IEC v ANSI Low Voltage Circuit Breaker Short Circuit Ratings 2

[gardengnome747]

Hi All,
There is a Switchboard on our project that is claimed to be rated to 65kA/1sec @ 480V as per IEC 61439. The Incomers to this Switchboard have Siemens 3VL MCCB's, and they have several short circuit ratings on the rating plate as follows;

Switching Capacity As per 60947-2
500/525VAC - Icu/Ics - 50/38kA

NEMA Breaking Capacity
480V - 65kA

I'm curious as to why there is such a large difference between the Icu Switching Capacity and the NEMA Breaking Capacity for essentially the same voltage level?

Is it possible to use ANSI/NEMA Circuit Breakers in IEC Rated Assemblies?

Appreciate any thoughts or experiences on this subject.

 

[veritas]

I would recommend you stay far away from Icu. I have never found much use for it in practice except when doing cascading and even then I am most cautious with it. Anyway, best to first define the difference between Icu and Ics first:

Icu = rated ultimate short-circuit current
This is the maximum symmetrical short-circuit current the breaker can interrupt and is tested as follows, O – t – CO.

O = breaking operation
t = time separating two operations equal to 3 minutes or the length of time needed to reset
the breaker, whichever is longer.
CO = making operation followed by a breaking operation

After the test, the breaker is tested to show that it is electrically safe. The dielectric test involves applying twice rated voltage, the minimum being 1000V.

Ics = rated service short-circuit current
Ics is the maximum current the breaker can interrupt three times and be returned to service with its operational integrity intact.

Ics is expressed as a % of Icu (25%, 50%, 75% or 100% of Icu).

It is tested as follows, O – t – CO – t – CO. Following the test a temperature rise, dielectric and tripping test is carried out. This is to verify that the breaker can be returned to service. In essence,

Both Icu and Ics are threephase symmetrical (r.m.s.) current duties.

I read two articles some time ago where these where compared and both authors advocated the use of Ics and Icu. I agree!

In your case, Ics = 75% Icu. Thus ensure that Ik" < 38kA (or at the very minimum Ib < 38kA) if using the IEC 60909 study.

With regards to your question, there would be a difference as the current ratings are expressed at different voltage levels. But this would not account for the 15kA difference.

I am not a NEMA expert so would leave it to someone else to comment. I have some literature about it and will read up meanwhile.

 

[gardengnome747]

Our short circuit calculations are based on IEC 60909, and the Ik" is above 50kA but below 65kA. It would be interesting to know if or how this would relate to the NEMA Breaking Capacity.

I have seen some literature from Rockwell that appears to have a similar rating scheme on some of their Circuit Breaker offerings also.

 

[cuky2000]

Enclosed is an excerpt regarding LV cable insulation that I hope help to bring some light to this thread.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1541001827/tips/LV_Cable_Insulation_133_dkk0mk.pdf[/url]

 

[veritas]

I read two articles some time ago where these where compared and both authors advocated the use of Ics and Icu. I agree!

Oops, should have read, "I read two articles some time ago where these where compared and both authors advocated the use of Ics and not Icu. I agree!

 

[ciy]

Dear All,

There is a question I wonder to know how select the breaker.

It assumed already have the calculated fault current by SKM.

Which do I select the Icu or Ics accordingly?

For example, the said calculated value is 42kA @480V InitSymRMS 3P.

IEC breaker rating Icu/Ics = 50kA/38kA @500V

50kA > 42kA , but 38kA < 42kA

So, this breaker rating is OK or Not?

Please advise. Thanks!

 

[gardengnome747]

ciy,
You should take a look at this thread, thread238-81200. It has good discussion on the Icu/Ics terminology and how to apply them in practice.

 

[Kiribanda]

ciy,
I my opinion the bkr should be rated for the prospective short circuit current at the location. In your case the calculated is
42kA whereas Ics=38kA. (In fact previous P1 & P2 sc duties given in IEC 57, were replaced by new Ics & Icu in IEC60947-2).
At 38kA it is suitable for two open operations (O-t-CO-t-CO) only whereas at 50kA this bkr is suitable for one operation
(O-t-CO) only. Now the question is at 42kA what is the number of operations given to this bkr. It is unknown, unless it is
tested by the manufacturer. Therefore, in my opinion you have to select a bkr with Ics=42kA or higher.

 

[veritas]

I agree, use Ics and not Icu. If you use Icu you need to actually test the breaker because you do not know if the fault current was > 42kA or not. In reality, how many plant operators are going to test the breaker, have the skills and/or equipment to do so? No, my vote goes for Ics.

 

[ciy]

gardengnome747, Thanks for your information.

Kiribanda and veritas, Thanks for your opinion.
I agree to select by Ics. It's safer selection.

However, have any guide or practice to highlight how to select CB according to Icu/Ics?
Please advise, if any.
Thanks again.

 

[veritas]

Ics is not the only parameter to consider when selecting a LV breaker. The 4 most important parameters are:

1. In = rated current (current breaker can carry continuously and is based on maximum load including allowance for future load increase as well as temporary overloads).
2. Ics = rated service short-circuit current, i.e. the interrupting capability of the breaker
3. Peak current, Im (making rating) - ability of the breaker to withstand the dynamic (mechanical) stresses associated with the initial peak. This is dependent on the X/R ratio at the breaker location. It is called a making rating as it considers the breaker closing (making) onto a fault.
4. Icw = rated short-time withstand current, in other words how much throughfault current the breaker can carry for a given time. Usually stated for 1s but other times are possible.

To start off with you would need to do a fault study. I use software that can do the IEC 60909 study for me. I then match the breaker parameters with the study results as follows:

Ics > Ik" (some would use Ib from the IEC 60909 results but I go more conservative)
Im > Ip
Icw > Ik or Icw > Ik" (strictly speaking you should work out Ithermal as per IEC 60909 which considers m and n factors. Practically I found this rather tedious and the results are not much different if Ik or Ik" is used). I use Ik if there is no AC offset (Ik" = Ik) and Ik" if there is AC offset (near to rotating machines).

Just a word regarding Ip. For a given Icu rating of the breaker, Ip = n*Icu. The value of n depends on Icu and is given in Table 2 of IEC 60947-2. You would be well advised not to use the n value in the table blindly. The values in the table are for a given power factor. So for example, Icu = 65kA, n = 2.2 in other words the peak rating is 2.2 * 65 = 143kA. This is based on an X/R = 4.89 or a pf = 0.2. If your X/R is larger than this, which frequently happens in my experience, then you would need to work out your own "n" value.

The formula for n = sqrt(2)*(1.02 + 0.98*e^(-3R/X)) as per IEC 60909. Note Ip = n*Ik". I had an example recently where the fault study gave Ik" = 60.04kA and Ip = 147.36kA. This meant n = 2.45. If I had gone purely on Ik" then a 65kA swbd would seem ok but the peak current doomed the 65kA option.

I worked around this by using a current-limiting MCCB on the incomer to the swbd. This dramatically reduced the peak current to well within the ability of the 65kA swbd.

Fianlly, note that In, Ics and Icw are rms currents whilst Ip is an instantaneous value.

I would be very interested to see something similar to the above write but for the NEMA world.

 

[dpc]

The breaker is dual rated for IEC and ANSI/NEMA. You need to use the rating that applies to your installation. Higher voltage reduces the interrupting rating. Also, there is absolutely no "1 second" rating for molded case circuit breakers under ANSI. They must have an instantaneous trip. Some MCCBs may be voluntarily tested for a 30 cycle withstand rating, but must still be fitted with instantaneous trip.

 

[veritas]

MCCB's have a 0.3s Icw (rated short-time withstand rating) in IEC 60947-2 since they're current limiting. ACB's typically have a 1s rating. I found some of the older mccb's do have 1s ratings. These were not classified as CL.