RCBO's not tripping - off-grid Victron inverter - external ground relay

DIY
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1

Hi,

I have an off-grid Victron system that runs power to a pretty conventional cabin.
It’s come as a literal shock to realise the RCBO’s aren’t functioning properly.

I’ve been doing a bit of reading and think this whole situation might revolve around the “External ground relay” or rather a lack thereof.

Equipment rundown:
5kW PV
Victron MPPT
9.8kWh PylonTech LiFePo4 battery bank
MultiPlus II – 48/5000/70-50
Cerbo GX
Generator

We dug out and made a high quality grounding pit.

The AC1 Output of the Multiplus runs directly to a distribution board and has been (perhaps erroneously) treated just like a regular mains connection. Inside the distribution board is a few RCBO’s and the neutral is bridged/bonded to ground.

The assumption is: Using an external ground relay is going to fix this.

The questions are:

  1. Why are we treating AC power from the Multiplus inverter differently from mains? Would my system be described as ‘IT’ as opposed to TT or TN?

  2. What should the specifications of the ‘helper relay’ and the ‘external ground relay’ be?

  3. Will PowerAssist and PowerControl affect the appropriate sizing of the external ground relay if using this system with a generator?

  4. Should bridging/bonding of neutral and earth ONLY be done at the first distribution board and NOT at any downstream?

Please have a look at the description and wiring diagram I’ve provided.

I think the documentation states the ‘ext ground relay’ output of the Multiplus outputs 24v @ 200ma. I’m assuming this is 24vdc. Also, I’m running off the assumption both these relays should be normally open (NO) SPST type relays.

Helper relay: Will receive 24vdc @ 200ma at the coils. The power pins will then be used to connect the 230vac Multiplus line out to the coil of the ‘Ground relay’ and we will use a 2A inline fuse.

Ground relay: Sized to Multiplus, Will receive 230vac at one side of the coil. The other side of the coil will go to the Multiplus AC out neutral. The power pins will go to Multiplus out GND and neutral. This will effectively short the neutral to ground when the ground relay is triggered.

Thanks heaps

ext_relay
ext_relay1920×1041 123 KB

2

Can you please add an AC wiring diagram?

The MultiPlus already has a ground relay inside that is activated by default. There should no need to install an external relay.

Disconnect the output and measure the voltage between L-N , L-PE and N-PE.
With active ground relay L-N and L-PE should be 230V and N-PE 0V.
If the internal ground relay is open L-N should be 230V and L-PE and N-PE should be around 115V.

3 Likes
3

Thanks for the quick response Matthias and apologies for my slow reply.

Access to the installation is limited as the property is very remote. I will try to get there in the next week but I want to come prepared.

Would you please have look at the following list and let me know if I’ve missed anything or am approaching the diagnosis in a dumb way?

  1. I want to establish whether my testing methods are going to give me usable results. I’m currently testing the functionality of the RCBO’s by taking an insulated wire and shorting earth to the RCBO’s output. I know it’s dodgy but I don’t have a multi-function tool.

  2. I also want to establish whether the Types/Classes of RCBO are appropriate for the installation and if they could be causing the symptoms. I’d have to check when I’m up there next week but I think they were Type AC. I will probably change them to Type B after I work out what the problem is as there is LED lighting and a solar system.

  3. When I say the RCBO’s aren’t tripping that’s not the best way to describe it, sometimes when shorting it, it will trip. But more often it’ll feel like it’s latching or arcing, the RCBO won’t trip and then after about a second or two the Multiplus overload protection will kick in. If that makes sense. I’ve tried swapping out the RCBO’s with the same results.

When I head up there soon I’d like to get everything to bare bones and try to get a really simple test setup working correctly. Like, strait out of the inverter into a very simple circuit. Does the attached wiring diagram for the test setup look sane to you?

Thanks a bunch

ac_diagram
ac_diagram1791×2016 152 KB

4

Thanks

5

Can someone please weigh in on the AC wiring diagram? Thank you