SWITCHBOARD SHORT TIME RATINGS

[CJMEE]

I would appreciate any forum input on the switchboard design issues presented below. I apologize for the length of this thread, but please read on as I believe the issue needs widespread discussion.

I am well aware of the differences between low voltage switchboards and switchgear knowing that Switchboards are tested to a 3 cycle standard and switchgear is tested to a 30 cycle standard. I am also aware that LVPCB's by definition are tested to 30 cycles. Thus the LVPCB and the Switchgear in which the breaker is placed is a fully rated system for 30 cycles when the switchgear is applied within its short circuit ratings.

On the other hand, it seems to me that our industry has not provided concise information to adequately allow users to configure switchboard products since in recent years switchboards are often supplied with main breakers equipped with solid state LSI features. These service entrance main circuit breakers often times include MCCB's as well as LVPCB's and retain their UL 891 Switchboard Listing.

My concern is based on the fact that unlike Switchgear ratings that include both a short circuit withstand rating AND a short time withstand rating, Switchboards do not have a known (or at least published) separate short time withstand rating. Yet most people handed a one line diagram that includes a main service entrance switchboard with a 4000 amp LVPCB main breaker and three 2000 amp LVPCB feeder breakers where all four breakers are equipped with LSI trip units will attempt to set the system up to include short time delays between the main and the feeders.

In the above example and in an effort to provided selective coordination, it has been my experience that a large number of people don't realize they can "turn off" or don't understand what "turning off" the instantaneous feature of the main 4000 amp breaker really does. They of course will still have instantaneous over-ride protection on the newer LVPCB designs for a short circuit on the bus immediately downstream of the 4000 amp main breaker.

But what if the available short circuit current ahead of the selectively coordinated main breaker does not reach the instantaneous override rating of the breaker? For example say the available short circuit current is 50KA and the instantaneous over ride value of the main breaker is 65KA. A short on the load side of the 4000 amp breaker will fall into the short time rating of the main 4000 amp breaker for which there is no published data for the switchboard bus structures. Damaged switchboard bus structures and bus supports are then possible before the 4000 amp breaker clears the 50KA fault.

Likewise, for a through fault condition where the selectively coordinated breaker settings are arranged for a short time delay between the 4000 amp main and the three 2000 amp feeders, the current that flows through the main bus may also result in damaged bus and bus structures due to this short time delay requirement.

In my experience, designers typically don't know about zone interlocking protection or if they do they don't request it often enough under these types of circumstances. Perhaps with focus on arc flash and selective coordination in the 2005 & 2008 NEC, people's understanding of zone interlocking will increase.

Zone Interlocking should help with the short circuit through fault condition described above to prevent short time bus bar and bus support damage in the main switchboard since the feeder breaker is supposed to trip immediately.

On the other hand, if the available short circuit current ahead of the selectively coordinated main 4000 amp breaker is high enough to reach even the Instantaneous Override rating of the main 4000 amp breaker or if the Instantaneous set point of the main 4000 amp breaker is set rather than over ridden, I don't see how a zone interlock system can be fast enough on instantaneous basis to respond to a 65KA through fault condition even with a zone interlock system in place. It seems to me that there would still be a race as to whether or not the 2000 amp feeder or the main 4000 amp breaker opened first because there is no way of knowing if the zone interlock system can communicate quickly enough on an instantaneous basis.

Any comments on this thread would be greatly appreciated.

Thank you.

 

[gepman]

You bring up an interesting point. I don't normally specify LVPCB's in switchboards, I use insulated case circuit breakers for mains instead (the type of industry that I work in usually does not want to pay the extra amount of the LVPCB's). However on the Square D units that I use the most the electronic trip unit is the same (6.oP). Addressing your last point first, Square D does not claim to have ZSI for Instantaneous, only for the S (short time) and G (ground fault) functions.

Regarding your first point, that turning off the I (Instantaneous) function could force the circuit breaker to allow a fault current less than the circuit breaker withstand rating but more than the switchboard withstand rating, I am not sure what the answer is. I know that the 6.0P has a "Withstand" module. Perhaps the manufacturer changes this "Withstand" module when it is installed in a switchboard instead of switchgear so it will trip sooner.

I know that there are some engineers out there that work for switchboard/switchgear manufacturers and they should weigh in on the subject.

 

[alehman]

The question would seem to apply under any scenario where the fault current does not reach the instantaneous trip value or the override value. Looking at the UL version of the Square D Masterpact 3000A frame, the instantaneous override is 65kA = 21.7 x 3000. The maximum instantaneous adjustment is 15 x 3000 = 45kA.

LVPCB's applied in UL 891 switchboards must be listed under UL 489, which requires instantaneous trip that cannot be turned OFF or instantaneous override.

There will always be a race situation if both breakers are equipped with instantaneous trip when the fault current exceeds the pickup on both breakers. ZSI is used only on short time and ground fault functions, not instantaneous. In a ZSI system, the "normal" condition for the upstream breaker is to trip at the minimum time delay. If it receives a signal from the downstream device, the delay time set on the trip unit is applied.

 

[CJMEE]

Thanks for the input.

My research on published circuit breaker data for Square D, Siemens & Cutler Hammer breakers helps to illustrate what I believe is a basic problem in our industry.

First off - Square D & Siemens do have UL 1066 LVPCB's and UL 489 breakers but they look exactly the same. The only way you can tell the difference is to examine the label on the front of the breaker.

Cutler Hammer choose not to produce a UL 489 Magnum breaker, instead they show two types of UL 1066 LVPCB's. CH has a UL 1066 Magnum and a UL 1066 Magnum SB (for switchboard). So if CH gear shows up, you are going to get a UL 1066 breaker installed.

Now because Square D, Siemens & Cutler Hammer don't want to produce more than one family of solid state programmers, the programmers available for their breakers are from the same family of devices whether they are used on the 1066 breaker or the 489 breaker (in the case of Siemens & Square D). In the case of Cutler Hammer, the solid state programmers are of course the same for both the Magnum LVPCB's and the Magnum SB LVPCB.

In my experience, UL 1066 LVPCB's are being supplied in UL 891 switchboards. In the case of two different manufacturers, I know for certain that nothing was said or done that would prevent the instantaneous trip function from being turned off. In cases where the instantaneous trip can be turned "off", the only way I know this is accomplished is via the front panel of the solid state programmer.

If you examine the data sheets for each of these three breaker manufacturers, you will find that all of the programmers available for each breaker family have an instantaneous function. In other words, you cannot purchase a breaker from any of these companies without an Instantaneous Function in the Solid State Programmer.

My research also shows for the US market, some different models of programmers allow you to turn off the instantaneous feature, other programmers do not.

Unless I have missed something in the published data sheets, ALL Siemens WL, ALL Cutler Hammer Magnum and ALL Square D Masterpack breakers have instantaneous over ride.

I believe the reason this is so is because unlike the days of GE AK/AKR/WAVEPRO & CH DSII where you could order breakers without Instantaneous Functions, today these three breaker manufacturers are not going to leave the breaker unprotected.

So with this said, back to my example. While I understand that ZSI works only with GF & ST functions and does not work with Instantaneous Trip functions, since each of these three breaker manufacturers include instantaneous over ride functionality, the breaker is going to trip on an instantaneous basis if the over ride breaker rating is reached.

My choice of words could have been better with regards to the ZSI issue.

All of the articles/instructions I find on ZSI indicates that that the instantaneous is not present or is turned off. However with the new breakers even if the user turns off the instantaneous function and uses ZSI for selective coordination, the instantaneous over ride is still present.

Thus my statement still seems to be valid in that even if ZSI is present and the instantaneous functions turned off, if a through fault is high enough to reach the instantaneous over ride value, there will be a race between the two breakers and the advantage of using ZSI on a short time basis is lost.

 

[alehman]

Thank you for the information on the instantaneous functions of the newer breakers. I have not yet tried to use any of these in a LVPCB switchgear (UL1066) applications. I had for whatever reason assumed the 1066 versions would have a no instantaneous trip option. My last switchgear job used Wavepro's. This seems to eliminate one of the primary advantages of LVPCB switchgear?

I'm looking at a cut sheet for the Masterpact. The table for the 3200A ANSI C37 version shows no instantaneous override for the 65 and 85kAIC versions. The 100kAIC version shows and inst. override at 85kA and the 200kAIC version is 117kA. The UL 489 version (3000A frame) shows inst. override of 65kA for the 100 and 150kAIC versions.

As for the C-H Magnum, I don't think UL permits 1066 rated breakers in switchboards, but I've seen them installed in switchboards? I need to check into that.

I agree with respect to your statements on ZSI. With ZSI installed, instantaneous works as usual. If the fault current exceeds the instantaneous or override values, then ZSI will not come in to play and there will be a race for through-faults. If the through-fault current is less than the instantaneous, ZSI will cause the main to delay per its STD setting. The primary benefit of ZSI is to reduce damage for faults upstream of the branch device.

 

[CJMEE]

Ok:

Hopefully you are seeing why I think our industry is "failing" some here. In my experience, the people I run across for the most part have not yet grasped the implications of these issues.

Your assumption (and mine initially) was that 1066 breakers would not have instantaneous functions. I have no doubt that the reason WL, MasterPack, & Magnum all have instantaneous over ride levels on some or most models is because the breakers will self distruct at the higher levels. Where we used to get 85KA short time ratings out of DSII & AK/AKR without an instantaneous, I believe "those days are almost gone". CH will sell DSII if you want, but when you talk to them, they of course say Magnum is better.

How long GE will continue to stay with just Wavepro and how long CH will keep DSII alive will be determined by the marketplace.

I encourage you to research your own question regarding CH's use of Magnum 1066 breakers in Switchboards. You will find it to be true. I have seen it as well. The reason a 1066 breaker can be placed in a Switchboard IS because of the Instantaneous and/or Instantaneous over ride function of the breaker.

Your question of whether these new and improved breakers eliminate the primary advantages of Switchgear is what took me down this path in the first place. I said why are we dealing with Switchgear if the manufacturers are putting 1066 and 1066 like breakers in Switchboards.

The breaker people now doubt said that because the over ride is "way up there" and protects the breaker it will protect the 3 cycle switchboard through bus as well.

But... now bring in the issue of selective coordination and the presence of an instantaneous or instantaneous over ride function and you have the possibility of a race whether we are talking about a main switchboard lineup or switchgear.

I believe the breaker people marched off and left the switchboard people in the weeds so to speak. Now that these types of breakers are being supplied in switchboards, who is to say that the instantaneous will not be turned off, who is to say that the available short circuit current will reach the over ride current if the instantaneous is turned off, who is to say that the short time withstand values of the circuit breaker match the unknown short time withstand rating of the switchboard bus and as I proposed in my initial posting, who among us wouldn't try to coordinate a main service entrance switchboard section using LSIG when it is handed to us (of course not realizing the issues at hand).

Unless the responses I have obtained from the different manufacturers is outdated, the three manufacturers providing these types of breakers have not performed short time withstand tests on their switchboards.

If the lack of short time switchboard ratings is indeed accurate, then one "salvation" is to implement ZSI to prevent switchboard bus bar distortion and "live with" the possibility of a race with a large through fault condition that does reach the instantaneous or instantaneous over ride current levels.

Heaven help the installation if the ZSI doesn't work and a short time short circuit current ends up lasting .5 seconds and is impressed upon the switchboard through bus. Given my years of experience with Switchboard and Switchgear design, manufacturing and their field applications, I am also concerned in general about the ZSI modules and there ability to operate reliably during fault conditions since in Switchboard designs they will in most cases be powered off of the same power source they are trying to protect (fox is watching the hen house so to speak).

Please don't take this as an endorsement of WavePro or DSII, because as they say that is a whole other topic.

This is also why I believe the 2008 NEC issue of expanded selective coordination is taking us down a path that the electrical industry as a whole is not prepared to fully address at the main service entrance level.

I have a suspicion there is going to be a lot of pushing and shoving throughout the industry to test the limits of these technologies.

Am I off base here?

 

[davidbeach]

A few comments:

As far as I could tell when I was working with such things, all of the 1066 breakers with instantaneous override had the override set no lower than the short time withstand rating of the breaker. To me, the override was a non-issue in that application because in a properly rated breaker it would never operate and should something go wrong I'd rather an over trip than a blown up breaker.

The selective coordination requirements were proposed for the NEC by a certain fuse manufacturer and accepted by a code panel that just doesn't understand the concept. I'm not going to take the time to look it up to provide the reference, but I submitted several comments on the 2008 NEC to with the intent to remove the selective coordination requirements as there are often more important considerations, such as arc flash. In the code panel's response to one of my comments, they stated that the technical objections I made were not correct because products were available, using series ratings (in their response), that would allow selective coordination using breakers across a full range of ratings. That's not how series ratings work, at all.

 

[CJMEE]

David:

My research on published breaker ratings for 1066 breakers indicate that each manufacturer does indeed provide at least one breaker model number that equates to an equivalent "air frame" circuit breaker without instantaneous functions. In other words if desired, one can still purchase a new style 1066 breaker with a short time rating of 85KA and these breakers include an instantaneous over ride function above the 85KA value. In this way the breaker people have provided an "equivalent" breaker.

I think where the confusion comes in is that there are a number of 1066 breakers with ratings that are also lower than the traditional 85KA rating; there are 489 breakers that look like 1066 breakers; each breaker manufacturer uses different terms for instantaneous over ride functions; because of the way in which each breaker manufacturer implements instantaneous over ride sensing, the actual trip point can be less than the stated value; and there have been some "creative" marketing words used in the breaker literature for at least one breaker manufacturer.

It is unfortunate that one has to dig so deep to find the appropriate rating and the functions for each breaker manufacturer.

On the issue of your attempts to impact the 2008 Code I am sure that you are frustrated by the true lack of understanding of the issues such as the breaker versus fuse protection and the code people's understanding of series ratings. Unfortunately I believe these types of examples illustrate some of the problems with our industry.

While we can "hope" for better, I am sure that discussion forums like this will serve to help serve our industry on a positive note.

 

[davidbeach]

Fortunately I no longer have to deal with the NEC for my own systems (utility) but I also deal with all customer generation attached to our system, so I have to continue to deal with the NEC to some extent, but mostly to the point of not requiring something that can't be done per the code.

 

[alehman]

Take a look at Page 2 of this document. It indicates a UL 489 listing for the Magnum SB. But I don't see 489 mentioned in the literature on the breaker itself. It does say it is intended for UL891 switchboards however.

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=16825

 

[CJMEE]

The link you provided for the Magnum breaker is the same one that I reviewed in my comparisons. Here is what I saw when I examined the document several months ago:

The photo of the actual breaker label on Page 10 of the same document indicates the Magnum SB breaker is an Insulated Case LVPCB.

Table 27.1-4 on Page 11 right under the photo indicates these are UL 1066 breakers.

The text on these pages indicate the Magnum SB breakers are LVPCB's but with lower ratings more common to switchboard applications.

When I visited one of the CH Satellite plants several months ago to examine a number of 4000 amp UL 891 boards, the breakers being used were LVPCB's.

When I asked the plant manager why they weren't using the Magnum SB breakers, he asked me if I had seen a Magnum SB breaker yet, since he had not.

Notice one of the Magnum SB bullet points on page 10 indicates that the instantaneous functions will be "phased in".

The document provided in the link is a 2006 document.

And this is just one breaker manufacturer. Quirks like this show up in the data sheets for each of the other breaker manufacturers discussed in this thread.

 

[alehman]

Both the DS and SB versions are shown with the same UL file number. This listing is for "Low-voltage AC Power Circuit Breakers". Square D has one UL file number in their Masterpact literature for all versions. It is under the same listing (listed by Merlin-Gerin).

UL states the following for this listing:

REQUIREMENTS
The basic standards used to investigate products in this category are UL 1066, "Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures," ANSI/IEEE C37.13, "Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures," and ANSI/NEMA C37.50, "Switchgear - Low Voltage AC Power Circuit Breakers Used in Enclosures - Test Procedures."

 

[CJMEE]

This is why I am saying our industry is in trouble.

My guess is that the breaker people "won" out on the plan to produce one basic style breaker. No doubt one manufacturer started it and the other two followed suit to stay in the game. As far as I can see they all marched over the edge of the cliff together with regards to switchboard through bus protection. Lord knows what GE is up to.

The breaker people seemed to have forgotten the Switchboard people in the process.

The only way out at this point that I can see is for UL to require short time ratings for UL 891 Switchboards. Of course the switchboard people aren't going to like this because of the time and cost, but they pretty much shot themselves in the foot, so I say have at it.

The more I read from each of these three manufacturers the more I wonder who is writing this stuff. If you look at application notes from Square D, they are all based on using LVPCB's in Switchboards. One application note is so focused on increased selective coordination on a downstream breaker that they shoot themselves in the foot big time when they demonstrate how a LVPCB can be used as the service entrance main breaker. What they missed is that if a short circuit occurs on the main switchboard, they have dialed the instantaneous out of the LVPCB (i.e. turned it OFF) such that in their example the available short circuit current from the service transformer will never reach the override level of the LVPCB. Thus by definition, a short circuit at the main switchboard will fall to the short time delay portion of the curve and it is anyones guess as to whether or not the bus and metal stays together or becomes a mangled mess.

I did not think to look at the UL File numbers for each of the three manufacturers. Good idea. I looked at the Square D published information since I don't have access to the UL book tonight.

The Square D Masterpack info lists 1066 and 489 breakers. But are you saying there is only one UL File Number?

I believe you are close on the CH thinking regarding the Magnum SB breakers and the reduced energy let-through. My guess is that this was originally more for arc flash reduction and selective coordination rather than worrying about 3 cycle bracing (although it is bound to help). I still believe additional 891 and structure testing on a short time basis is the only way out.

I have come to the conclusion that for now the only way to help the situation on switchboards is to use ZSI and NOT set the instantaneous up to sky high values. While there may still be a race under some circumstances, it is better than the alternative of doing nothing.

 

[davidbeach]

and it is anyones guess as to whether or not the bus and metal stays together or becomes a mangled mess

Even waiting for short time to trip, I don't think this is the real problem, at this point the biggest problem is probably still mechanical due to magnetic forces rather than thermal.

 

[alehman]

Yes, I could find only one UL file number in the literature on the Masterpact.

So let me see if I understand. I'm not trying to defend what is happening, just understand the concern.

1. The current breed of LVPCB's are apparently listed only under UL 1066/ANSI C37.

2. UL 1066/ANSI C37 does not require instantaneous trip (I think I have a copy of some version of this document at the office. I'll see if I can find it tomorrow).

3. UL 489 rated breakers are required to have instantaneous trip, or at least an instantaneous override, set at some unspecified value.

4. UL 891 does not seem to require UL 489 listed breakers. However it does specify the three cycle short circuit test as a design criteria. Testing is not required however for known designs.

5. UL 1066 listed breakers now being installed in switchboards are being equipped with instantaneous trip, possibly with an OFF setting, and instantaneous override. The override may be set at a relatively high value such as 65kA.


So the concern now is that the new UL 1066 breakers have higher instantaneous override values than the previous generations of UL 489 breakers and some have an instantaneous OFF setting which invokes the override. As I said in an earlier post, I'm not sure I see this as a fundamental change in the design, but rather an increase in the potential severity of damage due to the higher settings that are available. In some cases those settings may exceed the available fault current, but that could be the case with a UL 489 breaker as well, could it not - just not as likely?

Since UL 891 does not specify the trip values, I'm not sure you could call this a violation of the listing so long as there is an instantaneous trip or override. It would give reason to question if it would pass an actual test (not typically required for 891 boards).


I'm still not understanding ZSI fixes this problem. ZSI applies only on the short-time functions. The best it will do is reduce the short-time delay to the minimum of its adjustment range. It doesn't change instantaneous trip behavior. And it only helps for faults between interlocked devices, not through faults.

I think all of the major manufacturers are putting LVPCB's in switchboards routinely, and have been for along time. I've seen many UL 891 boards with DS-II's (equipped with instantaneous trip).

I wonder if the manufacturers are using the following logic: Get the breaker listed as UL 1066 since that's more difficult to pass. Then assume it meets UL 489 since that's a less stringent short circuit design.


[I had another post today that you may have seen which I red-flagged due to concerns of singling out manufacturers.]

 

[alehman]

I agree that the manufacturers' technical literature is very confusing and sometimes self-contradictory. I think that is somewhat the result of poorly written standards and also manufacturers' marketing folks who don't understand the standards.