Short circuit ratings

[steve66]

I thought I understood short circuit currents pretty well. But then I realized something I don't quite get.

Take a main distribution panel and a subpanel. I calculate the available short circuit current at each panel. Say I get 40K at the main panel, and 20K at the subpanel.

I know the main breaker in the main panel needs a 40K rating. What do the branch breakers in the main panel need? Or are they considered protected by the main circuit breaker?

At the subpanel, I think the main breaker needs to be rated at 20K, and the branch breakers are protected by the main, correct???

Here is where I really get confused: What if the subpanel is a main lug only (no main breaker)?

 

[charlie b]

Re: Short circuit ratings

Think of it this way (but don't try this at home ): Suppose you were to connect a wire from the load side of a branch circuit breaker (in the main panel) directly to the neutral bar. Then stand as far back as you can and close the breaker (perhaps with a long stick). The fault current available through this branch circuit breaker will be the same as the fault current available at the main. In calculating the fault current available in a branch circuit, you don?t get to take credit for the resistance of any of the branch circuit conductors. Therefore, the branch circuit breakers in the main must be able to handle the entire 40 ka.

The same is true at the sub-panel. Whether it is a main breaker or an MLO, the branch circuit breakers of the sub-panel must be rated at least as high as the calculated 20 ka.

 

[cosmos]

Re: Short circuit ratings

I think that all the branch breakers within the main panel need to be rated for 40ka; similarly, all the branch breakers within the subpanel should be rated for 20ka, irregardless of whether the panels have main breakers or not. All breakers and wiring need to be able to withstand their short-circuit currents.

 

[ron]

Re: Short circuit ratings

The two previous responses are correct for fully rated systems. The main panel must be rated at 40kAIC and the sub at 20kAIC in this case minimum. The panel rating is comprised of the bus detail withstand values and the lowest of all the OCPD's installed in that panel.
So you might have a panel capable/rated of a 65kAIC rating, but there is a 10kAIC rated breaker in that panel, then that panel is considered rated at 10kAIC for my analysis. Of course if that 10kAIC breaker is replaced with a breaker rated at 65kAIC, to match the characteristics of the rest of the panel, then the panel is 65kAIC.

[ November 26, 2004, 11:24 AM: Message edited by: ron ]

 

[charlie]

Re: Short circuit ratings

The exception to all this is the series rated system (this discussion is assuming the 65 kA is available). If the panelboard is series rated for 65 kA, the main circuit breaker must be rated for 65 kAIC. In this case, all of the circuit breakers must be part of a tested series rated combination to be used. You may be able to use circuit breakers with 22 kAIC in series with the main if they are listed for that purpose with the main. In overload conditions, the smaller circuit breaker will trip normally. Under fault conditions, both the smaller circuit breaker and the main will trip together with both circuit breakers introducing dynamic (changing) impedance into the circuit so that the smaller circuit breaker will be able to handle the fault current that it sees.

 

[don_resqcapt19]

Re: Short circuit ratings

Charlie,

Under fault conditions, both the smaller circuit breaker and the main will trip together with both circuit breakers introducing dynamic (changing) impedance into the circuit so that the smaller circuit breaker will be able to handle the fault current that it sees.

I thought that was how series rated breaker systems always worked until I was reading this SquareD document a few weeks ago. The following is a quote from that document.

As a result of the potential simultaneous opening, some designers of electrical distribution systems believe that all circuit breaker coordination is lost when using series-connected ratings. In reality, using series-connected ratings provides virtually the same system coordination as a fullyrated system.

The document does go on to say that with high available fault currents that both breakers will open, but they say that same thing will also happen with fully rated systems.
Don

 

[ron]

Re: Short circuit ratings

Don,
Excellent point.
Sometimes the series rated combination is a Marketing decision, not engineering. Square D has a 10kAIC branch and a 22kAIC branch, and the 10kAIC branch has a series rating with many upstream devices getting higher AIC ratings, 18k and higher. They claim the 10kAIC device is capable of an individual fully rated listing for much higher than 10kAIC, but they already have a 22kAIC branch, so Marketing wise, they don't want two products rated for the same AIC, so they leave the 10KAIC QO breaker alone and offer it at higher AIC values in a series rating configuration.
The trip characteristics of the 10k and 22k device are exactly the same. So there coordination with an upstream device will be the same too, fully rated or series.

 

[charlie]

Re: Short circuit ratings

Interesting, I have never read or heard that before. The series rated system has always been written up the way I just described it. Learning is the biggest reason that I am here (I just hate to be wrong though).

 

[steve66]

Re: Short circuit ratings

It seems pretty clear now. The thing that brought this up was a MLO panel that needed a 32K rating. I was wondering if I spec it with a MCB, then can all the branch breakers be 10K rated. It sounds like the answer is yes, if they are part of a series rated combination.

I'll bet the panel is cheaper with a 32K main breaker than with all 32K branch breakers.

Steve

 

[ron]

Re: Short circuit ratings

Steve,
Yes it is cheaper with a series rating for the main and branches. I don't like to use the series rating because it is too often that the branch breaker gets changed, and not with the appropriate model, so the series rating in nullified. So I specify it fully rated, so there is less confusion during future modifications.

 

[templdl]

Re: Short circuit ratings

It has been of my experience that it is extremely important to have a person must be qualified and has the experience to do a coordination study to determine the available fault current at various points in the distribution system. After that devices rated for that fault current could be applied. It is very common for distribution systems to be updated from time to time to have different service entrances and other changes within the distribution system, which often results in greater fault currents. As a standard practice industrial facilities often commission coordination studies of their distribution system to assure that existing devises are rated for the application.
The alternative is to use devices that are listed by UL for series combination rated applications. Such devises have actually been tested as such.
Simply using let through current data that is often available would not be a substitute for actual testing.
The default is always that fully rated devices must be applied.

 

[lquadros]

Re: Short circuit ratings

Hi,

Going further, what happens with the components that are connected to these circuits which do not have a short circuit rating nor exceeding 5KA?
For example VFD or servo amplifiers? Will it get protected? and what about contactor overload combinations?

 

[coulter]

Re: Short circuit ratings

IQ -

I subscribe to the idea that the OCPD generally does not protect the appliance (end use device). Some exceptions - motor overloads for example. I'm thinking the OCPD (fault current protection part) is there to protect the conductors in the case of a fault in the end use device, and to protect the structure in the case of a fault in the conductors.

EX 1: If the device faults, the first thing one needs to do is get the fire put out. Too late to save the device, it already faulted.

Ex 2: This install was a temp 600kW gen w/ 1000A main CB, feeding a 500A feeder CB through parallel 4C - 4/0CU. The question was what protects the parallel 4/0. The 500A CB at the load end protects the 4/0 from overload. The 1000A CB at the other is there to put out the fire if there is a problem with the 4/0. If the 4/0 faults, there is nothing to save, the cable is electrical toast.

carl

 

[rbalex]

Re: Short circuit ratings

I'm surprized I didn't notice this thread earlier. The "basic" rule is:

110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.

The "interrupting rating" may be a "series rating" course. There is a hidden problem with this for "closed-transition" automatic transfer switches and secondary selective switchgear lineups. I'll discuss that more when I post my 110.9 Proposal.

A Hint: What is the "current that is available at the line terminals" during the time of a closed transition?

 

[wirenut1980]

Re: Short circuit ratings

rbalex, the fault current depends on the lengths, sizes, etc of both lines, but I can tell you it would increase because of the parallel paths.

 

[steve66]

Re: Short circuit ratings

Bob:

The fault current available could be as high as the sum of the fault currents of the individual sources.

At first I thought "What are the odds that a fault would occur during a transition". But it may be possible for a fault to cause a transition. (For example, the transfer switch sees a short as a loss of supply voltage). And if the transition is faster than any overcurrent devices, the fault could see the sum of the currents from both sources.

Steve

 

[rbalex]

Re: Short circuit ratings

David and Steve,

Gold stars to both of you.

So when was the last time you selected interrupting ratings based on both sources?

 

[arvinolga]

Re: Short circuit ratings

Can anyone explain how the kaic is computed for a certaing Main Distribution Panel?

Thanks

 

[ron]

Re: Short circuit ratings

Bob,
I have had this discussion with many of my colleagues, and we continue to disagree. Most of them will agree to consider both sources if it is an extended parallel, such as soft loading from generator to utility (30 seconds), but for a 100mSec transition, most of them will not consider both sources for the short circuit calculation. This is engineering judgment.

 

[rbalex]

Re: Short circuit ratings

Originally posted by ron:
... This is engineering judgment.

While I agree it should be, CMP1 doesn't:

1-150 Log #2969 NEC-P01
(110-9)
Final Action: Reject
Submitter: David Soffrin, American Petroleum Institute
Recommendation:
Add the following after the first sentence:
During the momentary paralleling of an automatically controlled power transfer between two sources, the single-source condition interrupting rating shall be considered adequate provided there is no intentional time delay and the parallel condition cannot be maintained. The initiation of the automatically-controlled transfer shall be permitted to be manual or automatic.
Substantiation:
The condition identified in the proposal is common in double ended substation arrangements where maintaining power to the loads is critical for operation or system safety. It is also a common practice in generating stations where continuity of power flow may be requried for public safety. These systems are designed for only a momentary parallel condition, typically only long enough for the closure of the paralleling breaker to initialize the opening of the designated breaker returning the system to single source configuration. During the brief parallel time (often only a few cycles), the short circuit rating of the switchgear feeder breakers may be exceeded. This added provision in 110.9 would recognize this arrangement and establish limitations under which it can be applied. IEEE 666, Design
Guide for Electric Power Service Systems for Generating Stations, section 4.6.1 specifically allows this arrangement and equipment rating.
Panel Meeting Action: Reject
Panel Statement:
The operational practice described is only one of a number of methods that may be permitted under 90.4. This issue is best addressed through evaluation of specific installations.
Number Eligible to Vote: 12
Affirmative: 12 Ballot Results:

Edit add: Even the CMP members that actually agree with the proposal (~3) have given up supporting it.

[ December 03, 2004, 10:14 AM: Message edited by: rbalex ]