I'm a new SKM PowerTools user, and I'd like to evaluate a couple of different scenarios involving fuses. In the case I'm looking at, the fault current at the load is just a little below 11kA. In the system's current configuration, there are expulsion type power fuses on the transformer's primary side. I've been advised that if I were to look at current limiting fuses instead, we might be able to get that fault current at the load below 10kA, but when I run a scenario with current limiting fuses, there's no change in the fault current. From what I can tell, that is because PowerTools ignores protective devices when analyzing fault current in order to present the worst case scenario.

Is there a way to get PowerTools to consider those protective devices instead of ignoring them so I can determine what effect a current limiting fuse will have on my fault current?

Thanks!

Hi Rob, If I understand the question, this goes way back decades ago to what was known as fuse let thru curves. There were graphs where you could look at a prospective fault current - 11 kA in your case, and then graphically predict the reduced "let-thru" current. 10 kA as you state here.

These graphs were originally used for series ratings of devices where someone would predict how much current limitation there would be and use the reduced current to evaluate interrupting ratings of devices downstream.

The industry moved away from this method as there were some quirky problems in the late 70's and early 80's (when I started in the industry) and UL came in and began testing instead of predicting via graphs. Because of that, I don't know of any software that would revert back to "let-thru" current in the calculations to see how much reduction there would be.

Also a practical consideration, transformer primary fuses tend to be quite oversize to reduce the likelihood of nuisance interruptions. That means that usually for an arc flash on the secondary of a transformer, it is likely the primary fuse would not interrupt quickly.

Probably not exactly what you wanted to hear but that's the background and reasoning behind the current limitation.

Thank you Jim. Inspired by your post, I sent a support email to SKM directly. I was extremely impressed with the quick turn around, but not so impressed with the result. They confirmed that the Short Circuit analysis ignores protective devices to give the worst case scenario, and at this time, there's no way to over ride that behavior. Still, if your friend has a different answer, or a suggestion as to a different method by which to approach this problem, I'd be eager to hear it.

1) You are unlikely to find series rated devices that are rated to operate at different voltages.
2) The primary current limiting fuses I know of are full range devices. They will operate in the high speed current limiting region for high current primary faults, but will operate in the time-overcurrent conventional region for through faults. Current-limiting through faults would be undesirable for coordination purposes.

The current limiting charts are still publish and used in the industry to protect electric components that do not have dynamic impedance (I.e., circuit breakers). You can still use the key-thru charts to protect bus’s ways and wires.

Additionally, SKM software does use the let- thru chart information in their software for arc flash calculation purposes. It’s an option in the fault current options screen.

If you still want to know the current limiting effect of a fuse (for short circuit calculations) at a specific locations, you will have to do it manually because SKM, even though it has the current limiting information chart, it uses it only fior calculating arc flash incident energy, not the reduced let through current.

Reading my recent published post, I realized I did not communicate properly in one regard. You cannot use current limiting effect in dynamic impedance devices like circuit breaker. You can in cables and busway.

Two things:
1) If you're paying maintenance fees to SKM use them. My experience since 1992 is that they're extremely responsive and now that they can take over your screen, they can diagnose anything. And if they can't, they'll take your model and come up with something. I've had nothing but excellent response form their tech support folks. &
2) If you're a registered PE then NEC 240.86(A) gives you the authority to make an assessment on an existing system where protective devices are used in a fault current environment that's higher than what they're rated for. It's then that you can apply let-through charts to pass judgment for pass/fail on devices. It's a unique position to be in and one that comes with the requirement of full disclosure through the associated documentation. Given that only some devices get tested and listed, and manufacturers would never test with another manufacturer's devices, it often presents an interesting opportunity.

The "dynamic impedance" was a term introduced in the 1980s to explain the situation. Basically it is the impedance in the arc created during a fault interruption by a device (circuit breaker). It would alter the results of using the let thru graphs previously mentioned. This is because the prospective current is assumed to be the bolted fault current but when a device interrupts, the rapidly changing arc impedance decreased that current as the device opened. It seems this would "help" the situation but what could happen is the prospective current may reduce to a low enough value where the upstream current limiting device did not perform as anticipated.