Series Rating - Fuse & Breaker

[RonShap]

Consider a 1600A current limiting fuse with 200kA available short circuit current on the line side.
The fuse manufacturer states that the maximum let-through at any current up to 200kA on the line side is less than 100kA on the load side.
Why is a series rating required for a downstream 100kAIC rated circuit breaker located within a few linear feet of cable (other than just the code requires it)? The breaker can never see more than 100kA since the fuse will never let more than that through?
It doesn't make sense to me. Any amount of dynamic impedance downstream of the fuse will only reduce the fault current and the resulting let-through even further below 100kA.

 

[alehman]

The rating of the breaker probably depends on its current limiting capability. Even though it is rated for 100kA *available* fault current, it is designed such that it limits the fault current to a value that it can safely interrupt.

If the available fault current exceeds the rating of the breaker, the fuse must operate faster than the breaker in order to protect it. The only way to assure this is by testing.

Many breaker manufacturers publish tables of tested series ratings with various types of fuses.

 

[RonShap]

I completely understand the concept of tested assemblies of a fuse and breaker to acheive a series rating.
I am wondering why in some circumstances.
If the fuse will act so fast to limit any fault to less than 100kA on the load side, then nothing downstream will ever see more than 100kA, and thus seems to not need a rating higher than 100kA for anything on the load side of the fuse.

 

[RonShap]

Would your opinion change if the downstream device is equipment with a withstand rating of only 100kA? If it can't protect itself? Like a safety switch?

 

[dpc]

thread238-118092

The breaker's rating and the fuses's rating are based on a set of assumptions. When the fuse is place ahead of the breaker, those assumptions are no longer necessarily valid. When the breaker contacts blow open before the fuse clears, it's a whole new ballgame.

See the second page of this:

http://www.bussmann.com/library/techspec/TechSpec12.pdf

Or do a Google search on "up-over-down method".

 

[alehman]

With current limiting breakers it's all about timing. If the breaker opens faster than the fuse, then the fuse may be unable to limit the arc current in the breaker before the breaker is damaged.

If you can assure that the breaker will not open before the fuse has a chance to clear the fault (possible with some non-current limiting breakers), then the fuse protects the breaker. The 2005 NEC has a new paragraph to cover untested combinations, but only with engineering supervision.

Many switches are rated only for application with approved fuses. Some non-fused switches have dual ratings such as 10kA for non-tested upstream protection and something higher if applied with listed protection upstream. If the equipment has a suitable true 3-cycle withstand rating, then I would probably be comfortable with the current limiting fuse as protection.

 

[RonShap]

If the fuse will alway limit the let through, based on the worst case testing configuration (shirt circuit power factor, short circuit closing angle and applied voltage) as claimed by fuse manufacturers for their let through tables/graphs to less than the rating of that downstream breaker, then why does it matter if the breaker begins to open or not? If it would begin to open, then the fault current would be less and thus the let through would be less. I don't completely see how the let through will ever go up during a "dynamic impedance" event, it seems that it could only go down based on higher downstream impedance.

I understand that the NEC requires series ratings, I'm just trying to understand why in some situations.

 

[alehman]

Maybe the confusing point is this - current limiting devices don't actually reduce the current. They simply cut it off before it can reach the peak of the cycle.

A current limiting breaker achieves it's AIC rating by interrupting the current before it reaches the peak. If the upstream fuse is too slow to act, the current will increase past the point where the breaker is capable of interrupting it before the fuse opens.

This is a classic point of confusion promulgated by fuse manufacturer's published "let-through" values. Those values only apply if the protected equipment doesn't also try to interrupt the fault.

 

[rbulsara]

I beleive its not on the basis of technical analysis, but having authetic certification of series rating which is only available by testing.

Field employed "series rating" based on let thru currents of upstream devices is NOT acceptable nor is the "up-over-down" method is acceptable by Code to determine short circuit ratings of the downstream devices.

Only recognized "series rating" in the USA are those parings of devices in series which are actually tested by UL (or equivalent NRTL) and puslished as such by the manufacturers. There is no other acceptable method of labling a series rating.

 

[rbulsara]

Or in other words, in this case, the avaialble short circuit current is still 200kA at the downsrream breaker even if it is passing thru a current limiting fuse. You can use a 100kA breaker if it is series rated for 200kA with the upstream fuse. But not any 100kA breaker.

 

[stevenal]

And the next overcurrent device downstream from the series rated 100 kA breaker must be able to interrupt..? Is a three level series rating required to avoid going back up to 200 kA for the third device?

 

[dpc]

Must be able to interrupt available fault current, unless it is a tested combination with the upstream breaker.

 

[davidbeach]

If you have devices A, B, and C in series and you have an A-B series rating you can, for C, use:

1. Fully Rated.

2. A B-C series rating.

3. An A-C series rating.

Best would be to have a nice chunk of impedance between B & C, like a transformer, and not have to worry about a series rating for C.

 

[advidana]

Check with another brand of fuse maker. They may have a different opinion on your problem.

 

[alehman]

Back on Rbulsara statement, "Field employed "series rating" based on let thru currents of upstream devices is NOT acceptable". That was the case until the 2005 NEC. 240.86 now reads:

240.86 Series Ratings. 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 in (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.

(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.

(C) Motor Contribution. Series rating 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.[/color blue]

I would agree that (A) is NOT good practice, but in these limited circumstances it is apparently permitted.

 

[rbulsara]

alehman:

Thanks for the update. But is it only for "existing installations"? I would still be hesitant to approve a combination that is not tested.

 

[alehman]

Yes, it is for existing installations only, apparently. I agree completely.