Please help work out why my main fuse blew

[Leon Close]

Hi,

I've had my SBMS0 system working without fault for about 3 years now. During the last several days of rainy weather, I switched off my inverter, which is the only load on the system besides the SBMS0. When I turned it back on, the inverter started as normal, but cut out after perhaps 5 minutes.

It appears that the fuse that supplies the inverter has blown.

Please see my childish circuit diagram below. The fuses are actually not shown but are class T. The fuse between shunt and inverter is 225A - this has blown. The wiring to the solar array is fused separately with a 150A Class T next to the corresponding shunt.

I assume something has failed within the inverter causing an overcurrent which blew the fuse. The fuse is a proper Littelfuse JLLN225A-P. The inverter is an Aliexpress cheapy that was recommended by Dacian when I was setting up my system. It has performed perfectly until now.

My electronic knowledge is quite limited so I will probably not be able to diagnose the inverter unless something big and obvious is the fault.

Can anyone suggest a strategy here? Should I just order a new fuse and a new inverter? Could something else be the source of the problem? I assume trying a new fuse alone is futile.

I would really appreciate some advice.

Thanks and best regards,

Leon

The blown fuse is the lower one in the photo.

[Dacian Todea (electrodacus)]

Leon,

Have you attempted to power the inverter after the fuse has blown ?

Based on the fact that fuse did not blown immediately at power ON but after 5 minutes could suggest a bad contact at the fuse leading to fuse heating. Was a large load on the inverter during those 5 minutes ?

Is it possible that water got inside the inverter ? or maybe high humidity that resulted in condensation ?

You can just use a 24V incandescent lamp or power resistor to precharge the inverter capacitors the use a small fuse or breaker even one of those 32A breakers from solar will be fine to connect the inverter. If the inverter works with that maybe even with a small load say 500W so it is below the 32A if you use one of those 32A circuit breakers (use longer 12AWG cables for the test) in case the inverter has some internal fault it will help reduce the current trough the circuit breaker and not damage the breaker.

If the inverter is working fine then the most likely cause was a bad connection at one of the fuse terminals maybe due to oxidation and the created heat under load reducing the fuse current limit so it could have melted the fuse wire even at much lower current than spec.

[Leon Close]

Thanks for the suggestion, I will see what I can do. I was wondering if starting the inverter after a few days drew a large current (to charge the capacitors) even though the inverter was never physically disconnected.

As an aside, I've been delighted with this system - 6.7Mwh through the PV system and 4.3 through the battery since I installed it, without an unintentional shutdown. The SBMS0 has stayed powered up the whole time - getting on for 3 years.

Leon.

[Leon Close]

Ok I have powered up the inverter using a circuit breaker instead of the main fuse, precharging beforehand with a power resistor. The LCD and fans are working, and I get "Error 09"which corresponds to "Bus soft start failed".

If I can replace a whole circuit board that may be an option, but I am not enthusiastic about a component-level repair.

I will do some research.

Thanks again.

On Tuesday, May 7, 2024 at 1:35:46 AM UTC+10 electr...@gmail.com wrote:

[Dacian Todea (electrodacus)]

Leon,

I made a short search on that error and yes it seems it is damaged. Not quite sure what was the reason. Maybe the procedure you used to disconnect/reconnect the inverter or maybe water condensation inside the unit.

The boards are fairly inexpensive that is not even worth the time attempting to repair.

[Leon Close]

Hi Dacian,

Thanks for your response. I have contacted one of the sellers and have been directed to this board: https://www.aliexpress.com/item/1005002988500971.html?pdp_npi=3%40dis%21USD%21%21US%20%24226.86%21%21%21%21%21%40214112dd17150480534372219e3925%2112000023089181077%21im%21%21

I think condensation is unlikely, as I have no moisture or corrosion issues elsewhere in the system. I did take the cover off to blow out dust a few months ago, but there was very little dust to remove and all looked clean. I remember when I originally opened the case to install the control wires for the SBMS0, there was some evidence the unit had received a shock - some of the board connections were bent, or maybe just poorly aligned during manufacture. Perhaps this has finally caused a failure.

[Dacian Todea (electrodacus)]

Yes that looks like that board but I remembered seeing the board at lower price when I looked maybe a year ago. For me it shows as 317CAD free shipping.

If you feel confident you can replace that board I will say it is still a good value.

Else there are newer models now for maybe 100CAD more.

[Leon Close]

Yes, more expensive than I was hoping. It may have been better to just buy a new inverter, but at least this way I shouldn't have to change anything else.