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Grading between Upstream Breaker and Downstream Fuse

Grading between Upstream Breaker and Downstream Fuse

Grading between Upstream Breaker and Downstream Fuse

(OP)
Grading between the Instantaneous element of an upstream non-current limiting breaker,
and downstream current limiting fuse is described in Pages 6 and 7 of the Ferraz Shawmut publication at the website :
http://ep-us.mersen.com/resources/media/articles/SC1-SelectiveCoordination.pdf
A brief summary is as follows :
Data required :
•    Prospective symmetrical rms short-circuit current on load-side of fuse.
•    Fuse characteristic curves showing peak let-thru current as a function of symmetrical rms short-circuit current.
•    Minus current tolerance of instantaneous element of upstream breaker.
( Upstream breaker Inst element is calibrated in rms current )
Method to determine Setting of upstream Instantaneous Element
•    Determine max peak let-thru current for a max fault on load-side of fuse.
•    Use fuse characteristic curves to determine peak let-thru current.
•    Set the upstream Inst element above  the fuse peak let-thru current divided by sqrt(2).
Take into account the Inst pickup current minus tolerance and the extra load seen by the upstream breaker due to other loads on the board.
Are there comments on the validity of the above method ?
ie. The "Up-and-Over" method for fuse peak let-thru current given the prospective symmetrical rms current on the fuse supply side,
The "Up-Over-and-Down" method to determine the fuse equivalent let-thru rms current given the prospective symmetrical rms current on the fuse supply side.
 

RE: Grading between Upstream Breaker and Downstream Fuse

The up-over-down method is not allowed to be used for purposes of evaluating downstream breaker interrupting ratings.


My method of setting the upstream relay's instantaneous element would not be influenced by the presence of the downstream fuse.  The only way the fuse limits current is by melting.  It has no magical current-limiting capabilities.  I would set the instantaneous above both:

Maximum expected inrush current
Maximum secondary fault current (symmetrical or asymmetrical depending on the primary relay)

Since most modern digital relays have dc offset filtering, the asymmetrical current can pretty much be ignored. Also, the digital processing does introduce a slight delay in the instantaneous element and these are generally actually slower than the old electromechanical trip units.   

RE: Grading between Upstream Breaker and Downstream Fuse

dpc,

I don't think Eddy is asking about evaluating downstream breaker interrupting ratings.  He has described the situation where he wants to ensure the fuse clears before the upstream breaker commits to trip.  Assume interrupting ratings are adequate for location.

Eddy - my two cents.  I hadn't seen the method you describe before, but have tried to track down a method before.  The above method hangs off the premise that a non-FCL breaker will only trip if it sees the peak equivalent of the rated (minus tolerance) instantaneous trip current - in the sub-cycle time zone.  If the downstream fuses clears the fault before it sees this, then the breaker won't trip.  Provided you use sqrt(2) and not 2.3 etc. to determine peak asym. current, this sounds reasonable but I don't know if it is that cut and dried - seems a bit too easy to me.

You might also like to check the schneider document (Complementary Technical Information 2010) lvped308005en, page 71 onwards.  It is more for FCL breakers upstream, but says you need to check time-current curve and energy let-through curve and has tables as well.  Obviously you really need the tables if the upstream breaker is FCL type.

RE: Grading between Upstream Breaker and Downstream Fuse

healyx,

Yes, I understand what he was asking - please read my entire response.  

I just wanted to point out the "up-over-down" method cannot be used for purpose of determining breaker interrupting duties in case he was also trying to use it for that as well.

 

RE: Grading between Upstream Breaker and Downstream Fuse

(OP)
Thank you dpc and healyx for your helpful posts.
As healyx stated "The above method hangs off the premise that a non-FCL breaker will only trip if it sees the peak equivalent of the rated (minus tolerance) instantaneous trip current - in the sub-cycle time zone. If the downstream fuses clears the fault before it sees this, then the breaker won't trip. Provided you use sqrt(2) and not 2.3 etc. to determine peak asym. Current"
This assumes the ACB responds to the peak asymmetrical current ( including d.c. offset ) and the breaker Inst trip setting is calibrated in rms, but responds to peak current where peak current equals sqrt(2)*rms current.   This  would need confirmation from breaker manufacturers.
The other assumption is that the published  fuse let-thru peak current vs initial symmetrical rms current is based on a X/R ratio similar the the network involved.
My fuse catalog shows fuses were tested at an X/R of 5.55.
The 2.3 in your posting healx corresponds to a X/R of  6.25.
( I"k asm rms = I"k ( 1 + 2 e^-2pi R/X ) ^0.5 )
The X/R can be much higher closer to large transformers and generators.

However in my application, the upstream breaker is shunt-tripped by a Micom P145 overcurrent relay. The attached file shows the P145 relay has Fourier filtering which filters all harmonics ( except inter-harmonics ) up to the 22nd and responds to the  fundamental.  Therefore we need to know the equivalent symmetrical  rms current "let-thru" by the fuse operating in its current limiting region.
Could the "Up-Over-and-Down" method be used to determine the equivalent symmetrical  rms current "let-thru" by the fuse operating in its current limiting region ?
We could then use the following to determine a P145 short-time pickup and short-time time delay to avoid tripping due to a fault on the loadside of a downstream fuse :
1.    Fuse total let thru  I^2t  at the max short-circuit current downstream from the largest outgoing fuse ( from fuse catalogs ).
2.    Determine the equivalent rms current let thru by the fuse ( up-over-and-down ),
3.    Use the equivalent let thru rms current to determine the I^2 t seen by the breaker.
4.    Set the breaker Short-Time Pickup current and Short-time time delay so that the breaker trip I^2 t is greater than the fuse total I^2t in its current limiting region.
 

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