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# Short Circuit Current

03/31/2017 11:59 AM

Dear all,

What is the deference between rated service breaking current(Ics) and rated short circuit breaking current(Isc) and short time current(Isd), and which is bigger than the other.

Thanks

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#1

### Re: Short Circuit Current

03/31/2017 1:38 PM

If you know what's happening in a circuit breaker then these are very self explaining terms.

If the manufacturer of these breakers does not have a white paper that explains terminology then look at another manufacturer's datasheet.

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#2

### Re: Short Circuit Current

03/31/2017 1:41 PM

http://www.electrical-installation.org/enwiki/Fundamental_characteristics_of_a_circuit-breaker

"The Long Time (current rating) (Ir ) is the long-time pickup setting multiplied by the sensor plug amperage (In ).

The short-time current (Isd ) equals the short-time pickup setting multiplied by the long-time pickup (Ir ).
The instantaneous current pickup amperage (Ii ) is equal to the instantaneous pickup setting multiplied by the sensor plug amperage (In ).
Ground-fault pickup values (Ig ) is equal to the ground-fault pickup setting multiplied by the sensor plug amperage (In ). "

http://www.schneider-electric.us/en/faqs/FA92740/

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#3

### Re: Short Circuit Current

03/31/2017 1:57 PM

See-for instance:Schneider Electrical IEC 60947-2 Low Voltage Switchgear Part 2 Circuit Breakers page 11-13

https://www.scribd.com/document/244109578/Estandar-IEC-60947-2-pdf

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#4

### Re: Short Circuit Current

03/31/2017 3:57 PM

Ics < Isd << Isc.

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#5

### Re: Short Circuit Current

03/31/2017 4:01 PM

Electrons have no deference for he who assumes a position in the midst of the circuit.

Rated service means just that. The breaker should not trip below that current, with a constant load.

Short circuit breaking current, depends on the physical geometry of the conductors employed, and is specific to each circuit layout.

Short time current is the current the circuit breaker can sustain for a "short time" without tripping. Again, this depends on the geometry involved since inductance comes into play in a transient situation. Depends on the geometry of the circuit breaker conductor also.

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#6

### Re: Short Circuit Current

03/31/2017 10:49 PM

Kasrop,

You have already received a very concise engineer's answer to the question. A brief explanation--the Isc is a rating based on the physical ability of the circuit breaker to interrupt a fault current up to that magnitude without creating a "mess". The breaker is not required to be able to be used after that event, but it must be able to open the circuit without disintegrating or losing pieces in a way that causes harm or damage to the rest of the electrical equipment.

In practice, any time a breaker interrupts a current of a magnitude that approaches its Isc rating, it should be taken out of service and tested to ensure it is still working within its designed abilities.

Isc is always the largest of any of the ratings, and usually is tens to even thousands of times greater than the other ratings. Typical Isc ratings for breakers might be as low as 10kA (and even 5kA) to 65kA and much higher for certain designs.

JMM

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#8

### Re: Short Circuit Current

04/01/2017 6:15 PM

"...The breaker is not required to be able to be used after that event..." Could you please provide a reference for that statement?

I doubt that your local utility replaces a \$750,000 high voltage circuit breaker each time it interrupts a solid line to ground fault. Nor does it take one out of service to inspect it unless there was some misoperation or alarm that would indicate that it needed inspection.

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#11

### Re: Short Circuit Current

04/02/2017 2:00 PM

RAMConsult,

I appreciate your contributions to this forum. I don't have the reference(s) in front of me today (Sunday) but will check tomorrow. I suspect the primary one will be in the U/L requirements for listing the device. I suspect a secondary one will be in the NFPA-70B publication that relates to maintenance of electrical equipment. If I get busy and forget, feel free to send me a tickler/reminder via PM.

Regarding the point you raise about replacing a \$750,000 high voltage CB, it is very seldom that the actual short circuit current comes close to or exceeds the rated Isc of the breaker. Certainly the engineers who have consulted on such an installation will have done their calculations to determine the available fault current on the supply side (including calculations to account for multiple sources, transformer contributions, and the many other things that enter in). The specification for the breaker will have been for one that has a tested and listed capacity higher than the calculated available fault current. If not, then the engineer has failed and should "be replaced" along with the breaker! (humor intended.)

Thanks--John M.

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#12

### Re: Short Circuit Current

04/02/2017 4:07 PM

NFPA 70B Chapter 15, Section 15.1.1.4 has wording similar to what you are quoting, but it is only for Air Disconnecting Switches, which by definition are never to be used to interrupt any type of fault. The other references to rated short circuit always use the word "should" instead of "shall"; i.e., advisory versus mandatory.

You are correct, when we did the calculations every effort was made to ensure that they were conservative and could never be reached during actual system conditions. The only time a utility grade piece of equipment was ever subjected to its rated short circuit conditions was under factory proof testing, never under actual operating conditions.

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#13

### Re: Short Circuit Current

04/06/2017 9:12 PM

RAMConsult,

I took the following from a UL page that was discussing panelboards and switchboards, in which it was discussing short circuit tests of breakers at their rated interrupting current. See:

"After being subjected to short circuit conditions, the mechanical condition of the equipment must be substantially the same as its condition prior to the test, no live parts can become exposed, and components within the equipment cannot be significantly damaged."

I suspect more is also in UL-489, which I have not purchased. Hope that clarifies what I had said earlier about circuit breakers.

--JMM

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#14

### Re: Short Circuit Current

04/07/2017 10:41 AM

Thanks for highlighting one of the differences between a design/testing standard (UL) and a safety/operational standard (NFPA).

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#7

### Re: Short Circuit Current

04/01/2017 1:07 AM

This most certainly homework!

If you can search the web and find CR4, then you can search again and find the answer.
Or you can pay attention in class!

Go find the answer then come back if you don't understand it, or ask your teacher first!

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#9

### Re: Short Circuit Current

04/02/2017 1:15 PM

Don't worry, I got this. It's because this is V=IR, when V=0, then R≠0 so current must be I=0. I don't know if an EE would disagree to that. Common, challenge me?

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#10

### Re: Short Circuit Current

04/02/2017 1:48 PM

Gutmonarch,

You have to look at the circuit, not the point on it where the bolted fault exists. I say "bolted fault" because an arcing fault always has lower currents due to the resistance of the arc itself. The conductors leading to the fault all have an impedance so when current flows there will be voltage drop along each section. Assuming the source is very large, the current through the circuit will be determined by the source voltage and the circuit impedance. V≠0, because the source is supplying it. Because the impedance of the conductors is fairly low (remember you design the circuit for a voltage drop that is small, such as ≤3% or 5% at the expected current of the device being powered. But when you remove that device and substitute a short circuit for it, the current will be much larger. Therefore I>>0. Typically, on a relatively small branch circuit, the Isc can be 500-5000A, but on a feeder or the load side of a service the Isc can easily be up to 50,000A and more. At these fairly large values of Isc, the overcurrent protective device will trip (or blow) very quickly; many times in their instantaneous range of 0.01 to 0.02 seconds, so the total energy released in the short circuit is pretty small.

However, when you deal with an arcing fault, the Iaf is less than the Isc, by an amount determined by the spacing between the arcing conductors and many other factors. Under these conditions, it is common for the overcurrent protective device to trip (or blow) "much" later, and can in many cases do this tens or even hundreds of seconds later. At this point you get into an engineer's discussion of arc faults, arc fault energies, time/current curves, the type/size/shape of the enclosure, and many more factors. Study NFPA 70E as a beginning for that topic, but be prepared to read it many times before you get to understand its contents.

Thanks--JMM

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