Backup power supply issues

DIY
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1

Hi

I have 2 x Multiplus II 5000, 45Kwh seplos batteries, and a small solar array via MPPT.
We live in countryside so occasional power cuts when I want to be able to run off batteries.

Rather than split my house consumer unit into essential and non-essential circuits which would be too invasive, my approach is to use a generator changeover switch. Under normal operation everything is connected like a normal ESS with everything on the ACIn side of the inverters (grid, house consumers). In a power failure, I use the generator changeover to switch the house from the ACIn side of inverters to their AC1out essential output. That manual process allows me to load limit on the house consumer unit by turning off certain circuits.

Our incoming grid connection is bonded by the power company (PME, i.e. neutral and earth connected together upstream). I added an earth spike for when the grid isn’t available. All my earths are connected together at all times.

I have attached a schematic of the setup.

This has worked fine in “normal” operation for some time when both of the RCBOs in CU B (i.e. on the AC1out side) are isolated.

However, the system does not like it when I switch on the RCBOs in CU B and effectively connect together the 2 AC1outputs. Both RCBOs trip.

The only way I managed to get my house running from batteries today when I had a grid failure, was by only using the master inverter output (having its RCBO in CU B on, but the slave RCBO off) AND by actually disconnecting the wiring between CU B and the slave inverter. This implies it didn’t like having earths connected (and maybe neutral - I am not sure if my RCBOs are switching neutral or not. They are MK sentry ones described as “solid neutral type”)

What am I missing and/or doing wrong?

Is it OK to have all my earths connected up together as described.

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2

Have you set the multis up in parallel? When the multi is in inverter mode it will make its own Earth neutural bond with a relay.

3

yes, they are setup as a pair, in parallel

4

I have 2 multis in parallel. The AC in is not connected like that. The AC in has one breaker and the AC out goes to a consumer unit and the live neutral, ground is connected before the unit they do not have their own circuit with RCBO. Maybe something is going on with the AC wiring.

5

Can you provide more information about the RCBOs in question? How are your neutrals connected? How is the earth connected to the inverters and entire system?

The Multiplus II manual 4.4. Connection of the AC cabling calls for a fuse or magnetic circuit breaker on the AC In, and an earth leakage circuit breaker and a fuse or circuit breaker rated to support the expected load (RCBO equivalent) on AC Out 1 with no mention of what to do with AC Out 2. Maybe this is related, but I doubt it.

My personal opinion. Isn’t the split consumer unit route a more elegant solution with the advantage of automatic, near instantaneous, changeover to battery for AC Out 1 loads and the ability to push power back to AC In from the inverters?