    I will know more when I test everything with the ohm meter.
     
  5. groper
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    groper Senior Member

    The inverter you have will no doubt have overload protection built in. So if there is a short, then it should be protected regardless. Otherwise any over current circuit breaker will do a similar job.

    Using an isolation transformer doesn't help you either - regardless of who said what on another forum. Like I said before, the safest method of feeding an installation which has more than 1 electrical peice of equipment, such as a boat, is via an earthed distribution system which MUST include an earth to neutral bond at 1 single point on the supply side of the load center.

    If the above were not true, then every house in America would use an isolation transformer feeding each home switch board... It doesn't work to protect people when a situation arises where 2 peices of equipment become faulty at the same time...

    If you remove the dc negative to ac ground bond inside your inverter, there should be no problem making the ac earth to neutral bond without damage.
     
  6. groper
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    groper Senior Member

    Here is another link worth reading on the subject and compliance with regulations and permanent installations with inverters etc.
     

    Attached Files:

  7. philSweet
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    philSweet Senior Member

    I disagree.

    A non-isolated MSW inverter has a fixed relationship between the DC and AC.
    The DC is grounded at the battery. It is the relationship between the Live AC and DC that is the problem, not the AC ground. This is a direct result of the circuit design.
     
  8. groper
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    groper Senior Member

    And exactly how can that be?

    If you remove the ac ground to dc ground connection , then the ac ground is connected to nothing except the chassis. Adding the ac ground to ac neutral bond, and removing all bodds to dc negative, means any ac current, including faukt current, can't be back fed into the dc input side. So where is the issue?
     
  9. philSweet
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    philSweet Senior Member

    The issue is that AC voltage doesn't float - it is fixed in relation to the DC side. It's just a cheap and cheesy way to do things (or bloody elegant if you can live with it). The AC+ has a waveform that is opposite to the AC- relative to the DC.
     
  10. groper
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    groper Senior Member

    What does floating voltage have to do with damage to his inverter?

    a potential difference between neutral and ground is only 1 safety issue.

    Another safety issue is using an isolated supply ( which means the supply has no reference to earth, or any other connection to another circuit) to feed a distribution board with many sub circuits - as in a home or yacht. As soon as you have multiple circuits, then you should use an earth refereced distribution system with a neutral to earth bond upstream of the circuit protections devices. The laws over there, and over here, stipulate this must be the case for good reasons.

    Neither is a reason for damage to his inverter...

    The reason for damage, as I see it, is because of the ac ground to dc negative bond in that particular inverter. So if a fault current flows in the ac ground wire, it feeds this current directly back into the dc input side. Under fault conditions, it's no different to wiring a cable directly from the ac neutral or active, and connecting the other end of the cable to the dc input side of the inverter! It creates a short circuit through the inverter electronics! So my recommendation, to which you said "you disagree",was to remove the DC negative to ac ground bond....
     
  11. philSweet
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    philSweet Senior Member

    re - last para, that isn't the problem. It's in the design of the actual inverter circuit. The relationship (not a bond) between AC and DC is fixed. You can't have the AC+ at 120VAC and the AC- at 0VAC relative to the DC input. The machine doesn't work like that. Its +/- 60 volts, and the control circuit adjusts the PWM on the DC side so that the AC voltage remains +/- 60V. With an isolated inverter, the voltage feedback is isolated with optocouplers or some such, and the AC can float wrt the DC.

    If you hook a 120V non-isolated inverter up to your truck and run a cord to a tool and there is a fault where the AC- gets grounded (say, in a puddle), the truck frame will have 60VAC on it. This isn't due to any fault inside the inverter. This is why they are illegal in many places.

    There are serious issues with this type of inverter in the solar industry as well. There are direct conflicts with some protection systems (required by code) and the use of low voltage ground referenced signal/data wires such as USB and ethernet. It continues to be a mess. There is the potential (sorry) to end up with line voltage on low voltage comm wires when the protection system on the solar array activates. :mad:
     
  12. groper
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    groper Senior Member

    Your first para- when describing a sine wave, you have 120vac+ and then 120vac-, not 0 vac that's only half a wave. But I get what your saying. .. so these inverters sound like they output a 60 vac sine wave on one of the active conductors and then another 180deg out of phase on the other conductor. Is that what is happening here? If so , then I can see no safe way of installing one of these to feed a distribution board with multiple sub circuits...
     

  13. philSweet
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    philSweet Senior Member

    Yes. You can google "transformerless inverter circuit" and look at them. The only one's I've actually ripped apart were isolated, so I had to do some digging myself. They have developed some clever sync, freq, and voltage controls on the non-isolated units that are remarkably simple. It also looks like the MPPT implementation is tidier. They are running better than 95% efficiency at MPP from 30-95% rated power.

    So from a transfer efficiency standpoint, there is a strong motivation to go this route, especially with solar. But the external circuit protection is complicated and can cause problems. For instance, if the inverter communicates to a computer, the comm has to be fully isolated.

    And yes, I deserve a dope slap for my description.
     
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