Fault current reduction for lighting control

[ron]

Sorry to get involved so late ....
110.10 requires that the component ratings for the component (contactor) be selected so there is no extensive damage to the component.

No document says that you can use the current let through ratings of a fuse to achieve the component ratings unless it has been tested (tested combination). This is clearly stated in the NEC for OCPD's and the intent IMHO is the same for the withstand of a component. At least I would not my company's name on the project unless it was a tested combination.

 

[davidr43229]

No document says that you can use the current let through ratings of a fuse to achieve the component ratings unless it has been tested

There are several documents that say you can use the Current limiting effects to achieve a higher rating, without it being tested. UL508A SB ,IEEE without testing... just to name a few. Additionally here is an article from EC&M for your review.

Per UL248 listing, fuses are required to meet maximum allowable energy let-through values under fault conditions, which allows for protection of components.

http://www.comnews.com/stories/articles/1006/1006circuit_breakers.htm
Glad you joined us.
Just my $.02

 

[ron]

davidr43229,
I agree that a fuse can be used to protect downstream components. Circuit breakers are also capable of protecting downstream components, as installed in many feeder and branch circuits in this country.

So we agree that a fuse and breaker can each individually protect a component....
I don't have a copy of UL508A but I understand that they allow for use of let through current from a current limiting breaker or fuse to determine the SCCR of a component. This seems to be different for a control panel versus other equipment where there is no "standard" document to use.

I would think that a Bussman document is not one to use when trying to prove a standard or code issue, as it is a manufacturers document and not a national standard such as the UL Control Panel Document.

I've lost trust in certain fuse manufacturers and their intension's when certain code changes appeared to be occurring that specifically benefited that manufacturer and not necessarily the critical application (ie Article 700 and 701).

 

[davidr43229]

I don't have a copy of UL508A but I understand that they allow for use of let through current from a current limiting breaker or fuse to determine the SCCR of a component

Would you like a copy? I'll buy you 1.

I've lost trust in certain fuse manufacturers and their intension's when certain code changes appeared to be occurring that specifically benefited that manufacturer ie, Article 700 & 701

Those certain code changes were voted on after the submittal was accepted then by committes, panels and finally the general assembly. Or do you mean that Articles 700 and 701 are not beneficial to your liking?
Just my $.02
Bussmann+n

 

[iwire]

David there is no doubt in my mind Bussmans motive had more to do with sales then electrical safety.

The fact that it was voted in by others does not mean it is a good rule.

To believe otherwise would be the same as believing all members of the US Senate do what is best for the general public and are not swayed by the lobbyists.

Would you like a copy? I'll buy you 1.

A little rude don't you think?

 

[davidr43229]

David there is no doubt in my mind Bussmans motive had more to do with sales then electrical safety

I would ask you to prove that, rather than throwing accusations.

To believe otherwise would be the same as believing all members of the US Senate do what is best for the general public and are not swayed by the lobbyists.

Then we throw the baby out with the water? Or picking out only certain parts of the National Electrical code that suite you. I don't think so. It is a "Minimal Standard" that we use.

The fact that it was voted in by others does not mean it is a good rule

WHAT ?? they thought so.

 

[iwire]

I would ask you to prove that, rather than throwing accusations.

Sorry I am not going to able to prove that.

The fact that you are a sales engineer for Bussman means you are not without bias.

The fact that it was voted 'in' is not in any shape or form proof that it is a good section of code.

All it proves is they voted it in for whatever reasons.

If the CMP members where flawless we would not see the same sections of code re-written each code cycle.

Don't get me wrong, I have respect for the CMP members but no one including myself is always right.

 

[davidr43229]

iwire,
Respectfully....

The fact that you are a sales ENGINEER for Bussman+N means you are not without bias

What I have learned is that all people have their bias. However origionally stated Article 700 & 701 provide a safe and selectively coordination, which was added in Article 100 "Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their rating or settings".
For hospitals, Emergency Circuits, panic control and some gas stations in the event of another 911 senerio, I see nothing Biased with the end result. Why must everything be policital, or a "Certain Fuse Manufacturer" be made a scape-goat for "Not trusting anymore" ( quoting Ron here). I too maybe not always right, but by-gosh, by golly I am proud to be one who is with that "Certain Fuse Manufacturer.
Just my $.02

 

[don_resqcapt19]

David,
With all of the misinfromation provided by both the fuse manufactures and the breaker manufacturers to get their products into the code, I have no reason to take any maufacturer provided statement at face value. The policies of the manufactures have put a permenent taint on any of their information.
Don

 

[ron]

I apologize .... I have some egg on my face. Although my mom always told me if I had a problem go right to the source, I will honestly say that I didn't know that David worked for Bussmann. I guess I should have looked at his profile. Sorry for being what might have seemed aggressive.

I work with critical systems all the time, but generally different than 700 and 701. In a data center, OCPD selectivity is VERY important, but being able to recover from a accidental trip is more important. This is the same theory I would apply to a 700 and 701 application. Due to the code change in 700 and 701 requirement selectivity (in some jurisdictions down to the instantaneous region), it makes if very difficult to allow the on site staff the ability to easily recover from a trip..
I've been to too many sites where a fuse melts and there is no replacement. These are not run of the mill buildings, but sites where the mechanics/electricians are generally very competent and must make due if a circuit opens and have no replacement. That replacement, what ever it turned out to be, ends up staying there until someone gets around to buying a replacement fuse if ever.
I hijacked the thread, sorry!

 

[davidr43229]

Don,

Sorry for being what might have seemed aggressive.

No harm, no foul. We all express ourselves on here (professionally).
Dave

 

[davidr43229]

Don,

The policies of the manufactures have put a permenent taint on any of their information

I would agree for the most part. I always question how the component or combination was tested to properly apply it. I think our role here is to seperate fact from fiction and impart what we know and learn what we don't. I know for me, I have learned quite alot and I thank those people for sharing.
Just my $.02

 

[winnie]

Perhaps I misunderstand the history of 'current limiting fuses' and 'series ratings'. On this topic I only have a couple of ECM articles and discussions here (including stuff inferred from the present discussion and referenced articles) and at ECN under my belt, so please correct me if you know the field, and don't take this as in any way sure knowledge if you don't know the field:

My understanding is that the 'up-over-down' method used to be the standard method for using current limiting fuses to protect _anything_. By 'up-over-down' I mean using the fuse characteristic graphs that take the available short circuit current and used this to calculate the peak and rms 'let through' current, and then using the 'let through' current as the basis upon which downstream devices were evaluated as being acceptable.

At some point this was determined to not be a viable method to use for the protection of OCPD systems, and the use of 'series rating' (with actual testing) was instituted.

The requirement for series rating for OCPD devices calls into question the viability of using 'let through' current for other devices (thus Martin's original question), and David responds with a Bussman document. The Bussman document basically describes design methodology using current limiting fuses, and provides calculations for things other than OCPD devices and series rating tables for various panelboards and breakers.

IMHO the only thing missing from the document that David presented is that it doesn't provide a physical basis explanation for _why_ 'up-over-down' is applicable for everything else, but not for OCPD devices. We've discussed where in this thread, but I would have wanted to see it in the document on 'how to use current limiting fuses'.

Is my history of the engineering methods used on this topic even close to reality?

-Jon

 

[ron]

Jon,
I agree with your statement.
My understanding is that if the downstream impedance were to stay constant (not dynamic), then the let through and the associated let through curves would be valid.
Since it is not possible to keep the downstream impedance static, since there is always trying to open downstream or burn away, it is difficult if not impossible to use the up-over-down method to determine adequacy, since that empirical data is based on static downstream impedance.
I am surprised that the control panel UL standard allows for its use (thanks to David for that information), since there is lots of stuff in a control panel that would not stay as constant impedance during a fault as it attempts to withstand the fault until cleared.

 

[davidr43229]

My understanding is that the 'up-over-down' method used to be the standard method for using current limiting fuses to protect _anything_. By 'up-over-down'

You can not use the up-over and down method "IF" there is an another active overcurrent protective device that actuates within the 1st 1/2 cycle (.008)with the same circuit. This causes confusion and the active overcurrent protective devices get confused.
Said a different way the up-over-annd down method is acceptable for passive components.
Just my $.02

 

[jim dungar]

The general use of the up-over-and-down method for protecting most common circuit breakers has been discouraged since at least the early 80's. I wish manufacturers would put it into an appendix instead covering it as one of the first and primary topics of short circuit information.

And as far as the selectivity issues of NEC 700 and 701. One of my complaints is the NEC wording says all device must by coordinated which means that the primary protection of a transformer must be coordinated with the secondary main protection even though they both affect the same system zone. Another is the encouragement to use fuse selectivity tables without any clearly visible warning (i.e. like those for series breakers) about the substitution of a different manufacturer's fuse.

 

[hmspe]

You can not use the up-over and down method "IF" there is an another active overcurrent protective device that actuates within the 1st 1/2 cycle (.008)with the same circuit. This causes confusion and the active overcurrent protective devices get confused.
Said a different way the up-over-annd down method is acceptable for passive components.
Just my $.02

But every branch circuit supplied from a modern circuit breaker panel will have an active overcurrent protective device that actuates within the 1st 1/2 cycle, if you believe the breaker manufacturers' data. That means we can't use "up, over, and down" for much of anything in real life, doesn't it?.

Martin

 

[winnie]

But every branch circuit supplied from a modern circuit breaker panel will have an active overcurrent protective device that actuates within the 1st 1/2 cycle[...]
Martin

Perhaps I misread what David was saying, but I _think_ that he meant that the up-over-down method did not apply for evaluating the protection of OCPD devices 'downstream' of the current limiting fuse.

I see no problem with having a current limiting fuse downstream of a circuit breaker, protecting the remainder of the branch circuit or a device on the branch circuit. Taking the Bussman document at face value, the up-over-down method is applicable to everything except OCPD devices.

I presume that if a current limiting fuse is downstream of a circuit breaker, then there would still be a coordination issue (you wouldn't know which would blow first), but that the circuit breaker would have to be appropriately rated for its installation.

-Jon

 

[winnie]

Thinking more on this topic, I decided to search on 'up-over-down current limiting' to see if I could pull up some history of this method. I found the following articles:

NEMA document with a useful bit of history, a description of an analytical approach that could be used to determine series ratings, a description of 'dynamic impedance', and references to scholarly articles on the subject.
http://www.iaei.org/subscriber/magazine/05_e/nema.htm

Bussman document which explicitly states that 'up-over-down' is not valid for series rating of OCPD devices, but _is_ valid for 'static' devices.
http://www.bussmann.com/library/techspec/TechSpec12.pdf

EC+M article. 'Sidebar' at the bottom provides some history, and somewhat derisively points out that up-over-down was promoted by fuse manufacturers.
http://ecmweb.com/mag/electric_common_misunderstandings_overcurrent/

Eaton/Cutler Hammer document. Specifically warns against the use of up-over-down for protection of circuit breakers. Says 'It may be valid when the current-limiting fuse is sized to protect a passive bus bar system.'
http://www.eatonelectrical.com/unsecure/cms1/1C96944H02F.PDF

-Jon

 

[hmspe]

Perhaps I misread what David was saying, but I _think_ that he meant that the up-over-down method did not apply for evaluating the protection of OCPD devices 'downstream' of the current limiting fuse.

I see no problem with having a current limiting fuse downstream of a circuit breaker, protecting the remainder of the branch circuit or a device on the branch circuit. Taking the Bussman document at face value, the up-over-down method is applicable to everything except OCPD devices.

I presume that if a current limiting fuse is downstream of a circuit breaker, then there would still be a coordination issue (you wouldn't know which would blow first), but that the circuit breaker would have to be appropriately rated for its installation.

-Jon

So, how do the electrons know that the fuse is up stream or down stream from the breaker? Only one fuse counts? Seem to me that either there's a problem with fuse and breaker combinations or there's not.

I haven't had time to look at the links in your next post yet but will soon. They may answer whether in all cases with a fuse/breaker combination the fault current will be at or below the let-through of the fuse. If so, it would be appropriate to use the published let through as the basis for calculating the cable length required to reduce fault current to match an SCCR rating.

Martin