Lights tripping the inverter combo breaker

[Tradie]

I have an interesting challenge on a site which has a permanent bond, I know it is illegal because the site is TN-S, one of the very few sites which I still need to upgrade and fit the N/E relay, it will be done this week, in fact this issue could prompt me to go around and upgrade all the sites which we haven't fitted a relay, lets hear what others think about it.

What makes it interesting is the fact that when there is a live to neutral fault, the circuit breakers (smaller than the combo) in the essential DB dont trip, only the inverter output combo breaker trips. What are your thoughts on this issue, do you think the permanent bond could be the issue?

I have identified the circuit and will be investigating further to identify the cause.

[Tradie]

The Inverter is in normal operating mode, not UPS or islanding at the time of the fault.

Which means the supply star point in the sub station is connected to earth TN-S and there is a permanent neutral earth bond on the load side of the inverter output combo breaker.

The lighting circuit experiences a live to neutral fault, which resulted in the output inverter combo breaker tripping, my concern, why the 10 amp lighting breaker didn't trip, only the 25 amp combo.

There is an earth spike for the generator on the boundary, with an earth spike connected to the generator supply before the ATS, an earth spike next to the house, connected to the common earth bar which is where the neutral from the load side of the inverter output combo breaker is bridged.

Why didnt the 10 amp breaker trip?

If the neutral and earth were not connected at the inverter output, and a relay used instead, would the 10 map breaker trip and not the 25 amp combo breaker?

Could it be because the breaker are different brands?

Could it be that the fault level was so high that it bypassed the 10 amp breaker?

I have experienced 60 amp breaker tripping in the meter, but normally all the breakers in that line trip.

[GCE]

I would put it down to - the one CB saw the fault and disconnected quicker - smaller CB sometimes are a bit slower than others especially on a dead short

I have seen 15 and 25Ka CB come out quicker than a 20Amp on a dead short