Here's a weird one. I have a tenant with a 150A, 3-Phase, 480V service. The main switchboard is in the basement and the breaker panel is located in the tennant's space 50 feet away. There is also a 120/208 panel board next to the 277/480 volt panel in the same closet. The step-down transformer is suspended above.

Twice in the same year faulty HVAC equipment (480V 3PH) has grounded out and instead of tripping the branch circuit breaker, blows the main fuses in the basement. Once it was a 30 amp A/C device, the second time 9 months later it was a 60 amp duct heater. Again, neither time did the branch circuit breaker(s) trip, instead the main fuses blew. I don't suspect that the circuit breakers (newish) could be faulty although I have not had them tested. Could there be a grounding/neutral issue that is causing this problem... or possibly something else other than faulty breakers? Thanks.

Explain "main fuse"? What size is it?

The trip curves for breakers and fuses are very different. The fuse in general is a faster acting device. I'd compare the curves providing I understood the problem. "main fuse" was not defined and neither were the circumstances.

I suspect the fuses are "One Time". This should be listed on the label. If so, to prevent this from occurring again, use "Time Delay" fuses, of the proper voltage and amp rating, which seems to be 600 volt at 150 amps.

This will "coordinate" your overcurrent protection devices, that the proper device will open when necessary.

You mentioned "fault current". There is a fault current rating of electrical equipment, fuses, breakers, etc, but has nothing to do with this situation, so the term is out of context. An explanation is a bit out of scope of this forum, but we can discuss it if you like.

There are slower and faster acting fuses, and must be coordinated properly, esp when used in the same system as circuit breakers.

Get back with the info on the main fuse, as Kiss requested, but be sure to provide make and model , not just the amp rating.

The three main fuses are 600 volt, 150 amp. Fuses are and were during both episodes dual element, time delay. I'm guessing that breakers are simply not tripping. Can't think of anything else it could be.

Answer the question. What is the make and model of the breaker and fuse? You need to compare the trip curves of the fuses and the breakers