VSD SCCR without recommended fuses 1

[123MB]

Hi Guys

I have searched heaps of threads here, and on mikeholt.com about using MCCBs on VSDs/VFDs, even though the manufacturer recommends semiconductor fuses. I Haven't really found anything relating to my question though, here goes...

We have several 400V drives (largest being 315kW) with short circuit rating = 65kA "with the specified fuses". These fuses are gR semiconductor fuses which are current limiting (Siemens 3NE1436-2).

The customer has specified MCCBs on the line side of these drives. The MCCB is current limiting, but not as good as the fuse (Schneider NS400L).

The big issue in this application is that the prospective fault current at the location of these drives is 65kA... the same as the SCCR of the VSD with line side fuses. So without fuses, the fault withstand of the drive would definitely be less than the prospective fault current in this part of the plant.

Surely in this case it would be essential to use the recommended fuses... Thoughts?

Thanks

 

[LionelHutz]

Yes, you must use fuses.

 

[jraef]

You must have the fuses even if you use the MCCBs in fact. You will see this quite often, it's because on larger drives, the clearing time at that level of fault current for even a current limiting MCCB is too slow to keep the entire VFD from becoming a full time 3 phase bolted fault and exacerbating a situation that was originally only on one or two phases. So it's not really about the current limiting capability of those fuses, it's about the speed at which they clear. The current limiting aspect just happens to coincide with that.

Without the fuses, you will see that most larger drives are rated at very low levels, maybe only 5kA.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[dpc]

If the rating is based on fuses, you have to use fuses. Don't try to apply rational thought. We run into this all the time. A MCCB upstream with a fused disconnect downstream protecting the same piece of equipment.

 

[unclebob]

Just want to add that this has nothing to do with protection coordination, just with short circuit rating.