A vast majority of industrial machines are fitted with a NFB's, (non-fused breaker) and have all kinds of unbalanced loads due to some being 3 phase together with the single phase section of the machine.
Often the 1ph loads are spread between different phases in an attempt to balance, but it is usually never equal.
Max.

 

Lundwall Paul is right.....the first thing to do is some maintenance on that box.

After a good cleaning you need to install another 3 pole breaker in that hole, and secure those 3 loose legs!

Next, replace every connector lug with a new one.

After securing all the lugs.....paint each lug black with a magic marker.

Do not paint a rusty lug!

Now you can measure the connection temperature under load with a cheap laser thermometer.

The breakers in question are ganged together, so that if one leg trips....it will cut all three legs. It's a mechanical safety feature for a 3 phase load of 90 amps each leg.

If you don't want to use the circuit for a 3 phase load.....say....you want to install new lighting....you can take off the gang and use each leg independently up to 90 amps.

Where are your electricians from?

DO NOT do this yourself!

Find a good maintenance man.

 

If you take the tie bar off and put 3 single phase loads on the three lines, each would be protected for 90A. Things do get a little squirreled around when the 3 phase load is thrown into the mix. Obviously, a 3 phase load needs the tie bar.