The National Electrical Code article 240.86 addresses series ratings with the following:

Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in (A) or (B), and (C).

(A) Selected Under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment. For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.

During the early 1980's there were some problems with series ratings and tested combinations were introduced. Series rated test standards in accordance with U.L were developed and circuit breaker manufacturers begin to provide tables with their listed series ratings like we see today. 240.87(B) states

(B) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.
Informational Note to (A) and (B): See 110.22 for marking of series combination systems.

240.87 (C) addresses the situation where short circuit contribution from motor's a.k.a. "motor contribution" may occur between the line side and load side devices that make up a series rating. The consideration is as follows:

(C) Motor Contribution. Series ratings shall not be used where (1) Motors are connected on the load side of the higher rated overcurrent device and on the line side of the lower-rated overcurrent device, and
(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.

An example of a series rating may be a breaker that normally has an interrupting rating of 22,000 Amps but when protected with upstream fuses of a specific size and class, may have a series rating of 100,000 Amps. This must be either a listed combination or determined under engineering supervision as stated above in 240.86 (A)

Those that consider using series ratings should thoroughly review the requirements of 240.87.

With all of that information, here is this week's question.

Does your facility/client's facility use series ratings?
- Yes
- No
- Multiple sites - some do
- Don't know
- Doesn't apply to me

Yes. I use it all the time. See my response to your last poll question. The majority of the time it's when we have just over 10ka available at branch panels with 10 KAIC breakers.

Square D, Siemens, GE and Eaton all publish tables with tested combos for their breakers and the fuse manufacturers have them too for fuse/breaker combos.

See my other response for examples of the required labels when applying a series combo. If anyone wants the .doc version of those labels posted in the last poll thread let me know.

Thanks for your response to last weeks question!

Series ratings have had quite an evolution. Back in the very early 1980's the transition was underway from using the "up-over-down" method. This was a graphical method to determine how much current limitation an upstream current limiting device would have for a specific fault current. It was around that time (I was working for Square D back then) that some breaker failures were occurring with many breaker companies which raised the red flag.

U.L. became involved and a new test procedure for series ratings was introduced. Basically it takes the "guess" out of what happens with the up-over-down graphs and instead requires tested combinations of devices.

The actual tests used U.L. "Umbrella Value" fuses. These were fuses of a specific class/rating that had the highest let thru current and I^2T (thermal energy) that a device could have. Much worse than an actual specific device. That way you could use any manufacturer's fuse of a specific type and size since the worst case was used. As an example if you use a 400A Class J fuse, the umbrella values were the worst case so you could then use anyone's 400A Class J fuse.

Today as most know, about all series ratings are based on tests and are listed combinations. The NEC does allow "engineered" series ratings for existing systems. However, there are a lot of documentation/engineering and P.E. requirements for this.

I have used medium voltage circuit breakers with backup fuses. This is essentially a tested combination since the manufacturer is selling the series combination as a unit. This is obviously very different from "roll your own".

Second place I've used a "series combination" and in this case it isn't to attempt to push for higher interrupting ratings, is with using say a panelboard downstream of a transformer which is then connected to multiple subpanels / MCC's that have their own main breakers. The idea here is that the first panel is essentially never used except for testing purposes. It's protective value is bringing incident energy down to reasonable values. The subpanels/MCC's have their own main breakers as primary disconnect/lockout points. This avoids the problem of having huge incident energies at those respective panelboards and MCC's.