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MV breaker interrupting multipliers for fault current DC offset

MV breaker interrupting multipliers for fault current DC offset

MV breaker interrupting multipliers for fault current DC offset

(OP)
I believe is was reading in a reference book that MV breakers need to have a multipler applied to their interrupting ratings depending on the system X/R ratio in comparison to the breakers tested X/R ratio (usually 15)in order to accound for a higher current needing to be interrupted due to the DC component decay of the fault current.  With a higher X/R ratio the Aysmmetrical fault current will decay more slowly so will be higher at the a given interrupting time (3 cycle, 5 cycles, etc...) with a system X/R that is larger than the tested X/R.

I know this is applied to low voltage breakers but am not quite sure after reading if this is also applied to an MV breakers "interrupting rating"?  In other words if an MV breaker has a 29kA interrupting rating does this value need to be adjusted for higher X/R ratios?

I know an MV breaker has an aysmmetrical "momentay" or "close & latch" rating" which is a multiplier (usually 1.6 aysmmetrical or 2.7 peak)of the interrupting rating but I understand this a the breakers ability to close into a fault or ride through a downstream fault and not necessarily interrupt it?  Is this correct?

Thanks

RE: MV breaker interrupting multipliers for fault current DC offset

If the actual X/R ratio results in a power factor lower than the ANSI or IEC test power factor, the interrupting rating must be de-rated.  This is true for both MV and LV breakers.  Many electrical analysis programs can do this automagically.  And you also have to calculate the fault duty using the applicable standard - it is not a straight Ohm's Law calculation method.  

The close & latch or momentary rating is based on closing into a fault or withstanding a momentary fault, not interrupting it.  

 

RE: MV breaker interrupting multipliers for fault current DC offset

(OP)
Thanks dpc!

To determine the multiplying factor manually is this where you need to look at the curves that are published for the different cycle breakers  with the X/R ratio on one axis and the multiplying factor on the other axis?  You then find the curve relating to the cycle breaker you have and find where this point intersects with the system X/R value in order to determine the multiplying factor?  As you mentioned most software programs do this automatically I was just curious how to do it manually?  I assume these curves are all based of an equation(s) that accounts for amount of current remaining after DC decay?  

Does this multiplying factor apply for both total current breakers and symmetrical current breakers?

Also I was curious why total current breakers have their close and latch values listed as an aysmmetrical value rated at 1.6x the symettrical value while the newer symmetrical current breakers (k=1) have their close and latch value rated as a "peak" value at 2.7x the symmetrical current value?

RE: MV breaker interrupting multipliers for fault current DC offset

The testing standards have evolved over time.  ANSI C37 has some different wrinkles for the old total current breakers.  You need to refer to the ANSI standard on short circuit calcs and follow their cookbook recipe for each type of rating.  Conrad St. Pierre's book on SC calculations covers this in great detail - I highly recommend it.  

The methodology is complicated, but not really technically difficult.  It's more than can be covered in an Eng-Tips response, though.

For IEC-rated breakers, there is a completely different standard and calculation method.  This is covered in Conrad's book as well.  

 

RE: MV breaker interrupting multipliers for fault current DC offset

(OP)

Thanks dpc!

I have Conrad St. Pierre's book and agree that it is a very good book.  The questions I posed above came mostly from looking through chapters 3,5,7,&8 in the book.  I will have to go back and look at these chapters more closely and re-read them until the material sinks in.

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