Current limiting fuses

[Grouch1980]

Hi all,
I read articles and literature on current limiting devices and series rated combinations. just a couple of questions as I'm putting some of the pieces together:

1) I understand you can't use current limiting fuses to protect a downstream panel (directly on the load side of the fuses) with modern molded case circuit breakers due to dynamic impedance of the breakers affecting the operation of the current limiting fuses. That being said, if you do try to accomplish this, then the CL fuses and downstream circuit breakers HAVE to be a series rated combination that way the CL fuses operate as intended?

2) If you place CL fuses in a disc. switch, and it feeds a breaker panel, but the panel is not directly on the load side of the disc. switch... there's an ATS switch in between the disc. switch and breaker panel, or the disc. switch feeds a bus duct which then feeds the breaker panel... do these CL fuses and the breakers in the panel have to be series rated? or does series rated only apply if the breakers are directly on the load side of the fuses with nothing else in between?

Thanks!

 

[Grouch1980]

and 3) If you have a service switch with CL fuses, which feeds a downstream switch & fuse distribution board, will the fuses in the distribution board (which can now have a lower AIC rating because of the upstream CL fuses) affect the operation of the upstream CL fuses? I assume yes since the downstream fuses also have dynamic impedance. If that is the case, can we even have this setup? We can't make this a series rated combination since series rated is only fuse to circuit breaker, or circuit breaker to circuit breaker.

 

[jim dungar]

I read it as: if you are expecting the CL fuses to reduce the fault current to breakers, then they need to be series tested.

'Modern' molded case breakers have been on the market place for almost 60 years.

 

[Grouch1980]

I read it as: if you are expecting the CL fuses to reduce the fault current to breakers, then they need to be series tested.

'Modern' molded case breakers have been on the market place for almost 60 years.

That's what i'm reading as well. ha! yes, at this point I guess there's no need to say 'modern' anymore.

 

[Grouch1980]

just curious if anyone had any thoughts about questions 2 and 3? or question 1 as well.

 

[don_resqcapt19]

just curious if anyone had any thoughts about questions 2 and 3? or question 1 as well.

Both 1 and 2 require a series rated combination of OCPDs.

 

[Grouch1980]

Both 1 and 2 require a series rated combination of OCPDs.

oh, so it doesn't matter how far downstream the breaker panel is then... there could be other equipment in between the upstream fuse and breaker panel, and they still have to be series rated? That's good to know.

 

[jim dungar]

oh, so it doesn't matter how far downstream the breaker panel is then... there could be other equipment in between the upstream fuse and breaker panel, and they still have to be series rated?

If you are not relying on the Current Limiting effect of the fuses, series ratings are not required.
Series rated protective devices, cannot have other equipment between them, if the other equipment may try to open during a fault. An ATS using circuit breakers might be a problem while a latched contactor style might be okay.

 

[Grouch1980]

If you are not relying on the Current Limiting effect of the fuses, series ratings are not required.
Series rated protective devices, cannot have other equipment between them, if the other equipment may try to open during a fault. An ATS using circuit breakers might be a problem while a latched contactor style might be okay.

So if the 'other equipment' between them doesn't include devices that try to open during a fault, then they can be series rated?

 

[jim dungar]

So if the 'other equipment' between them doesn't include devices that try to open during a fault, then they can be series rated?

Maybe.

I get the feeling there are significant details you are omitting or glossing over.

 

[Grouch1980]

Maybe.

I get the feeling there are significant details you are omitting or glossing over.

yeah, it's really a generic question... 'maybe' is the right answer... dependent on what this equipment is will determine if the answer is yes or no.

how about my 3rd question? (it's the 2nd post on this thread)... i'm curious about that set-up....

 

[RB1]

Fuses don't have dynamic impedance.

 

[Grouch1980]

Fuses don't have dynamic impedance.

aah. I haven't come across anything that said one way or the other. with that being said, then I'm assuming you can use CL fuses at the switch to feed a downstream switch & fuse board?... and no series rated combination.

 

[Grouch1980]

could someone explain to me why fuses don't have dynamic impedance like circuit breakers, but rather passive? would really like to learn it. I'm pretty sure it has to do with the way they operate, but can't piece it together... :?

 

[Grouch1980]

could someone explain to me why fuses don't have dynamic impedance like circuit breakers, but rather passive? would really like to learn it. I'm pretty sure it has to do with the way they operate, but can't piece it together... :?

anyone?

 

[RB1]

Sorry Grouch I don't get out much anymore. The way I understand it, the contacts of a circuit breaker blowing apart in the instantaneous range is what causes a circuit breaker to have dynamic impedance. Many years ago it was an acceptable practice to use the "up over and down method" to calculate the let through current of a fuse and use that number to establish the interrupting rating of a circuit breaker installed downstream. That method is still acceptable for withstand ratings, but not for interrupting ratings. You can't count on the breaker contacts staying put for the length of time it takes the fuse to clear. I am sure there are a lot people on this site that can explain this better than I did, but I figured I would take a stab at it.

 

[Grouch1980]

Sorry Grouch I don't get out much anymore. The way I understand it, the contacts of a circuit breaker blowing apart in the instantaneous range is what causes a circuit breaker to have dynamic impedance. Many years ago it was an acceptable practice to use the "up over and down method" to calculate the let through current of a fuse and use that number to establish the interrupting rating of a circuit breaker installed downstream. That method is still acceptable for withstand ratings, but not for interrupting ratings. You can't count on the breaker contacts staying put for the length of time it takes the fuse to clear. I am sure there are a lot people on this site that can explain this better than I did, but I figured I would take a stab at it.

Thanks for the info RB1. Interesting. my understanding is that a breaker immediately displays dynamic impedance as soon as the contacts open, whether in the overload region or instantaneous region. No matter what region you're in, the contacts start opening and that starts to increase the impedance immediately... so i would assume the same goes for a fuse no?