fuse impedance

[Grouch1980]

Hi all,
I know there's various threads regarding current limiting fuses and series rated equipment. However, I'm confused regarding the following:

Does a fuse have static or dynamic impedance?

For example: Can you use current limiting fuse charts on a line side fuse protecting a downstream distribution board with fused switches, without the combination being series rated? I know you can't use those charts on fuses protecting downstream circuit breakers, because the breakers have dynamic impedance while they are opening... the combination would be have to be series rated.

Thanks!

 

[petersonra]

Hi all,
I know there's various threads regarding current limiting fuses and series rated equipment. However, I'm confused regarding the following:

Does a fuse have static or dynamic impedance?

For example: Can you use current limiting fuse charts on a line side fuse protecting a downstream distribution board with fused switches, without the combination being series rated? I know you can't use those charts on fuses protecting downstream circuit breakers, because the breakers have dynamic impedance while they are opening... the combination would be have to be series rated.

Thanks!

I don't think UL does any series fuse testing.

However, I don't recall any prohibition on putting fuses on series. You just end up with the AIC rating of the individual fuse.

 

[Grouch1980]

so that means that fuses have static impedance then? you can use the current limiting fuse charts on the line side fuse to protect the downstream fuses?

 

[petersonra]

so that means that fuses have static impedance then? you can use the current limiting fuse charts on the line side fuse to protect the downstream fuses?

I did not say that. They have some impedance. It may not correlate to anything like their current limiting capacity might indicate.

 

[GoldDigger]

If a fuse opens before the first peak of a sinusoidal current waveform resulting from the fault, the peak current before the circuit is interrupted will be lower than the number you would get from calculating from the peak voltage of a sinusoidal applied voltage waveform and an effective instantaneous resistance.

 

[electrofelon]

For example: Can you use current limiting fuse charts on a line side fuse protecting a downstream distribution board with fused switches, without the combination being series rated?

No, it has to be series rated. IMO for SCCR it is not quite as clear.

 

[petersonra]

No, it has to be series rated. IMO for SCCR it is not quite as clear.

For aic, no. For sccr of a piece of equipment that is listed, it is up to ul.

 

[Grouch1980]

ok so from what i got with all the answers.. fuses can be placed in series, with the line side main fuse feeding a fuse/switch distribution board; the line side fuse can be current limiting, bringing down the available short circuit current to a value that the fuses in the distribution board can handle. and the combination does not have to be series rated.

do i have this right?

 

[electrofelon]

ok so from what i got with all the answers.. fuses can be placed in series, with the line side main fuse feeding a fuse/switch distribution board; the line side fuse can be current limiting, bringing down the available short circuit current to a value that the fuses in the distribution board can handle. and the combination does not have to be series rated.

do i have this right?

I do not see anything prohibiting that, as long as the current limiting fuse is properly selected.

 

[petersonra]

ok so from what i got with all the answers.. fuses can be placed in series, with the line side main fuse feeding a fuse/switch distribution board; the line side fuse can be current limiting, bringing down the available short circuit current to a value that the fuses in the distribution board can handle. and the combination does not have to be series rated.

do i have this right?

I don't think so. For sccr purposes as part of a listed system, maybe. For AIC purposes no.

 

[electrofelon]

I don't think so. For sccr purposes as part of a listed system, maybe. For AIC purposes no.

I am not aware of any fuse-fuse series ratings, and the NEC article on series ratings is under part VII of 240 which is "circuit breakers", so I am not seeing anythings prohibiting this, at least per NEC. Of course the CL fuse needs to be selected properly.

 

[Grouch1980]

I am not aware of any fuse-fuse series ratings, and the NEC article on series ratings is under part VII of 240 which is "circuit breakers", so I am not seeing anythings prohibiting this, at least per NEC. Of course the CL fuse needs to be selected properly.

that would make sense... I looked up the section under 240... it only covers circuit breakers on the load side for series ratings... no mention of fuses on the load side.