Low Current, High ACB Rating

The current sensing elements used inside of that breaker that give info to the trip unit are accurate within a range that goes down to 40% of the maximum setting. So a 2500A breaker will have sensors that can be used down to 1000A. Below that, the output value of the current sensors, like CTs, becomes such a small value that it cannot be effectively used.

The local Siemens engineer knows what he's talking about. Replace the breaker.

In addition to what JRaef says, there is a risk of the ACB getting into the "critical current" zone. Not that it is very likely in this case, the numbers don't seem to indicate that. However, every switching device has a critical current, whose value is much lower than the max short circuit current, but which the device struggles to break. Mainly because the magnetic forces are too weak to blow the arc away, and the arc persists. Most easily seen in fuses, where the standards specify a test for I₂, the critical current, at which the arcing I²t is the maximum. Prudent for you to use a properly sized breaker.

What is the cable rating? The breaker is there to protect the cable!

We have 3 runs of 400sqmm copper cable.

3 runs of 400 mm² for 350A? i sure am missing something here

His wiring is well protected with 350 AMP breakers.

On that basis he does not need to reduce his breaker setting. I have no idea why he should want to, then I'm funny that way.

The utility services demand it.

That too, Copper!!!!

Buy the correct panel.

Why not just place a sub-panel?

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CC8QFjAC&url=http%3A%2F%2Fwww.siemens.com.br%2Ftemplates%2Fget_download2.aspx%3Fid%3D571%26type%3DFILES&ei=mnjeU5fJMMybyASB3oGACA&usg=AFQjCNH-GeOwNqUYmdzu5FFaGvzuScN8HA&sig2=tDNhUNGjy4S1y3GCgUohLw&bvm=bv.72197243,d.aWw

For some strange reason I'm confused by the OP's reasoning in this decision.

Although 3 x 400mm² is small for 2500A, they're ridiculously over sized for 350A

The setup was for higher currents. Is it possible to modify the ACB for current loads?

Try to install additional CT in the circuit of ratio 400/1. Have Simple IDMT relay similar to CDG31 relay. Wire up the relay to Shunt trip coil of ACB. This will retain your ACB intact so that in case you revert back to your original load of 2500A you can restore. This suggestion will not cost much comparing to cost of replacement of ACB.

Can we use an Overload relay instead ?

HIRE AN ELECTRICIAN.

You really need one.

Indicate type and make of overload relay.CDG31 type relay will protect over current, earth fault and more reliable than bimetallic overload relays.

When a high (fault)current flows, 400/1 CT will burn.

It is more likely to saturate. Not burn.

However, the OP does not seem to be reading all the posts. He seems to be only concerned with using a cheap bimetallic overload relay in combination with a suitable CT and make do.

Like buying a Ferrari, and running it on kerosene.

That makes perfect sense!

Instead of in series if you put in parallel,you can any of the two according to the load

NO! If connected in parallel, most of the current will pass through the larger breaker, because it has heavier conductors. Because of interconnection variations, it is impossible to predict what fraction of the total current will pass through each breaker, but the smaller breaker will certainly not open until the total current is several times the rating of the smaller breaker.

In series, all of the current must pass through both breakers, but since the larger breaker has heavier conductors, and is set to open at a much larger current, It is essentially just a short piece of heavy wire, and the lower current breaker will always be the one to open in case of an over-current.

In the original post you said the load was 350 Amps, so you were considering a 350 Amp breaker. There must always be some room for variation. For a 350 Amp load, you need at least a 400 Amp breaker.

In my comment(23) the word "select" was omitted.You can select any of the two CBs according to the load.

I am considering to add 500 A cts, which will be hooked on to the electronic release of the breaker.i suppose that should work. Only problem Siemens ct for that rating doesn't fit on the busbars.

To 'select' one or the other would require either major switching or rewiring. Since you mentioned bussbars, the lower current CB probably won't connect to them correctly anyway. You must do some rewiring/rerouting.