Short Circuit Current used for Cable Sizing Calculation

[Schwatzernov]

I have the question that when we do cable sizing calculation based on short circuit current, the short circuit current used is the short circuit current at the end of cable(terminal of motor orload) or it should be at the beginning of cable connection to circuit breaker ?

I understand that the point for calculating the short circuit current should be at the end of cable as it should be through fault ? because if we use the point at the beginning of cable connection with CB, it seems to be unrealistic as this current does not pass through the cable yet.

Is there any standard recommend the point of short circuit current for the cable sizing application ?

Thank you

Best regards

 

[alehman]

I don't know of any standard. The best way is to plot the cable damage time-current curve with the overcurrent protection curve.

 

[jghrist]

I'd say use the available fault current at the first manhole for cable in conduit. A fault any closer than that would require pulling in new cable up to the first manhole anyway. For direct buried cable, use the fault at the breaker. You would want to be able to repair a faulted cable anywhere on the cable run and be assured that the cable is not damaged between the source and the fault.

 

[Alex68]

It was the same question I asked myself some weeks ago.

First I thought I should consider the through fault since it doesn't make sense considering the current of a fault at the beginning because the cable must be changed anyway.

But a faulted cable ought not to be a cause of danger or damage also for the equipment at the beginning. So it must be able to withstand to a fault close to its beginning.

So my last and actual opinion is that I must consider the fault current at the beginning of the cable.

 

[davidbeach]

Nah, it's just like a transformer damage curve - for through faults. If the cable itself faults it is already damaged. The damage curve/withstand curve is for the fault current the cable can supply to the fault without damage occurring to the cable.

It can make sense to consider chunks of cable where portions could be replaced if damaged, but if you have a cable fault at the source end the cable is toast whether it was above or below the withstand rating. Any fault before the first point the cable is accessible and could be spliced is going to require removal and replacement anyway, so use the through fault current; just choose the right through fault.

 

[Schwatzernov]

In my opinion, I also considering the same consideration as those of transformer.However in case of transformer, it is clear and have IEEE standard support to use the through fault current, but in case of the cables , it is not so clearly mentioned what is the base point whether internal or external of cable portions.Does anyone can tell me the technical support document such as textbook or standards ?

Thank you very much

 

[PHILEC]

Let me explain how short circuit current flows on a motor feeder cable. If fault occurs on the beginning of cable termination (Fault 1), the fault current flowing on the cables is the motor contribution which is reduced by the cable impedance. On the otherhand, when the fault occurs on the end of cable or near the motor terminal (Fault 2), the fault current flowing in the cable are comes from the utility or generator contribution and the contribution from other motor loads which is higher. The fault currents calculated on Fault 1 is higher compared to Fault 2 but the fault current flowing in the cable is higher in Fault 2 compared to Fault 1. In practice, we normally used the fault current on the bus where the motor is connected using the scenario where the system will have the maximum prospective short circuit current.

Reference ANSI/IEEE STD.242-1986.

Hopes this clarifies the issue.

 

[HamidEle]

The short circuit current should be the through fault occuring at the the beginning of the cable, which is the worst case. This current equals the bus fault current minus this motor contribution from this feeder.In practice we can just use fault current on the bus where the motor is connected using, where the system will have the maximum prospective short circuit current.

 

[Senselessticker]

Maybe I'm missing something here....but....

Why is cable sizing being determined by SC availability?

The purpose of knowing the SC current is to properly size the interuprt rating of the OP device...right? When you change the conductor size....you change the SC availability all the way downstream of that cable.

Please let me know if I'm missing something here...

Thanks,

 

[davidbeach]

Senselessticker, conductors have a withstand rating, just as transformers have a withstand rating. For conductors it is a straight (on log-log anyway) line of constant I²T. I rarely found any problem with it; good system design doesn't usually expose conductors to more fault current than they can withstand, though it can happen. What you look at for parallel conductors can become an issue. I always treated it as a withstand, assuming that a faulted conductor was already damaged, but also looked at N-1 conductors in each parallel run. That way the current in the good conductors feeding a fault in one of the parallel conductors at the far end of the run would be evaluated. That would be the worst condition - far end fault being carried by N-1 conductors.

I used SKM and eventually got them to add an N-1 option for cable damage; for a long time it had been 1 or N as the only choices.

 

[rbulsara]

davidbeach and OP's hunch in the original post are correct.

For cable withstand rating, calcaulate SC at the end of the cable, as it is for through faults only. The cable is not supposed to be damaged in delvering the fault current.

The fault current at the beginning of the cable is for the breaker/fuse assembly protecting the cable and not for the cable.

Senslessticker:

In medium and high voltage applications, SC withstand rating often dictate the size of the cable as normnal load currents are small (as the voltage is high, but SC currents could be very high. You need to check both.

 

[davidbeach]

In medium and high voltage applications, SC withstand rating often dictate the size of the cable as normnal load currents are small (as the voltage is high, but SC currents could be very high. You need to check both.

I'll beg to disagree. 50kA to 100kA, and vastly higher on network systems, are common at 208V and 480V. In utility work, away from generation stations, fault currents are generally much lower.

The utility I work for designs our distribution substations, with few exceptions, to provide a maximum of 20kA at the distribution voltage. Many transmission busses won't draw even 20kA for a bus fault. The energy content of the fault is far greater, but the current associated is often surprisingly low if one is used to building systems. We're looking at ways of keeping the distribution voltage ring bus of a potential new substation below 40kA, but that is an issue only because there will be 200MVA of transformation feeding that ring bus. But even at 40kA, the large conductors used for the distribution circuits will not have withstand issues.

 

[PHILEC]

Rbulsara, Through fault is not clearly mentioned ANSI/IEEE STD.242-1986 it only dictates to use the maximum available short circuit currents which is the subtransient currents for cable protected by instantaneous overcurrent relay and medium voltage switchgears. For cable protected by fuses or low voltage and instantaneous trip circuit breakers the asymmetrical current is used.

In practice we normally used the calculated currents in the bus where the cables is connected because this is the highest fault current available that the cable will withstand to lessen the effect of equipment damages if the fault occurs on the cable it self (especially in the location under the substation cable space).

Normally when we calculate cable size by load currents we rarely encountered resizing of cable size due to short circuits due to a very conservative derating factor on cable ampacity. That is my experience in Oil and Gas Plants especially Aramco projects.

Cheers!

 

[Senselessticker]

davidbeach and rbulsara,

Thanks for the insight. I just learned something new!

Thanks,