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We have a LV Switchgear which feeds the VFD via MCCB (all enclosed in the same LV Switchgear), the MCCB is rated for the available fault current in the LV Switchgear but the VFD is not?
I guess it is okay.

Any opinions?


I guess your VFD has its own breaker/ fuse, right? Most VFD manufacturers guarantee that their products will not blow out when a short circuit happens down the line from the VFD load terminals. Besides, the distance from the VFD to its load is enough to drop the fault current to a very manageable level due to the line impedance. Anyway, please post your specific case and we will help if your setup is okay. A picture means a thousand words, they say.


Normally VFD manufacturer recommends the MCCB/ Fuse required at the LINE SIDE of his VFD unit.
Any fault at the line side of the VFD unit (MCC bucket) is looked after by that MCCB/ Fuse.
Therefore, the interrupting capability of this MCCB/ Fuse should be equal or greater
than the calculated maximum fault current duty at the MCC bucket.
But if there is a fault on the LOAD SIDE of the VFD, those fault currents are limited
(typically to 1.2xFLC) to protect the power electronics of the VFD unit. Those faults will be
looked after by the built in protection functions of the VFD unit.


All devices need to be rated for the available fault current, possible by using a coordinated protection device.


No, in general the VFD must have a short circuit rating equal to the available fault current at the VFD. A standard MCCB upstream doesn't eliminate this requirement. I would review the VFD manufacturer's installation instructions. Often a current-limiting fuse is required upstream to deal with short circuit protection.


Dear Mr. NickParker (Electrical)(OP)14 Sep 22 08:54
".... We have a LV Switchgear which feeds the VFD via MCCB (all enclosed in the same LV Switchgear), the MCCB is rated for the available fault current in the LV Switchgear but the VFD is not?...I guess it is okay ".
1. Your installation is typical in any industry.
2. On cost reasons, (the VFD by itself) are NOT designed with [high short-circuit capacity]. Therefore, the manufacturers always recommend the appropriate/tested? short-circuit protective device to be placed ahead of each VFD; either (a)* semi-conductor fuse (200kA), or (b)** Current-limiting CB. The manufacturers know better and in most cases tested by independent lab with certification. Refrain from any other alternatives, for safety reasons.
3. The incoming ACB or the branch standard MCCB which is rated for the available fault currents in the LV switchgear but the VFD is not... would NOT protect the VFD.
4. For safety reasons, strongly recommend to use the OEM recommended/documented (a)* or (b)**
5. FYI: Attention!, on the market there are numerous fuses* with same curve, but may NOT be so for the breakers**.
Che Kuan Yau (Singapore)

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