Series Ratings VS Current Limiting Fuses & Breakers

[xptpcrewx]

It would be extremely rare, if at all possible, to find a molded case circuit breaker that meets these requirements. About the only place to use this section is with the older 'iron frame's draw out switchgear breakers.

Agreed. Power circuit breakers are what I’ve seen for series rated systems applied outside of tested combinations.

 

[xptpcrewx]

Not true either. At one time there was a calculation (up, over, down) but it turns out to be faulty and no longer accepted. The theory is correct but in practical reality because of dynamic impedance the only practical way to predict it is by actually testing a combination.

What is not true either?

I’m not saying the calculation method is good or accurate. I’m simply stating it’s allowed by engineering supervision. Whether it’s faulty or no longer accepted would be determined by the engineer in responsible charge. However, most engineers trying to avoid liability would not get involved with reinventing the wheel. So much easier and less risky to recommend fully rated equipment. The cost is someone else’s problem.

 

[electrofelon]

Huh? This doesn’t make a whole lot of sense to me.

I was referring to an average non engineer Joe using a CLF, looking at the let thru charts. I am not disputing the under engineering supervision provision, but again my understanding is that is quite rare that an engineer would use that.

 

[mbrooke]

I have argued the same thing: why can't a CL fuse be considered to lower the avaliable fault current just like a transformer or conductor would? I think the wording in 240.86 could be improved. And/or perhaps we need a definition for "available fault current" that would explicitly prohibit using a CL fuse to reduce the AFC.

Basically, the problem is no one has been able to accurately compute or model the behavior of two OCPD's in series so actually testing the combination is really the only way to see if it works.

Current limitation of CLFs only occurs as the fuse itself begins to melt, otherwise they can essentially be thought of as a short piece of copper.

During a fault at point "C" it is possible for a breaker to begin unlatching at point "B" before the CLF begins to melt at point "A".

While we wait for the fuse to work its magic, the breaker at point "B" may be graciously grenading.

 

[EC Dan]

This is an area I'm still really trying to understand as well. Let's say I'm not concerned about underdutied OCPD so I'm not trying to raise AIC, just SCCR of a panel, which has components that render the panel with an SCCR rating of 5 kA. Let's say I have 20 kA fault current and a 100-A T fuse has an instantaneous leak-through of 5 kA at 20 kA RMS fault current. Does adding this fuse in the feeder not allow the SCCR of the panel to become 20 kA? I'm especially unclear whether ONLY the fault current values and peak-through currents in table SB4.2 can be used (since 20 kA is not listed, then this application wouldn't quality without a PE verifying it). Also, even if I'm not concerned about the AIC of a breaker, does the fact that the fuse is in series with the original feeder circuit breaker introduce uncertainty about the ability of the fuse to limit peak let-through current to 5 kA at 20 kA fault current?

 

[petersonra]

This is an area I'm still really trying to understand as well. Let's say I'm not concerned about underdutied OCPD so I'm not trying to raise AIC, just SCCR of a panel, which has components that render the panel with an SCCR rating of 5 kA. Let's say I have 20 kA fault current and a 100-A T fuse has an instantaneous leak-through of 5 kA at 20 kA RMS fault current. Does adding this fuse in the feeder not allow the SCCR of the panel to become 20 kA? I'm especially unclear whether ONLY the fault current values and peak-through currents in table SB4.2 can be used (since 20 kA is not listed, then this application wouldn't quality without a PE verifying it). Also, even if I'm not concerned about the AIC of a breaker, does the fact that the fuse is in series with the original feeder circuit breaker introduce uncertainty about the ability of the fuse to limit peak let-through current to 5 kA at 20 kA fault current?

In general, if it is a UL508a listed panel, the panel fabricator can use this technique to increase the SCCR of the panel components other than OCPDs.

BUT, you have to use the charts UL gives you in SB4. You cannot do your own thing, regardless of how logical it might seem to you.

The table shows Class T fuses can be used up to 200 kA of available short circuit current. For instance, a 30 A class T fuse used in a feeder circuit with 50 kA or less of available short circuit current results in a peak let through of 5 kA according to the chart. For a 100 A class T fuse it is 9 kA.

Whether a PE would be willing to sign off on a such a thing as you are suggesting is something else. There is just no generally accepted procedure for doing so that would allow for it.

 

[EC Dan]

In general, if it is a UL508a listed panel, the panel fabricator can use this technique to increase the SCCR of the panel components other than OCPDs.

BUT, you have to use the charts UL gives you in SB4. You cannot do your own thing, regardless of how logical it might seem to you.

The table shows Class T fuses can be used up to 200 kA of available short circuit current. For instance, a 30 A class T fuse used in a feeder circuit with 50 kA or less of available short circuit current results in a peak let through of 5 kA according to the chart. For a 100 A class T fuse it is 9 kA.

Whether a PE would be willing to sign off on a such a thing as you are suggesting is something else. There is just no generally accepted procedure for doing so that would allow for it.

If a panel is not UL listed and is not required to be listed, can UL508A supplement SB be used to determine it's SCCR rating in accordance with NEC 409? I understand going the extra step in the example I described above may be limited to the panel fabricator, but specifically can manufacturer listed tested combinations be invoked to assign an SCCR outside the scope of a UL listing? For example, in the situation above, if a 100A T fuse is a tested combination with the service disconnect switch on a panel that increases the SCCR of that switch from 5 kA default to 100 kA, is that something that can be done outside of a UL shop?

 

[petersonra]

If a panel is not UL listed and is not required to be listed, can UL508A supplement SB be used to determine it's SCCR rating in accordance with NEC 409? I understand going the extra step in the example I described above may be limited to the panel fabricator, but specifically can manufacturer listed tested combinations be invoked to assign an SCCR outside the scope of a UL listing? For example, in the situation above, if a 100A T fuse is a tested combination with the service disconnect switch on a panel that increases the SCCR of that switch from 5 kA default to 100 kA, is that something that can be done outside of a UL shop?

If a disconnect switch has a series rating with a CL fuse, that is completely separate from a UL508a listing. Such a series rating is a different UL listing, but it is also valid.

In fact, I do not think that in general anyway, you could invoke a UL508a rating to a device like a disconnect switch. It likely does not qualify as an industrial control panel according to UL508a so you could not list it under UL508a.

 

[EC Dan]

If a disconnect switch has a series rating with a CL fuse, that is completely separate from a UL508a listing. Such a series rating is a different UL listing, but it is also valid.

In fact, I do not think that in general anyway, you could invoke a UL508a rating to a device like a disconnect switch. It likely does not qualify as an industrial control panel according to UL508a so you could not list it under UL508a.

It's a UL 98 panel mount rotary disconnect switch, not a separate safety switch. But thank you for the responses, it's more clear to me now when the various methods can be applied.