Hello.

I have recently come across some "hybrid" inverters (do not export, but have SUB mode) that have input and output Neutrals internally connected, always. These are some 12v and 24v models by Powmr and Anenji (probably all made by Sumry, AFAIK), in the 1000-1600w range.

I did some extensive testing, all showing that the Neutrals are permanently connected internally.

For example:

1. Inverter not connected to anything (no battery, no AC IN, no AC OUT, no PV connected). Just out of the box.

I get continuity between both Neutrals on the inverter ports. Resistance is 0 Ohms.

2. Inverter connected to battery only.

Same results. I also can measure full 230v between AC IN Neutral and AC OUT Live.

I tried different output settings (SUB/SBU), but all the same.

3. Various other tests.

 

My problem is this: I want the AC OUT to be N-G bonded when grid is down.

With my MUST inverter, this was possible just by adding an external NC relay to do the N-G bond when grid is down.

But with these Sumry inverters, it's not possible, because when grid goes back up, the main RCD (from mains DB) trips because the NC relay takes a few milliseconds to open back up, but because the Neutrals are permanently connected, I get an imbalance (double N-G bonds).

I also have a double pole breaker on the inverter AC IN. I want the output N-G bond to work properly if I choose to turn off the AC IN breaker (meaning I loose the grid's Neutral).

My question is: Is this design of inverters common and recommended? 

It seems they have no safety relay for the incoming Neutral.

I suppose they did this permanent connection in order to achieve N-G bond from mains/grid even when running on battery only, but there are various use cases where you could loose the mains Neutral (like I mentioned, either if AC IN breaker is shut off, or if main DB RCD trips, etc...).

My main DB RCD switches Neutral. For technical reasons, I cannot connect the inverter before the main DB RCD.

Second question is how should I go about N-G bonding the output (so that output RCD will trip on leakage) in this case? Any Ideas? 

 

Thanks.

Edited December 19, 20241 yr by meetyg

Not normal, earth neutral bonding can and should only be at one place.

Bonding can only be permanent for full off-grid inverters.

Output setting however is preference only, you need to measure this when inverter is running . Inverter normally has relay when running on batteries. When Grid AC is on this should be disconnected. Again a bonding can and should only be on one place.

 

 

Of course I understand this, but it seems that design of these inverters complicates things.

Not sure if the "smaller" inverters (you've mentioned <=1600w) has different design ( and safety) mechanisms but I won't try making it work with manual switches , relays and other options. I would rather get a bigger inverter and use this one for camping or similar use. I've sold my Axpert 5kva for R3500 recently and there are some available on this forum as well.

 

I had the same question on battery packs like Ecoflow as they don't even have an earth, but apparently these units manage potential leakages in other ways...

 

 

 

Just for reference, I have solved this issue. The solution is tested on two different installations and is working well.

The solution is to disconnect grid input to the inverter, while N-G bonding the output of the inverter. This is made possible by one relay that has 2 NC ports and 2 NO ports.

Although only 3 terminals are used, I could find one that exists with 2 NO and 1 NC ports.

You connect grid input (L and N) to the NO ports, and the N and G output to the NC ports.

Of course you connect grid supply to the power of the relay.

When grid is up, the relay holds the input to inverter, so inverter has grid input, but N-G of output is open (not connected).

When grid goes down, the input to inverter is opened (disconnected) and N-G bond is connected.