Elevator Controller SCCR

[jim dungar]

. Can you elaborate? I don't understand what you mean.



No . Do they say something different than Bussmann?

When a device is listed its SCCR is determined by testing or by specific procedures which are part of the listing standard. Those standards address the use of fuses in conjunction with showing the SCCR on the equipment lable.

Other fuse manufacturers are less forward in the language they use when discussing current limiting fuses. This brochure from Mersen does not list a reduction in available fault current as one of the benefits of its fuses.

https://ep-fr.mersen.com/sites/mersen_fr/files/BR-Amp-Trap-2000-Fuses-Brochure.pdf

You can find more information on page P33 and P63 of Mersen's ADVISOR, their technical guide.

 

[wwhitney]

Dynamic impedance is the reason.

Sort of basic question: I get that the above means that a certain result was determined when the downstream circuit elements behave like a fixed impedance, and that if they don't, that result may not apply.

But what result is it exactly? Is it that a CL fuse that is supposed to only let through say 5kA may let through more than 5kA when the downstream impedance is dynamic?

Or is that the CL fuse will still limit the maximum current to 5 kA, but that the fault clearing behavior will be changed by the dynamic impedance?
I.e. equipment with a 5kA SCCR has that rating based on some implied maximum fault duration, and the interaction of the 5kA CL fuse and the downstream dynamic impedance may cause that fault duration to be exceeded?

Or is it something else?

Thanks,
Wayne

 

[jim dungar]

Dynamic impedance occurs during the interruption of an arc, as the arc grows in length the impedance increases. If the impedance is great enough the current through the fuse may decrease enough that it falls out of its current limiting range causing it not to follow its apparent Let-thru curve.

 

[wwhitney]

If the impedance is great enough the current through the fuse may decrease enough that it falls out of its current limiting range causing it not to follow its apparent Let-thru curve.

Let me rephrase the question: say we have a situation where current limiting fuses can be applied to protect some equipment in accordance with its SCCR because there is no dynamic impedance issue. During a fault, we get some worst case current vs time graph (fault current curve) at the equipment's terminals.

Now we add some dynamic impedance to the circuit that is going to be a problem. The fault current curve changes to something that is no longer in accordance with the SCCR of the equipment. How exactly does it change? What important metric of the curve gets worse (peak current, fault clearing time, area of the curve, etc)?

Thanks,
Wayne

 

[jim dungar]

What important metric of the curve gets worse (peak current, fault clearing time, area of the curve, etc)?

You would have to ask a fuse manufacturer.
My guess is peak current is lowered while the clearing time increases.

 

[Strathead]

You would have to ask a fuse manufacturer.
My guess is peak current is lowered while the clearing time increases.

I am curious, are you stating that using a fuse let through chart to determine the available fault current is a violation of code in your opinion, or a violation of code as a fact?

 

[jim dungar]

I am curious, are you stating that using a fuse let through chart to determine the available fault current is a violation of code in your opinion, or a violation of code as a fact?

Let me say, in over 40 years as a registered PE I can't remember ever using let through values to determine the available fault current in order to comply with an equipment's Listed SCCR. For those that do short circuit studies for a living, it is definitely not an accepted industry practice.

In my experience only one fuse manufacturer regularly promotes the Up, Over, and Down methodology independent of their published procedures for determine the available short circuit current at an equipment's line terminals. In my opinion their literature is in conflict with itself.

 

[electrofelon]

Let me say, in over 40 years as a registered PE I can't remember ever using let through values to determine the available fault current in order to comply with an equipment's Listed SCCR. For those that do short circuit studies for a living, it is definitely not an accepted industry practice.

In my experience only one fuse manufacturer regularly promotes the Up, Over, and Down methodology independent of their published procedures for determine the available short circuit current at an equipment's line terminals. In my opinion their literature is in conflict with itself.

I trust what you are saying Jim, so maybe the NEC should state that method is prohibited, and maybe also nix the definition they provide for current limiting fuse as it doesnt seem to serve any purpose other than to confuse the issue.

 

[jim dungar]

I trust what you are saying Jim, so maybe the NEC should state that method is prohibited, and maybe also nix the definition they provide for current limiting fuse as it doesnt seem to serve any purpose other than to confuse the issue.

Why?

Does the NEC tell you how not to pull conductors through conduits, or what fasteners shouldn't be used to mount equipment?

 

[electrofelon]

Why?

Does the NEC tell you how not to pull conductors through conduits, or what fasteners shouldn't be used to mount equipment?

You are right: let's keep the NEC confusing and ambiguous, why change now. Not sure what I was thinking.

 

[jim dungar]

You are right: let's keep the NEC confusing and ambiguous, why change now. Not sure what I was thinking.

The NEC already says to follow listing instructions and it gives a list of typical items to be considered when determining available fault current. The fuse manufacturers call the result of their fuse action Let-Through current which is not an NEC defined term.

Adding language for untrained persons goes against 90.1, so I guess we will always have some ambiguity in the NEC. This is why electrical continuing education should not be restricted to just 'changes in the NEC'. I am glad sites like Mike Holt's Forum exist.

 

[JoeStillman]

.... In my opinion their literature is in conflict with itself.

I'm curious, where do you find it self-contradictory?

 

[jim dungar]

I'm curious, where do you find it self-contradictory?

The two links, in post 31 and 32, appear to present two different methods for dealing with available short circuit current that is higher than an equipment's listed SCCR. In particular Appendix 4 never mentions the use of current limiting fuses as a method for complying with NEC SCCR requirements.

 

[Strathead]

The two links, in post 31 and 32, appear to present two different methods for dealing with available short circuit current that is higher than an equipment's listed SCCR. In particular Appendix 4 never mentions the use of current limiting fuses as a method for complying with NEC SCCR requirements.

what document is this appendix 4 in

 

[jim dungar]

what document is this appendix 4 in

The one about complying with the 2017 NEC, it is in post 32.

 

[petersonra]

And I think it is a little irresponsible for Bussman (Eaton) to keep ignoring that issue. I see this document trotted out almost every time contractors, who don’t fully understand the context it was intended for, but have to deal with low SCCR values on equipment from lazy suppliers. All Eaton needs to do is add a preamble explaining the purpose of this paper and point out that the methods can’t be used in the field by users and contractors.

annex 5 clearly states this:

Panel designers can easily achieve high equipment SCCRs. UL 508A Supplement SB is a rule-based analysis method for determining an industrial control panel’s SCCR which is too detailed to explain in this publication.

That seems clear that the context of annex 5 is only UL508a.

 

[Strathead]

annex 5 clearly states this:


That seems clear that the context of annex 5 is only UL508a.

For those people like me who have some knowledge, but not nearly the expertise in deep theory as you or Jim, are really struggling to come to the conclusion you have. I was told by an engineer a few months ago that the fuses in an elevator disconnect reduced the available fault current. After 4 pages of discussion and trying to digest the information in the publication Jim referenced I can marginally see where your conclusion is. I also think that without a statement that “these let through tables shall not be the sole justification to comply with NEC 110. At this point I will not use the tables myself, but I also won’t argue with an Engineer who does.

 

[jim dungar]

For those people like me who have some knowledge, but not nearly the expertise in deep theory as you or Jim, are really struggling to come to the conclusion you have. I was told by an engineer a few months ago that the fuses in an elevator disconnect reduced the available fault current. After 4 pages of discussion and trying to digest the information in the publication Jim referenced I can marginally see where your conclusion is. I also think that without a statement that “these let through tables shall not be the sole justification to comply with NEC 110. At this point I will not use the tables myself, but I also won’t argue with an Engineer who does.

Page 14 is all about elevator panels.

620.51(D) - An elevator control panel must now be field marked with the maximum available short-circuit current along with the date the calculation was made.
620.16(B) - if the SCCR of an elevator control panel is not equal to or greater than the available short-circuit current, the elevator control panel must not be installed."

Notice, Eaton/Bussman does not say to just put current limiting fuses in front of the control panel. They also do not consider any protective device action when calculating available short circuit currents.

 

[electrofelon]

Notice, Eaton/Bussman does not say to just put current limiting fuses in front of the control panel.

I think we are just going in circles, but arent they indeed saying that and also the NEC does in the definition they provide? Am I misunderstanding what "let through current" means?

 

[jim dungar]

I think we are just going in circles, but arent they indeed saying that and also the NEC does in the definition they provide? Am I misunderstanding what "let through current" means?

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.

Eaton/Bussman says you need to first determine the available fault current only then can you use their literature to determine the apparent Let-through current. But they also clearly say if the available fault current exceeds the SCCR you cannot install the elevator panel.