I have four variable-frequency drives (VFDs) in total supplied from one largest VFD in system. The other three VFDs don't have connection to ac grid. These three VFDs are supplied from common DC bus.

I don't understand exact reason why I must fuse each inverter DC input. Some documents say it is done for protecting VFD against overloads and some documents say it is for protecting other VFDs in case of short-circuit that may happen in any VFD. Short circuit theory seems logical to me but I can't understand the one says it is against overload since VFD output already protect itself from overloads and there is no connection at the front end of these drives.

What is meant by overload then? (They are supplied by common dc bus only.)

Is there anyone who has comment on it ?

  • \$\begingroup\$ This is a flippant response, but I still get a chuckle about what my professor said once: Size the fuse to blow at a lower current than the wiring. You want the fuse to protect the wiring, not the wiring to protect the fuse. \$\endgroup\$ – Kate Moon Jun 8 at 15:49

Every wire run needs an individual fuse to make sure that no matter what the fault condition, the fuse will operate (blow) before the wire gets too hot for safety. The wire size and fuse size must be coordinated. In the US, the allowable capacity of different wires is given in the National Electric Code (NEC) ampacity tables. Presumably other countries have similar published guidelines.

The NEC does not try to guess what kind of faults may occur or how much current will flow. It simply requires an over-current device sized appropriately for each run of wire.

If you have 4 VFD's and each one gets a 20 A 600 V DC fuse for its 10 AWG wire run, you cannot consolidate those fuses into a single 80 A fuse because now the individual wire runs are not adequately protected. A 50 Amp fault on one VFD would not cause the 80 A fuse to operate (blow). You may think a 50 Amp fault is impossible. The NEC does not care. They want fire protection to be as simple and reliable as a rock rolling down a hill.

NOTE: even though I say "fuse" it doesn't have to be a fuse. It could be a breaker, but in this case it may be difficult to find a breaker rated for such high DC voltages. In an AC system, breakers would probably make the most sense.

  • 1
    \$\begingroup\$ Hot wires start fires. The fuse is there to make sure the wires can't get too hot. \$\endgroup\$ – mkeith Jun 8 at 2:19
  • 2
    \$\begingroup\$ There’s the maintenance consideration as well. If one VFD causes the fuse to blow, how will you determine which one did it? How would you replace the faulty VFD without turning them all off? It could mean the difference of a production line stopping or just a minor impact. Think about one fuel tap for four engines on a jet plane. \$\endgroup\$ – Kartman Jun 8 at 2:42

There must be a clear distinction between safety devices and VFD features. The VFD "can" limit the overload until it gets "mad". Beside this you always do need a short circuit protection, so why bother if the VFD has an internal software overload limit, if you need an additional circuit breaker for the event of SC.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.