fuse coordination

A

aLWAYS_lEARNING-112532

Member
Location
Fl
Occupation
E
when a feeder breaker feeds a fused disconnect, which one should be coordinated to activate first. fuse or breaker? I understand they are in series so the outage will be the same regardless, but should the breaker trip before the fuse blows? I believe its easier to reset the breaker than to install a new fuse, is this correct?
 
aLWAYS_lEARNING-112532 said:
when a feeder breaker feeds a fused disconnect, which one should be coordinated to activate first. fuse or breaker? I understand they are in series so the outage will be the same regardless, but should the breaker trip before the fuse blows? I believe its easier to reset the breaker than to install a new fuse, is this correct?
Click to expand...
Not acceptable practice this case
 
Deltaforce said:
Not acceptable practice this case
Click to expand...
What is not acceptable.

It is common to have a breaker feeding fuses, especially when VFDs are used.
As long as the items have been applied within their AIC and SCCR, I would want the normal operation to be the breaker. I have seen too many incorrect fuses installed under the guise of getting things back into operation.
 
With VFDs I hope they are not protecting with CBs that are the same size as the recommended fusing.😬
IDK how you could blow a fuse of those sizes without already having damage short of rapid disassembly.

I would think looking at time current curves would help but why are they in a feeder? Are you using fusing at taps?
 
Deltaforce said:
If "want the normal operation to be the breaker," intentionally causing a larger, more disruptive system outage on short VFD branch.
Click to expand...
Maybe.
There are many times (1) breaker feeds (1) set of fuses, in these cases the amount of area affected are identical. This is most li,key the situation the OP descibes.
 
don_resqcapt19 said:
There is no need for any coordination for OCPDs in series, but for cost purposes I would have the breaker trip first.
Click to expand...
I the VFD world, this is becoming SOP because of a recent change in the UL listing rules for VFDs. VFDs used to be listed under UL508C just like any other electrical power device, but recently UL changed top align with the IEC requirements, and VFDs must now be listed under UL81800-5-1. As part of that change, fuses are now almost always required, even if you have a circuit breaker (it has to do with possible short circuit failure modes of components). But because this will ONLY require fuse protection AFTER something else in the VFD has already failed. they are NOT required to be the Branch Over Current Protection. They CAN be, but it's not required. So when people do not WANT the fuses to be the only thing, because they are super expensive, the drive mfrs are testing and listing the drives with the MAXIMUM size fuses allowable in the NEC, then purposely size a breaker closer to the input amps of the VFD, specifically so that the breaker should trip FIRST, rather than wait for the expensive semiconductor fuses to clear.
 
Jraef said:
I the VFD world, this is becoming SOP because of a recent change in the UL listing rules for VFDs. VFDs used to be listed under UL508C just like any other electrical power device, but recently UL changed top align with the IEC requirements, and VFDs must now be listed under UL81800-5-1. As part of that change, fuses are now almost always required, even if you have a circuit breaker (it has to do with possible short circuit failure modes of components). But because this will ONLY require fuse protection AFTER something else in the VFD has already failed. they are NOT required to be the Branch Over Current Protection. They CAN be, but it's not required. So when people do not WANT the fuses to be the only thing, because they are super expensive, the drive mfrs are testing and listing the drives with the MAXIMUM size fuses allowable in the NEC, then purposely size a breaker closer to the input amps of the VFD, specifically so that the breaker should trip FIRST, rather than wait for the expensive semiconductor fuses to clear.
Click to expand...
But for high level fault, fuse to clear, avoid catastrophic failure on vfd components failure
 
Deltaforce said:
But for high level fault, fuse to clear, avoid catastrophic failure on vfd components failure
Click to expand...
Not really, because with all of the built-in protections of a VFD, the VFD itself will trip if there is a short in the load, so the only thing that will make that fuse clear is if there was ALREADY a catastrophic failure in the VFD components somewhere, so you have not really prevented anything other than a full meltdown.

This was basically the change in the UL listing process to meet the IEC standards. The previous UL standard understood this, so the SCCR listing did not consider the highly unlikely possibility of a full blown line-to-load thermal runaway meltdown, the IEC standard that UL has now conformed to, does. So the fuses were added “just in case”, like adding a full body harness to your belt and suspenders…
 
Jraef said:
the only thing that will make that fuse clear is if there was ALREADY a catastrophic failure in the VFD components somewhere, so you have not really prevented anything other than a full meltdown.
Click to expand...
Not able find any web reference in support above.
But many web references high speed semiconductor fuses limiting damages and vfd repairable thereafter
 
Deltaforce said:
But many web references high speed semiconductor fuses limiting damages and vfd repairable thereafter
Click to expand...
That is probably fuse manufacturer marketing information, from decades ago.
I would guess the majority of VFDs are less than 50HP 480V and are not built to be repairable.
 
You must log in or register to reply here.
Top