series rating

[augie47]

I hope someone here can explain this to me in terms I can understand (a challange, I know)
I understand that fuse/breaker and breaker/breaker "listed" combinations can be used to allow lower AIC breaker to be used on a higher AIC system.
In addition, we often require documentaion to 10kAIC for components by use of current limiting devices, or sysyem components (wire length,etc).
In this reagrd, I have a couple of questions:

(a) If a fuse "let-thru" curve shows a "let-thru" AIC of less than 10k (such as a 250 amp class J on 50,000) why could one not use any 10k rated breaker regardless of series listing?
(b) If a 10k AIC breaker is allowed as the load side breaker on a series rated system, does that indicate that a component witha 10k withstand is protedcted on the load side of the secod breaker ?

 

[jtester]

a) If a fuse "let-thru" curve shows a "let-thru" AIC of less than 10k (such as a 250 amp class J on 50,000) why could one not use any 10k rated breaker regardless of series listing?

It is not common for the downstream breaker to start to open while the CL fuse is operating. When this happens, the impedance of the fault circuit changes and the interrupting characteristics of the CL fuse change. You must be sure that the total energy let thru the CL fuse doesn't exceed the rating of the breaker, not just the max current let thru.

b) If a 10k AIC breaker is allowed as the load side breaker on a series rated system, does that indicate that a component with 10k withstand is protedcted on the load side of the secod breaker ?

Not necessarily. Different 10 ka breakers have different energy withstand characteristics, and some would work, and others wouldn't.

Bussman has a great paper on protecting breakers with CL fuses. I don't know if it is available on the net, I've seen hard copies of it.

Jim T

 

[templdl]

Yes, it appears that you have a good idea.
This is the way that I always looked at it.
The important thing to remember is that us normal guys may not be qualified to do a coordination study so would have to depend on product that is series rated but the manufacture and listed by UL for the application
Breaker manufactures must perform test to verify that a select group of breakers are able to be applied within a given available fault current.
Breakers do have current limiting abilities but can not be considered current limiting by definition. But the current limiting capability that they do have can be put to good us when protecting down stream breakers that are tested for use with them. Most beaker nmanufacturers publish a chart which list the available ka and the breakers that are allowed to be applied as a series rating. Some even will include series rating with specific fuses.
Then there is the qualified person who can do coordination studies, studies which calculate the fault current which is available at each point in the distribution system bases upon cable and equipment, etc. If this is done breakers can be applied at their nameplate kaic at these points.

 

[jim dungar]

Actually there are two components to current limiting.

Let through is the total amount of current over a period of time. But there is also the peak current (Ip) which must be considered. Ip is not usually available from standard fuse curves.

Another factor to consider is the internal impedance of the breaker (contacts and other materials) and it's effect on the let-through fault current. It is not uncommon for the upstream breaker to have enough internal impedance that the downstream breaker is the only device to operate.

 

[augie47]

let-thru

let-thru

thanks, guys. I'm begining to understand. guess i forget "a man has to know his limitations".
From an inspection stand-point, 10k seems to be a somewhat "threshold" figure (a lot of the equipmet I see has and interrupting or withstand rating of 10k) If you have a system with a curent limiting fuse that has a "let-thru" of less than 10k (at the given available system AIC) is there any way an inspector in the field can feel confident that componets (breakers, starters, contactors, etc) that are rated 10AIC are properly protected.

We have never worried much once the system reached the 10k level but that has been by practice.

 

[jim dungar]

When using any series combination that includes molded case breakers, an NRTL test is mandatory.

Yes, I know that the NEC now recognizes an engineered solution but it is all but impossible to do given the speed that molded case breakers operate at. It is not unusual to find miniature breakers (like Square D's QO family) that can operate at speeds approaching those of current limiting fuses, however their Ipeak is still very high

 

[templdl]

Let through data is not commonly available and is like pulling teeth because it is somewhat proprietary and relatively useless for the most part.
I had provided let through data to one OEM that wanted to know if testing for a series combination was practical. This is common when obtaining an interrupting rating for a combination starter as an example. But the testing still must be done to verify.

 

[Krusty]

Coordination vs Series Rating

Coordination vs Series Rating

:shock: Rather than due the research, thought I would through this out to the forum. Apparently series rated systems can be coordinated, because the impedance of the upstream breaker protects the lower AIC rated downstream breaker, but the downstream breaker opens ahead of the upstream, and so is coordinated.

My simplistic view was that in a series rated system the upstream higher AIC rated breaker opens upon a fault that is higher than the rating of the lower rated downstream breaker, thus it protects the lower rated downstream breaker at fault levels above its rating.

Now I learn series rated systems can be coordinated as well. Can someone provide a concise explaination of how this works?

Thank you.

 

[ron]

Sometimes a breaker ("A") is capable of passing a fully rated test at 22kAIC, but a product ("B") within that manufacturers line is already fully rated and listed for 22kAIC, so the "A" breaker gets fully rated at 10kAIC and series rated at 22kAIC with particular upstream protection. So, the fact that breaker "A" is series rated at 22kAIC and fully rated at only 10kAIC, does not mean that "A" requires "B" to trip simultaneously. It is a marketing issue as to why some items are tested in the way they are.
Series rated OCPD's can be selectively coordinated in many cases, but like most, are not selective in the instantaneous (short circuit) region.

 

[jim dungar]

It is all about misconceptions of what fault ratings mean and how they are determined.

Be warned this is a very simplified explanation.

Normally higher interrupting rated devices are just like normal devices except they are built with stronger glue and rivets. Very very rarely do they open faster than their lower rated equivalents.

An overcurrent protective device (OCPD) is tested with a specific specific circuit which includes 4 ft of cable on the load lugs of the device (the circuits are similar for either a breaker or a fusible switch). Among other things the cable is used to make sure actual conductors are not damaged during a fault interruption. The fault level for the test will occur at the line side of the device.

An OCPD is usually tested to specific levels (i.e. 10K, 22K, 42K, 65K etc) of current - not to the maximum of the device. It is possible that the device might have a 10KAIC rating but it actually might be able to clear a non-standard level of 13,800A.

In a series rating the upstream device and the down stream device both have 4 ft of conductor on their load lugs (again to verify that everything stays connected). So the end result is that during a series rating test the downstream device sees the added impedance of 4 ft of conductor on its line side as well as the internal impedance of the upstream device. Again, the fault level is set to occur at the line side of the upstream device.
So a downstream device can clear a higher level fault all on its own as long as the extra impedance lowers the fault current to a level below its actual maximum rating.

 

[Krusty]

Series rated selective coordination

Series rated selective coordination

Thank you Jim and Ron for your responses, they make sense. However, for emergency systems, circuit breakers that cannot be coordinated in the instantaneous region would be in violation NEC 700.27. Would it be your position then that standard circuit breakers are not suitable for systems requiring selective coordination?

 

[hardworkingstiff]

For the 600-amp to 1,000-amp single-phase panels (exterior) that I install at marinas, in your opinion am I making a mistake in having the panels series rated instead of fully rated?

 

[jim dungar]

Krusty,
Maybe, Maybe not.
What if the available fault current is less than the instantaneous trip point?

I believe it is possible to have tested combinations of devices that provide both series ratings for fault currents while maintaining selective coordination up to specific levels of available fault current.

hardworkingstiff,
Are these engineered solutions?
Personally I would investigate series ratings during my short circuit analysis.

 

[ron]

Krusty,
Jim mentioned one of the key questions. What is the available fault current? If it is low, then selectivity in the instantaneous region isn't a problem. See the interesting article below.
http://ecatalog.squared.com/pubs/Circuit Protection/0100DB0403.pdf
Square D also provides a chart to help determine selectivity outside of the standard TCC curves, even with higher fault currents
http://ecatalog.squared.com/pubs/Circuit Protection/0100DB0501.pdf

 

[rbalex]

Bussman has a rather ?in depth? paper on series rating:

http://www.bussmann.com/library/docs/series rating SPD.pdf

Which ultimately says:

Series rated combinations allowed per 240.86 should be used sparingly. The most suitable and often the only proper application of series rated combinations is for branch circuit, lighting panels.

 

[jim dungar]

Not arguing with some of their presentation but, I am not suprised a fuse manufacturer says do not use breakers when fuses are an option.

 

[hardworkingstiff]

hardworkingstiff,
Are these engineered solutions?
Personally I would investigate series ratings during my short circuit analysis.

Jim,

They are engineered in the sense that I tell my supplier what size panel/breakers I need and the fault current we are working to and Square D puts together the equipment.

What do you mean you would investigate series ratings? Do you mean you would check it out as an option, or are you indicating you have a concern that would preclude you to not use series ratings?

Thanks,
Lou

 

[ron]

It would be considered, as there are reasons not to use series ratings. As was mentioned, selectivity of the OCPD needs to be considered. As well as motor loads and where they fit into the distribution, as the code prohibits the use of series ratings in some cases considering motor load placement in the system relative to the OCPD's.

 

[jim dungar]

Hey, the more time I spend "engineering" the more hours I can bill.:wink:

No seriously, there is almost never a one size fits all answer.
My point was to not blindly apply any single method. But, I have been using series ratings for 20+ years, including on services, and cannot remember a situation where a lack of coordination became a gotcha.

 

[augie47]

Jim, The particular situation involved a 250 amp Siemens panelboard. The customer had requested that the AIC at his equipment be at less than 10k.
The supplier installed class "J" fuses ahead of a panelboard to lower the let-thru current to less than 10k, howver his 14 kaic breakers (BQD) are not series rated with 250 amp fuses (or the breker suppling the bussduct) so he is changing the panel from main lug to main breaker (to obtain series rating).
I think the prime problem is that we, as inspectors in this area, have for years asked for documention to the 10k level and not concerned ourselves below that point. Once there was a current limiting device in the circuit that lowered the aic to 10k, we carry it no further....anything downstream of that device that had a 10k rating was acceptable.
From what I am learning here, I assume, that the same procedure would apply to starters and comtactors with a withstand rating; ie: if a starter has a withstand rating of 5 kaic, it should be protected by a device which has been listed WITH THAT STARTER to deliver no more than 5 k, or can any curent limiting device with a 5k let-thru be acceotable?