Cerbo GX broken on connection of the negative battery wire on Multiplus II

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grizzlysoft asked Mar 18, '22

Cerbo GX broken on connection of the negative battery wire on Multiplus II

Hi,

I had my brand new Cerbo GX and a BMV 700 connected to a 48V battery. The BMV 700 was connected to the Cerbo GX with a VE.Direct cable. Also a Multiplus II inverter was connected to the Cerbo GX using a VE.Bus cable. The inverter was connected only to the positive battery connection. At the moment I've connected the inverter to the negative battery connection, I've heard a spark like sound from the Cerbo GX. Since then is not working anymore. The fuse on the positive wire is ok.

Is this a wrong way to connect the Cerbo GX? Since the VE.Direct and the VE.Bus are isolated, it should not matter if I disconnect/connect the negative supply of the inverter (or any other connection on the inverter or on BMV 700). Then why has it been damaged by connecting the negative supply of the inverter?

cerbo gx multiplus ve.bus

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Answer 1 · 2022-03-18T16:20:08Z

wkirby answered · Mar 18, 2022 at 04:20 PM

The negative cable of the inverter should always be connected, the first cale to be connected beore any others ideally.
As far as I remember the VE.Bus is not isolated on the CerboGX.
You should never allow yourself to get into a situation where the negative of the inverter gets disconnected whilst the positive is still connected in an energised system. Negative is common to most components in the system, so alternative, undesirable high current paths can be found and then things stop working.

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grizzlysoft commented · Mar 18, 2022 at 06:34 PM

While I agree that disconnecting the negative cable is not a wise thing to do, still, if the VE.Bus of the Cerbo was truly isolated, there should have been no current through it when I connected the inverter. But seeing the effect, the only logical conclusion(beside that I had a defective Cerbo with broken isolation) is that indeed the VE.Bus is not truly isolated. Which contradicts the statements in the manual:

Isolation

Because the Cerbo GX is connected to many different products, please ensure that proper care is taken with isolation to prevent ground loops. In 99% of installations this will not be a problem.

The VE.Bus ports are isolated
The VE.Direct ports are isolated
The VE.Can ports are not isolated.
...

https://www.victronenergy.com/media/pg/Cerbo_GX/en/installation.html#UUID-d0b5f24e-6d63-52ca-6d7b-088ae73d40b1

I will reconsider the connections, taking into consideration that the VE.Bus and VE.Direct are not actually properly isolated on Cerbo GX.

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