Method(s) used to limit fault currents
to 10,000 amps on branch circuits

If the circuit composition leaves you with a high short circuit availablity at a piece of equipment, you are pretty much stuck with it. You could install an in-line reactor, but it is large, cause voltage problems and mostly not worth it. If you could rerun the feeder or circuit to the equipment, take the long road, since conductors have lots of resistance.
Mostly you need to deal with the availability of the fault current. Many times you could research a fuse to put in series with the down stream breaker to achieve series rating. Don't fall into the false perception that a current limitting fuse by itself will limit the available current, 'cause it will not. You must have a listed combination to have a series rating.

This is a very important point and I don't want anyone to overlook it.
Don

I don’t agree 100% with this statement - "Don't fall into the false perception that a current limitting fuse by itself will limit the available current, 'cause it will not."

For example, Current Limiting Fuse X in the sketch below will reduce the 50 kA available fault current (AFC) at the transformer, to the let-through rating of the fuse, 17 kA, at Switchgear A. The Withstand Rating of the switchgear can then be less than 50 kA, let’s say, 30 kA.

I agree with this statement - "You must have a listed combination to have a series rating."

A series rated system is a listed combination of circuit breakers, or fuses and circuit breakers, that can be applied at available fault levels above the interrupting rating of the load side circuit breakers, but not above that of the main or line side device (formerly known as a Cascaded System).

The series rated system has only one advantage - lower initial cost compared to a fully rated system.
However, they have many disadvantages that often preclude their use. These disadvantages include:
1. No Selective coordination. The fault shown in the sketch will shut down Panel B.
2. Not permitted for feeders supplying motor loads (if motor load exceeds 1% of the lowest interrupting rating in the series combination).
3. Limited to listed combinations available from manufacturers. Typically applied only for feeder and branch circuit loads.
As a result of these limitations, series rated systems are normally only used for:
1. Lighting feeders and other non-motor applications.
2. Non-critical loads where selective coordination is not needed.

My understanding is that the Y – Z series combination would not be code compliant.
The 10 kA breakers in Panel B are a violation because of the 200 amps of motor loads, but the combination would be OK (when listed) if the motor loads supplied were less than 1% of the interrupting rating of the downstream breaker Z.



Ed

Ed,
Unless the exact combination of fuses and breakers have been tested, listed and marked on the equipment by the breaker manufacturer, you can't use the X-Y system shown in your drawing. The code does not permit the use of a current limiting fuse to protect downstream breakers with a lower interrupting rating. The breaker interrupting rating must be equal to or exceed the fault current that is available at the line side of the breaker without the reducing effect of a current limiting fuse. Often the length of the feeder from the source will provide enough limiting to permit the use of lower rated breakers. The fuse people have been trying to get the use of current limiting fuses for this purpose into the code for the past few code cycles and have not succeeded.
Don

this is interesting.

we have to have the AFC , if anything else , to comply with 110.16 (a component of the calc)

but in collecting the #'s for the AFC, if the impedance (of both the serving x-former, as well as the R value in terms of entrance conductor distance) is often minimal.

this results in much umph, so i was under the impression that 'series rated' was the only out..... my bad....

The siemens sentrons i've installed are only good to 200K IR ......how does one combat infinity?

Not sure what you mean.
There would be few, if any, distribution transformers capable of delivering more than 200,000 amps fault current. The 200k AIC Sentrons would be OK almost anywhere.

My point was simply this - Current limiting fuses do reduce the available fault current that downstream equipment is exposed to.

And, (this is an observation, not a criticism) the NEC people are being very cautious in recognizing series rated combinations.

Ed

Ed,
Actually this is just a turf war between the manufacturers of fuses and breakers. There is no question that a current limiting fuse can do exactly that and could protect the downstream breakers, but the breaker manufacture does not want to be in the position of saying a competitive product will protect his breaker.
Don

perhaps i'm confused Ed, it would not be the first time.

the last 400A single ph 240/120V service i installed included fuses, a choice over breakers for what may in fact be erroneous reasons??

a contributing factor was the proximal x-former, it's unknown impedance, and a short OH run of minimized R value.

please continue, i can be taught.....

I understand one of the reasons that a certain type ( catalog number )CB must be tested by NRTL is to assure that the CB does not start to open the circuit before the fuse opens when the downstrean CB is of a less Kair rating than the upstream fuse.
Such as, the fuse is for 100Kaic and the CB is for 10Kair on a circuit where there is 25,000 of Available Short Circuit Current (ASCC).
If the CB starts to open the circuit before the fuse opens the circuit, the 10Kair CB would be seeing the 25,000 amperes, well above it's rating and could explode.

The Idea of Series Rating is for the up-stream OC device to open at the same time or before the down-stream OC device starts to open.

Locally, the 50KVA 120/240V transformers have 23640 Fault Amperes at it's terminals which is typically used for services to two or more dwellings. Many times the ASCC with-in the dwelling is well over the 10Kair CB's used.

[This message has been edited by Gwz (edited 12-15-2002).]

Yes, there is an article in EC&M Magazine about the very thing Gwz has described? http://www.ecmweb.com/ar/electric_dont_felled_higher/

This is a quote from the article -

"If the downstream circuit breaker starts opening on a high-current fault within the first half cycle, it may "fool" the upstream, fully rated device into not opening, because the fault current steadily decreases as the downstream contacts open. The result is a downstream device clearing the entire fault, even one well beyond the device's rated interrupting current. The outcome: self-destruction."

Apparently this was not a problem with older breakers that took 2 or 3 cycles to open. Some newer breakers can begin to open in the first half cycle, creating the problem you mentioned.

Re the "turf war". The breaker manufacturers are scrambling to develop ways to cause breaker contacts to open in less than a half-cycle, and thus produce a current limiting circuit breaker. They can see that, with ever higher available fault currents, current limiting fuses/breakers will be the OCPD of choice.

Ed

The fuse manufacturers have been using the term "newer" style breakers for over 25 years.

Very few (except large drawout power breakers) will stay closed for 2-3 cycles. When breakers are this slow they are usually listed with a withstand current rating rather than an interuppting rating.

Almost all molded case "loadcenter style" miniature breakers begin opening within the first 1/4-1 cycle. This is why the NEC requires actual testing. It is not unusual to find some small amp breakers that are faster than some large amp fuses.