Its very hard to say without looking at it. If it fails under no load and the neutral is disconnected I’d go for the device. But if neutral is connected and it has loads attached i’d look at the load. I am not an expert RCDs have always been neutral issues in my previous experience 

If you do find the fault I interested

It’s a machine fault that is blowing RCDs. I replaced the RCD and it ran for 4 days and now will not reset load free. I believe there a earth leakage fault >200mA. I have not considered frequency as being a problem. Having spoken with the operator that had increased the speed.

The only time I seen this happen was a three phase RCD that was three pole with single phase loads on it. Got it replaced with a four pole RCD and was okay

These are 4 pole RCDs

3 phase. No neutral 

Neutral installed to the machine but not utilised 

Theres a good document out there that shows how type b rcds are good for picking up high frequency hamonics for mode four ev chargers. They can burn out type a rcds

Is that not ya problem? You may need a Delta RCD without neutral

Can you do a PAT on the iron - i,e, strap all 3 phases and neutral and test insultion with respect to CPC or chassis at 250V DC to begin, maybe 500V later

Any DC fault that will fire an RCD of 300mA rating should show up pretty smartly as a few thousand ohms, I imagine there is either a construction fault - pinched insulation that has now squeezed out, or there is something like a water heater element leak,

The way you damage an RCD that quickly  is to make it break far more than its nominal rating, not a gentle trip at a a few times the nominal. Or it has been breaking and being reset far more then the user has reported.

Mike.