Elevator Controller SCCR

[Strathead]

Let-through current is different than available fault current. This is clear in the referenced literature. You need one in order to find the other.

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Jim, it may be clear to you, but it is confusing as heck to me, and apparently electrofelon as well. I still have no concept WHY let-through current isn't limiting the current available to components on the load side of the fuses. Can you explain that in master electrician terms (not engineer terms)?

 

[jim dungar]

You cannot determine the apparent let-through current without first knowing what the available fault current is.
Equipment listed SCCR, like those from UL, are all based on the available fault current not the apparent let-through current.
To comply with NEC 110.3 you need to follow the listing procedures and thus use the available fault current value.

The actual why is apparent let-through current not the same as available fault current is a pretty heavy engineering discussion. It involves things that happen in time frames of less than 1 cycle (16 milliseconds) and not more than 3 cycles. The mathematics are difficult which is one reason most listed combinations are determined by using actual testing.

This is kind of like having a discussion, with an untrained person, about why a white grounded conductor and a green grounding conductor are not the same thing at an outlet as they are at a service.

 

[Strathead]

You cannot determine the apparent let-through current without first knowing what the available fault current is.
Equipment listed SCCR, like those from UL, are all based on the available fault current not the apparent let-through current.
To comply with NEC 110.3 you need to follow the listing procedures and thus use the available fault current value.

The actual why is apparent let-through current not the same as available fault current is a pretty heavy engineering discussion. It involves things that happen in time frames of less than 1 cycle (16 milliseconds) and not more than 3 cycles. The mathematics are difficult which is one reason most listed combinations are determined by using actual testing.

This is kind of like having a discussion, with an untrained person, about why a white grounded conductor and a green grounding conductor are not the same thing at an outlet as they are at a service.

This is beginning to make some sense and I appreciate your tolerance. However, I look at the let through charts on the document that I cited, and it has the prospective short circuit current and the let through current. So I still don't see how the charts aren't applicable. They explain that the test in actual conditions. short circuit current available 15,000 60 A J type fuse, lets through 2,000A. No indication that it matters what equipment is beyond the fuse, it still lets 2,000 A through. No indication in this document that the let through will be more than that under the test conditions. If that isn't what actually happens then Eaton is irresponsible to even publish this document.

 

[jim dungar]

This is beginning to make some sense and I appreciate your tolerance. However, I look at the let through charts on the document that I cited, and it has the prospective short circuit current and the let through current. So I still don't see how the charts aren't applicable. They explain that the test in actual conditions. short circuit current available 15,000 60 A J type fuse, lets through 2,000A. No indication that it matters what equipment is beyond the fuse, it still lets 2,000 A through. No indication in this document that the let through will be more than that under the test conditions. If that isn't what actually happens then Eaton is irresponsible to even publish this document.

Primarily, those curves and charts can be used by equipment and control panel manufacturers as they design their products. The designed products are then tested and Listed as requiring the appropiate fuse be installed upstream from the equipment. For an example look at the SCCR of any non-fusible disconnect switch.
They can also be used with items, like bus bars and conductors, that can tolerate fault currents for periods up to 30 cycles, like used with heavy industry and utility switchgear.

 

[Strathead]

Primarily, those curves and charts can be used by equipment and control panel manufacturers as they design their products. The designed products are then tested and Listed as requiring the appropiate fuse be installed upstream from the equipment. For an example look at the SCCR of any non-fusible disconnect switch.
They can also be used with items, like bus bars and conductors, that can tolerate fault currents for periods up to 30 cycles, like used with heavy industry and utility switchgear.

Nowhere in that document does it say that. It also clearly states that the clearance is in les than 1/2 a cycle, so why would up to 30 cycles even enter the picture? And once again, in less than 1/2 cycle the current interrupted at a very low value compared to the AFC. I apologize if I am sounding thick headed, and I greatly respect and appreciate your knowledge, but nothing you say seems to give any valid reason not to use the let through tables. To summarize. The document I presented clearly states, that the charts are applicable to the fuses in less than 1/2 a cycle. They list the available current and the let through current. They also indicate that all values are through actual testing and the results are conservative. They go through the steps to use the charts with no mention that they aren't applicable to every day applications. The only thing my brain comes up with that COULD be contrary is, whether 1/2 a cycle is quick enough, but I have never heard you mention this potential issue.

 

[wwhitney]

Nowhere in that document does it say that. It also clearly states that the clearance is in les than 1/2 a cycle, so why would up to 30 cycles even enter the picture? And once again, in less than 1/2 cycle the current interrupted at a very low value compared to the AFC.

As for the general question of using those let through charts to protect only downstream passive components, I'm unclear on the answer.

But the document says "Clearing time: less than 1⁄2 cycle when fuse is in it’s current-limiting range (beyond where fuse curve intersects A-B line)." So that, I think, is where the issue of dynamic impedance comes in. If another OCPD (or other circuit element) starts opening within that first half cycle, then the additional impedance may drop the fault current down below the fuse's current-limiting range, and neither OCPD may open in under a half cycle.

In which case while the peak current may be limited, the clearing time could be more than on a system where the AFC was only the let through current. Which means that comparing the SCCR to the let through current is not necessarily conservative.

In other words, AFC and let through current are not interchangeable notions.

Cheers, Wayne

 

[Strathead]

As for the general question of using those let through charts to protect only downstream passive components, I'm unclear on the answer.

But the document says "Clearing time: less than 1⁄2 cycle when fuse is in it’s current-limiting range (beyond where fuse curve intersects A-B line)." So that, I think, is where the issue of dynamic impedance comes in. If another OCPD (or other circuit element) starts opening within that first half cycle, then the additional impedance may drop the fault current down below the fuse's current-limiting range, and neither OCPD may open in under a half cycle.

In which case while the peak current may be limited, the clearing time could be more than on a system where the AFC was only the let through current. Which means that comparing the SCCR to the let through current is not necessarily conservative.

In other words, AFC and let through current are not interchangeable notions.

Cheers, Wayne

I believe my non-engineer brain is understanding what you are saying, but under the scenario highlighted in red, aren't you saying that the available fault current would then be below the let through threshold and no longer above the SCCR of the components?

 

[wwhitney]

but under the scenario highlighted in red, aren't you saying that the available fault current would then be below the let through threshold and no longer above the SCCR of the components?

Jim or another engineer should really double check my previous post and this post, but I think the answer is:

An SCCR of, say, 10kA doesn't mean the equipment can withstand 10kA indefinitely. It means that if the AFC is 10kA or less, and the equipment is properly protected by an OCPD with an AIC of at least 10kA, then the equipment should avoid blowing up. And during that fault, the instantaneous current could exceed 10kA for a short time period. But the OCPD should limit the time period of high fault current.

Compare that to a system with an AFC of 20kA but with a fuse with a let through current of 10kA. If there's dynamic impedance so that the fuse doesn't act as quickly as it otherwise would, I'm not sure if the peak current can be greater than in the 10kA AFC case or not. But the issue is that the clearing time may be longer, and so then the equipment may see worse fault conditions than in the 10kA AFC case. Therefore a 10kA SCCR doesn't provide the same guarantee of the equipment not blowing up in this system.

Cheers, Wayne

 

[jim dungar]

Nowhere in that document does it say that. It also clearly states that the clearance is in les than 1/2 a cycle, so why would up to 30 cycles even enter the picture? And once again, in less than 1/2 cycle the current interrupted at a very low value compared to the AFC. I apologize if I am sounding thick headed, and I greatly respect and appreciate your knowledge, but nothing you say seems to give any valid reason not to use the let through tables. To summarize. The document I presented clearly states, that the charts are applicable to the fuses in less than 1/2 a cycle. They list the available current and the let through current. They also indicate that all values are through actual testing and the results are conservative. They go through the steps to use the charts with no mention that they aren't applicable to every day applications. The only thing my brain comes up with that COULD be contrary is, whether 1/2 a cycle is quick enough, but I have never heard you mention this potential issue.

That document simply says

If both the let-through currents (IRMS and Ip) of the current-limiting fuse and the time it takes to clear the fault are less than the withstand rating of the electrical component, then that component will be protected from short circuit damage.

Which is not really helpful if you don't know the timing associated equipment with withstand ratings. The majority of equipment ratings are based on 3 cycles, while large draw out style equipment may have 30 cycle ratings.

I have continually said you need to apply equipment per its Listing procedures.
Eaton/Bussmann agrees with me. This is from their 'comply with NEC' document page 5. Notice how it does not say you can perform item 2 in the field.

When apparatus are SCCR marked, there are two types of markings:
1. The main OCPD is part of the equipment (see Figure 1A). In this case, the equipment manufacturer includes the OCPD in the equipment and the SCCR marking states the SCCR rms symmetrical amperage and voltage.
2. The equipment does not have a main OCPD and it relies on a field installed OCPD device on the lineside (see Figure 1B). In this case, the SCCR marking states the SCCR rms symmetrical amperage, voltage, and required OCPD type, and amp rating (often stated as a maximum). For example, the SCCR marking may state “100 kA rms symmetrical, 480 V, and maximum 400 A Class J fuse.” This requires 400 amp or less Class J fuses to be field installed on the equipment’s lineside in order to comply with the SCCR marking.