TCC coordination - Fuse let-thru current

[Designer69]

Hello, Please see attached image.

I have a TCC showing the coordination of some fuses and a breaker.

As can be seen, the fuses clear before the breaker so that portion is fine. But the TCC's show the let-thru current of some of the fuses is within the breaker trip region. One is even in the instantaneous region.

My question is.. does this even matter? The let-thru current may be within the breaker's trip region but wouldn't the fuses open before the breaker anyway? So I would think the let-thru current of the fuse would be irrelevant to the breaker since it's not enough time to open it.

I believe these are Current limiting fuses so let's assume that is the case.

Thank You!

 

[ron]

Let through has a lot to do with what happens before 0.01 seconds which isn't shown.

Let through is essentially useless when doing coordination because of possible downstream dynamic impedance.

 

[jim dungar]

..wouldn't the fuses open before the breaker anyway?

Many molded case circuit breaker begin to open well before .01 seconds,which is all you can see on the curve.

If the fault current exceeds the point where a protective device 'touches' the bottom of the TCC, the 0.01second line, then the TCC can no longer be used to determine coordination.
If you are trying to meet NEC selective coordination requirements, you will need to check with the breaker manufacturer for Listed series combinations.

 

[Designer69]

I got this from Stevenengineering.com....

"Fuses that are current-limiting open severe short-circuits within the first half-cycle (180 electrical degrees or 0.00833 seconds) after the fault occurs."

That says for severe short-circuits, but it looks like the fuse let-thru current isn't long enough in duration to be able to trip a molded case breaker. So therefore to me it appears to be irrelevant.

 

[ron]

That says for severe short-circuits, but it looks like the fuse let-thru current isn't long enough in duration to be able to trip a molded case breaker. So therefore to me it appears to be irrelevant.

You also don't know what the fault current that the fuse would see, whether it is "severe enough" because there may be other fuses or circuit breakers in between the current limiting fuse and the actual fault location in the pathway, that would have started to open but may have not completely opened by the time the current limiting fuse opens, so that is called dynamic impedance. As fuses begin to open or breaker contacts begin to part, it imposes impedance in the fault path, which would change the fault current at the current limiting fuse below its limiting range, making it not "severe enough".

 

[Ingenieur]

If critical you need the fault current characteristics
with the steady state fault i and system x/r you can estimate
-peak i
-fault freq (which can give you the time of the peak)
treat it like a switched ckt but the switch is a ph-ph or ph-gnd fault
but this is for a bolted fault with 0 impedance
typical peak 1.5-2 x ss
peak time 1/4 x 100 to 500 Hz, perhaps kHz depending on ckt
so peak as fast as 4 x 10^-3 sec
with a decay to ss of 3-5 times that

in reality it will have some Z and likely be arcing
so little more than a guess lol

 

[Designer69]

Thanks Guys. Well it seems that a "severe" short-circuit will open a CLF fuse in approx. 0.00833 seconds which seems to be significantly faster than a MCCB can open. (Therefore no need to include fuse let-thru current in coordination)

So I guess I just need to find out what is the CLF fuse open time for a "non-severe" short-circuit. I'll try to contact a vendor.

 

[Bugman1400]

Does your MCCB use a thermal device to open or an electronic trip? Even if the CLF opens before for the breaker, if the electronics see the same fault and a trip is latched in, the breaker may still open even after the fuse clears.

 

[Ingenieur]

What is the available fault i at the cb?
at each fuse feeder?

can't really see the tcc, too small

 

[jim dungar]

Thanks Guys. Well it seems that a "severe" short-circuit will open a CLF fuse in approx. 0.00833 seconds which seems to be significantly faster than a MCCB can open. (Therefore no need to include fuse let-thru current in coordination)

Don't 'drink the koolaid'. Breakers are not slow.

Many, if not most, molded case circuit breakers unlatch and begin opening their contacts before current limiting fuses clear the circuit.

For the vast majority of analyses, knowing fuse let through current serves very little purpose. I can't think of anyone except a product designers that might routinely need this data. Even most fuse-fuse coordination is done using look up tables.