Short circuit rating on MCC 1

[JuanBC]

Hello,

I would like to read your opinion regarding the following design criteria:

Let's say we have point in a network where the available fault current (three-phase symmetrical sub-transient RMS current Ik'') is 50 kA

Is it correct to install an MCC rated "30 kA 1 sec." (symmetrical RMS) provided we install a limiting protective device upstream? Or shall the MCC be rated >"50kA 1 sec"? (i.e. 65 kA)?

Thank you,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[waross]

The source has 50 kA Available Short Circuit Current.
The MCC is rated for 30 kI ASCC.
Consider the impedance of the feeders to the MCC.
There may be enough impedance in the feeders to limit the ASCC at the MCC to below 30 kI.
An old school trick is to calculate the length of feeder required to reduce the ASCC to the rating of the MCC.
The extra length of cable may be run past the MCC and then doubled back.
The extra cost for the longer cable is often by far the most cost effective solution.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JuanBC]

Hi Bill,

I really like your trick but, just as a theoretical exercise, let's assume that 50kA is the calculated ASSC at the MCC.

Is the idea of using a upstream limiting protective device technically correct?

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[waross]

I'll stand aside. Others here are more able to give an accurate answer. This has been discussed in other threads.
If the MCC has a main breaker there may be coordination issues with a current limiting device.
Note that 30 kA for 1 sec is a thermal limit.
There will be an absolute limit for the maximum instantaneous asymmetrical fault current based on the ability to withstand mechanical forces.
This will be rated as an ASCC with an allowance for a typical X/R ration of the source.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

In general no, that doesn't work. What is your "limiting" upstream device and how do you know what it is limiting the current to? For upstream protective devices, the only way that this is acceptable is if the upstream device and the downstream devices in the MCC are tested combinations. If the MCC bus bracing is only 30 kA, then you still have a problem unless MCC manufacturer has some test data to so that this will work.

Bill is correct that fault current can be reduced by longer cable run or addition of a current-limiting reactor. That is the foolproof solution. But relying on "current-limiting" of an upstream device - typically a fuse is problematic.

 

[JuanBC]

Yep, I know full coordination will be quite difficult, if not impossible, to achieve
Thanks for your point of view!
Regards,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[JuanBC]

Hi dpc, thank you for your input

What is your "limiting" upstream device and how do you know what it is limiting the current to?

The first idea I came up with was to use the "Current-limiting curves" that manufacturers provide for current limiting protective devices (prospective kArms vs kApeak) but that will only give me an idea of the mechanical forces involved and not the thermal energy developed.

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[dpc]

Use of the current limiting curves to determine current limiting for downstream circuit breakers ("up-over-down" method)has been pretty much deprecated, especially by the breaker manufacturers. The dynamic fault impedance of the breaker itself complicates the determination. Hence the reliance on actual test data. Most smaller molded case breakers are tested for use downstream of certain classes of current-limiting fuses. But not as common with larger breakers.

I'm not clear if your concern is the breaker short circuit ratings or the bus bracing in the MCC.

 

[ElecEE8]

Hi JBC,
We all may have better understanding from the purpose of short circuit study when we need to sizing/selection new equipment or evaluate the existing equipment fault withstand rating i.e. switchgear, CB, etc..
SC study, following definitions normally apply;
1) Initial symmetrical short circuit current Ik” – rms value of the a.c. symmetrical component of a prospective fault current applicable at the instant the short circuit occurs.
2) Peak short circuit current Ip – maximum possible instantaneous value of the prospective fault current.
3) Symmetrical short circuit breaking current Ib – rms value of an integral cycle of the symmetrical a.c. component of the prospective fault current at the instant of contact separation of the first pole to open of a switching devices.
4) Steady-state short circuit current Ik – rms value of the fault current which remains after the decay of the transient phenomena.

Practically, Prospective asymmetric peak fault currents and the symmetrical short circuit break currents have to be calculated and these are related to the equipment withstand capabilities;
1) Ip 10 ms peak asymmetric current used to establish the required making capacity of circuit breakers.
2) Ib the symmetrical short circuit breaking current is used to establish the required breaking capacity of circuit breakers and the short time withstand ability of busbars for all fault types.
3) The 60 ms rms asymmetrical break current has also been considered and compared against switchgear ratings for three phase faults.

Rgds
ElecEE8

 

[JuanBC]

Hi,

dpc: If I am understanding you correctly, current limiting curves are... useless? Or do they have some use that I do not know?
My concern is about the whole MCC, the main breaker and the bus-bars

ElecEE8: Is it accurate to say that Ik'' is always greater than Ib? We usually only specify the value of Ik'' and the CCM's mfr design it (I think that the usually assume a "Kappa" K=2.55 to determine Ip but I do not know what they use as Ib)

Sincerely,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[waross]

The Available Short Circuit Current is a defined rating.
It is not the symmetrical current.
The Available Short Circuit Rating is based on a fault current at a specified X/R ratio.
That is, a component rated at 30 kA ASCC will be able to withstand the Asymmetrical current associated with an Available Short Circuit Current at a specified X/R ratio.
It is an artificial rating that facilitates the use of the supply % impedance to select suitable equipment.
Said another way:
The Available Short Circuit Current rating is NOT the steady state current after the decay of the initial offset.
It is a component rating.
The Available Short Circuit Current is calculated the same way as the symmetrical current and is numerically the same.
However the Available Short Circuit rating is based on, but not the same numerically, as the initial fully offset fault current at a specified X/R ratio.
A breaker that can withstand a given symmetrical current is not the same as a breaker that can withstand the asymmetrical or offset current associated with the same given symmetrical current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JuanBC]

Hi Bill,

Do you know if the concept is the same in both the IEC and ANSI world? Or it only exist under ANSI?

In the IEC world, IEC 60947-2 states that:
"
2.15.1 ultimate short-circuit breaking capacity ;A.K.A. "Icu"
a breaking capacity for which the prescribed conditions according to a specified test sequence do not include the capability of the circuit-breaker to carry its rated current continuously

2.15.2 service short-circuit breaking capacity ;A.K.A. "Ics"
a breaking capacity for which the prescribed conditions according to a specified test sequence include the capability of the circuit-breaker to carry its rated current continuously"

I'm not sure if the right thing is to compare Icu/Ics ratings with Ik'' or to "another" value




JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[dpc]

The fuse let-through curves are indeed worthless for determining if a downstream circuit breaker will be protected by the current-limiting of the fuse.

 

[waross]

Thanks for the heads up JuanBC.
Difference sides of the pond, or different hemispheres.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[veritas]

I found this thread extremely interesting and a very important for those responsible for LV network design. I would like to add the following: The IEC and NEMA have different ways of defining breaker ratings. The same fault current waveform but different ways of interpreting the results.

As I've stated in another post, I would advise that Icu be ignored (except for cascading applications and even then be careful). Best is to use Ics. Ik" should be less than Ics as a rough rule of thumb. Yes, one could use Ib but the difference between Ib and Ik" is usually negligible unless Ib is considered say 5 cycles after Ik" and the ac decay is extremely large. If there is no "near" generation I would go for Ik" and not Ib as it is always the safer more conservative one to use.

NEMA uses the asymmetrical rms current which is really the symmetrical with the DC offset included. In any event, one could convert IEC to NEMA values and vice versa.

The fuse let-through curves are indeed worthless for determining if a downstream circuit breaker will be protected by the current-limiting of the fuse.

I wonder what fuse manufacturer's may say about this statement. I was informed by one that the peak let-through curve of a fuse may be used to determine what the "reduced" downstream fault level is for network designers. Consider the following curve.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1541041293/tips/fuse_let-through_curves_kf0qwg.rtf[/url]

Thus a 4kA prospective fault current is tamed to a 420A one when using a 50A fuse. In other words, the downstream network can be designed as if the fault level is 420A and not 4kA. The danger of this of course is that the fuse needs to be replaced with the correct one once operated.

I actually used cb cascading to design an LV swbd with 6kAmcb's where the prospective fault level is 23kA. For this I consulted the manufacturer's tables. I used lesser rated cables as well. Now this could pretty well be the reason I'll earn eternal damnation but I do think current limiting could be put to very good use.