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#169763 - 10/15/07 06:30 AM Series Rated Breakers
Last Leg Offline
Member

Registered: 05/04/05
Posts: 41
Loc: Houston, Texas
I am looking for some 'white paper' type of explanation on Series Rated Breakers - how they work and the difference in their internal make-up from non-series rated breakers. A synopsis would be nice with more detailed information for further understanding.

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#169764 - 10/15/07 06:43 AM Re: Series Rated Breakers [Re: Last Leg]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
 Originally Posted By: Last Leg
I am looking for some 'white paper' type of explanation on Series Rated Breakers - how they work and the difference in their internal make-up from non-series rated breakers. A synopsis would be nice with more detailed information for further understanding.


Companies like Cutler Hammer and Square D have white papers on their websites.

I know of nothing special in the internal workings of a series rated device from any mnaufacturer. in general the only difference is that actual test have been peformed using two (or sometimes three) standard devices in series.


Edited by JBD (10/15/07 06:43 AM)

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#169768 - 10/15/07 09:18 AM Re: Series Rated Breakers [Re: JBD]
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
Last Leg,

AFAIK, there is no difference- they're just not quite as beefy. Series-rated breakers cannot open under full fall current, but can withstand it without exploding, provided an upstream (series) breaker opens in time. Thus, you only really need 1 full-fault current rated breaker in the panel, and the rest can just be series rated. The disadvantage is that you lose coordication; if you have a fault on any branch circuit, the main will open and you lose the whole panel. The advantage is that these breakers are often cheaper than full current-rated breakers.

As the series rating is dependant entirely on the properties of the full-rated breaker, you can't necessarily mix-and-match breakers.

I'd be interested in seeing a white paper, too, actually.


Edited by SteveFehr (10/15/07 09:19 AM)

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#169822 - 10/17/07 07:45 AM Re: Series Rated Breakers [Re: SteveFehr]
Last Leg Offline
Member

Registered: 05/04/05
Posts: 41
Loc: Houston, Texas
Wow, guess I really need to find something on them because it is so hard to undo misconceptions. I thought the whole idea of series rating was to be able to use a lesser AIC rating (not able to withstand high faults) because they worked in conjunction with the breaker above it to 'coordinate' the fault, or work together to minimize its 'boom' affect. However, I do admit I have no idea where that idea came from... guess I ought to read on it - think I've been wondering off and on now for about 15 year now !!

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#169847 - 10/17/07 07:35 PM Re: Series Rated Breakers [Re: Last Leg]
Dave T Offline
Member

Registered: 01/22/05
Posts: 157
Loc: Waukesha, WI, USA
Since you question I'll try to shed some light on it for you.

It all starts with a 100% rated OCPD such as a molded case circuit breaker that is UL listed for application at 100% of its rating. These breaker will have a solid state trip unit and usually will be at least a 400A frame if not larger.

100% rated breakers aren't any better than the its common counterpart. The only difference if that they have been UL list for 100% application only when used in an enclosure of installation that is also listed with it for 100%.

When you are so inclined to apply a 100% breaker and read the instructions carefully you will find that it must be applied with 90degC rated cable, the cable of which is applied at 75degC. Remember that cable must be sized to carry the load and breakers then protect the cable.

Normally, the cable is sized 100% of the non-continuos load plus 125% of the continuous load. The cable size selected must carry that load. The breaker is sized to protect the cables rated ampacity that is often more than the calculated load because the breaker rating is based upon the cable rating and not the load.

When applying 100% rated breakers it is 100% plus 100% of the continuous and non-continuous loads. You select the cable size based upon 75degC and you will note that it gives you the opportunity to select a smaller cable.

Smaller copper cable means lower cost. BUT, that cable MUST BE RATED 90degC and not 75degC. This may also give you the opportunity to step down a breaker frame size depending upon what cable size that you end up with and you get lucky. The next smaller frame size can also reduce cost.
So applying cable at 100% is a bit more than that.
I trust that this sheds some light on the application.

You may also not that 100% rated breaker have electronic trip units. The standard breaker will have mechanically thermal magnetic as well as the available identical electronic trip unit. The electronic trip unit does derate with ambient temperature that makes it ideal for 100% rated breakers. Electronic trip units don't make a better breaker but they do provide the ability to apply the LSIG features, digital read outs, communication, etc. Electronic trip units provide a lot more capability to coordinate.

To be honest, we in OEM sales are very intrigued by those who think they are getting a better breaker because it's 100% rated. We love to collect that extra money. I was a Sales and Application engineer for 18 years and am now a Sales Engineer for a dry type transformer manufacturer and we still supply 100% breaker because they are specified knowing that they can't be applied per their 100% UL listing. The customers don't have a clue. They can only be applied, as would be a standard breaker.

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#169853 - 10/18/07 06:37 AM Re: Series Rated Breakers [Re: Dave T]
Last Leg Offline
Member

Registered: 05/04/05
Posts: 41
Loc: Houston, Texas
Thanks Dave, I will have to read this more slowly to see the cause and effects of you explanation. However, somehow this almost sounds like a side line issue, maybe a selling point? I just googled again and found the following... it seems to be getting me closer to what I was looking for..
Thanks again.

http://forums.mikeholt.com/showthread.php?t=60801

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#169867 - 10/18/07 01:43 PM Re: Series Rated Breakers [Re: Dave T]
mikesh Offline
Member

Registered: 06/07/06
Posts: 614
Loc: Victoria, BC, Canada
 Originally Posted By: Dave T
Since you question I'll try to shed some light on it for you.

It all starts with a 100% rated OCPD such as a molded case circuit breaker that is UL listed for application at 100% of its rating. These breaker will have a solid state trip unit and usually will be at least a 400A frame if not larger.

100% rated breakers aren't any better than the its common counterpart. The only difference if that they have been UL list for 100% application only when used in an enclosure of installation that is also listed with it for 100%.

When you are so inclined to apply a 100% breaker and read the instructions carefully you will find that it must be applied with 90degC rated cable, the cable of which is applied at 75degC. Remember that cable must be sized to carry the load and breakers then protect the cable.

Normally, the cable is sized 100% of the non-continuos load plus 125% of the continuous load. The cable size selected must carry that load. The breaker is sized to protect the cables rated ampacity that is often more than the calculated load because the breaker rating is based upon the cable rating and not the load.

When applying 100% rated breakers it is 100% plus 100% of the continuous and non-continuous loads. You select the cable size based upon 75degC and you will note that it gives you the opportunity to select a smaller cable.

Smaller copper cable means lower cost. BUT, that cable MUST BE RATED 90degC and not 75degC. This may also give you the opportunity to step down a breaker frame size depending upon what cable size that you end up with and you get lucky. The next smaller frame size can also reduce cost.
So applying cable at 100% is a bit more than that.
I trust that this sheds some light on the application.

You may also not that 100% rated breaker have electronic trip units. The standard breaker will have mechanically thermal magnetic as well as the available identical electronic trip unit. The electronic trip unit does derate with ambient temperature that makes it ideal for 100% rated breakers. Electronic trip units don't make a better breaker but they do provide the ability to apply the LSIG features, digital read outs, communication, etc. Electronic trip units provide a lot more capability to coordinate.

To be honest, we in OEM sales are very intrigued by those who think they are getting a better breaker because it's 100% rated. We love to collect that extra money. I was a Sales and Application engineer for 18 years and am now a Sales Engineer for a dry type transformer manufacturer and we still supply 100% breaker because they are specified knowing that they can't be applied per their 100% UL listing. The customers don't have a clue. They can only be applied, as would be a standard breaker.


Dave
Are you answering a question in another thread or did I miss the question? I am OK with you description of 100% rated breakers but it does not have anything to do with series ratings. A 100% breaker might be part of a series rated combination but that is it.
A series rating would permit a breaker with a lower Fault current rating to be applied where the fault current exceeds the breakers interrupting ability.

A few Codes ago this rule was introduced by Westinghouse (Cutler Hammer) They claimed that the practice of using fuse or breaker curves to determine let through current in a fault was fatally flawed. The principle for let through is that the fuse or breaker would interrupt or control the fault current let through below a level that a lower rated breaker could interrupt.
As an example a building has a 1200 amp 3 phase 120/208 service and the utility can supply with a circuit with an available fault current of say 46,000 amps. Normal breakers are rated for 10'000 amps and if the full 46,000 amps is available a breaker rated at 10,000 amps would not stop the flow of electricity and the current would flow right on through this breaker causing it's destruction and likely the destruction of the panel and bus etc. The theory was if you placed an HRC fuse ahead of these breakers the fuse would limit the let through current, proven under its individual fuse curve. So we went along merrily for years using lower rated breakers that would be protected by the fuse.
Cutler Hammer said there is a problem here and it was related to the dynamic action of impedance and energy as the fuse was clearing the fault. As 2 over current devices open under fault the impedance of the fault changes IE it tends to go up which limits the fault current. OK that seems like a good thing. As the fault current changes so does the interrupt time and the energy required to clear the fuse drops thereby increasing the time to clear the fault. Now the little 15 amp regular rated breaker might see more than 10KA and blow up.
Cutler Hammer wanted all O?C device makers to be required to prove that their breakers would both be able to clear a fault and not blow up.
Fuse curves are still important as are current limiters but for different reasons than in series faults. Many electrical components are not designed to interrupt fault currents higher than 5000 amps like many motor starters. Those 400 amp cord sets for show power are only rated to be used where the fault current is lower than 10 ka but the distribution might be rated higher and in a fault the cables would become a mechanical hazard by moving under fault conditions. Current limiters can help reduce that potential fault to below 10,000 amps. Current limiting devices are fully rated for the available fault current.

In Victoria our downtown core has 3 network services.
1 a high voltage network (12,500Volts) a 347/600 voltage network and a 120/208 volt network.

The beauty of these networks is the reliability of the utility grid. With several (as many as 6 or 7) utility transformers feeding each grid in parallel a single transformer failure can almost be ignored until a scheduled repair can be made. Getting a damaged underground distribution transformer changed can take days even if a spare is available. On a network the damaged transformer can be isolated and removed without shutting the network off. The downside is the high available fault currents because the system impedance is so low.
Our 600 volt network has places on it with fault current very close to 100,000 amps. Can you imagine having to wire a building with all 100KA rated o/c devices? Might cost more than the building.
So we install a series rated combination Maybe a 1200 amp main rated for the system fault current and some lower rated breakers maybe 400 and 800 amp 42KA distribution breakers to some branch circuits panel boards with 10KA rated breakers. In a branch circuit fault all 3 breakers might have to trip to protect the 15 amp breaker without damage to the breaker. The problem here is there is no coordination and the building is now in the dark for a 15 amp breaker tripping. Oops the fault was an electrician sticking a wire against the box to identify the circuit and now the computers have all crashed.
Coordination must sometimes be maintained like in a hospital or a data center so those distribution breakers need to be fully rated for the incoming fault current of 100ka Installing 15a branch breakers with a 100ka rating would still be outrageously expensive so now we install some impedance into the system by adding high Z transformers and reducing panel boards to 200 amps or less and ensure longer wire runs to add some impedance. There are other ways to ensure coordination through electronic trip signals etc.
This explanation is getting a bit long and is still spotty. For a more detailed explanation by a real engineer you should go to the cutler hammer web site and look for some of their technical papers on series rating.

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#169869 - 10/18/07 02:55 PM Re: Series Rated Breakers [Re: mikesh]
Last Leg Offline
Member

Registered: 05/04/05
Posts: 41
Loc: Houston, Texas
Thanks Mikesh,
That almost answered my question. I am still wondering... is there acturally an interaction between the breakers for them to trip together, or is there some time/current curve that they are set at, when set up as a system, that meets the criteria to make them trip.... or am I still missing it.
I will look up Cutler Hammer's tech papers on it.
Thanks again.

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#169877 - 10/18/07 06:46 PM Re: Series Rated Breakers [Re: Last Leg]
Dave T Offline
Member

Registered: 01/22/05
Posts: 157
Loc: Waukesha, WI, USA
I do appologize guys,
I cut and pasted the incorrect document, was distracted and then didn't review my post and clicked send and turned off the computer for the evening.
The following is what I intended to post on series rated breakers:

I have always understood that you first must establish the KAIC available at the service entrance which is commonly provide by the power company. The disconnects, i.e., the Main breaker, must be rated with a minimum kaic of the available fault current. With fully rated systems all otehr devices must be also.
However, in that main panel the feeder and/or branch devices may have ratings that are considerably less than the main device, that is if these devices have been series rated with the main breaker. What the manufacturer has done is to test the main breaker with the downstream devices to assure that the main breakers current limiting capability (not necessarily a "current limiting" circuit breaker) will be current limiting enough to protect those down stream breakers from a fault current that could damage them.
However, the manufacturer simply cannot make this determination but the combination must pass UL tests and be listed as such. Thus, this panel is considered to be series rated. Likewise, down stream panels and breakers can be series rated with those that are upstream provided that they, too, have been tested and listed by UL to be applied as such. Manufacturers will have a table of some sort which shows which breakers can be used is series with others.
One thing that must be noted is that you cannot mix manufacturers products unless they too have been tested and listed with one another which is not likely. But quite often fuses may be tested and series rated with breakers by some manufacturers.
One of the goals of using series rated devices is to reduce the cost of the installation by using less expensive OCPDs. I'm sure you're well aware of the premium that must be paid for higher kaic devices. The manufacturers make you dig deep in your pocket for those 65 and 100kaic breakers.
When a spec. hasn't disallowed series rated breakers then they would be allowed. I have been able to reduce my proposal price significantly by using series rated devices in lieu of fully rated devices..

Also, "physically thermal magnetic" breakers are very hard to coordinate at best. These are non-electronic trip type breakers.
They will almost all certainly coordinate thermally (overload) but magnetically (fault, instantaneous)it is often pot luck which breaker in series with the other will trip when all see the same fault if the magnetude of the fault is within that calibration of those breakers. There is no coordination.
Neither series rated nor 100% rated breakers provide any advantage in the coordination in this regard as they all have the same trip calibrations. Being that 100% rated breaker have electronic trip units those have adjustable pick-ups and delays (LI, LSI, LSIG) can provide a level of coordination when set properly. Some even will communicate with one another letting each other know that they see the fault allowing the upstrean breaker to delay giving the down stream breaker the time to trip first.

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#169878 - 10/18/07 06:47 PM Re: Series Rated Breakers [Re: Last Leg]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
 Originally Posted By: Last Leg
Thanks Mikesh,
That almost answered my question. I am still wondering... is there acturally an interaction between the breakers for them to trip together, or is there some time/current curve that they are set at, when set up as a system, that meets the criteria to make them trip.... or am I still missing it.
I will look up Cutler Hammer's tech papers on it.
Thanks again.


There is interaction, and it can be very complex. Some interactive items are:
Two sets of contacts resistance in series.
Additional cable is included in the series testing.
As one set of contacts opens it creates additional impedance which severly limits the current seen by the other breaker.

There are many misconceptions about and misapplications of series ratings even though they have existed for almost 20 years.

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