Understanding Current Limit Fuses and let through current 

Glad you found it helpful

Hi Zen, yes the fault will be isolated after approx 0.01 seconds. Prior to that it'll let through 22k in my example. It may not have been the best example as the time is so brief... as another imaginary example, let's say there was 25kA of fault current and that some current limiting fuse had a let through of 10KA and a trip at 0.3 seconds then you can see how the let through is more useful as the downstream device will have to withstand many cycles under a fault before the fuse will trip. Hope that helps!

Hi Jonathan, Is my understanding correct that Let through limiting fuse is not required to blow its own to get characteristic of limiting current to downstream? Just current pass through it and it already limited current to downstream without blown itself?

Glad you found it helpful. Yes, RK1 & RK5 fuses follow the same concept.

If the current dropped to 800A before it tripped then nothing should happen at 18 seconds. The fuse should theoretically trip at 800A but over a longer period. As the trip is thermal (melt) the heating up at 3000A for say 17 seconds would be relevant with respect to the trip point if dropped to 800A. If it dropped to say 799 Amps at 17 seconds then it should never trip, in theory. Circuits are not supposed to be more then 80% loaded so a properly designed circuit with an 800A fuse shouldn't have more then 640A of constant draw.

Thanks for you comment. Certainly some standards so however NEC (2017) focuses on the symmetrical rating. See article 100 Definitions for Short Circuit Current Rating.

@@graceyj20 How to evaluate this? I have a scenario where using current limiting fuses would normally consider the symmetrical RMS even major fuse manufacturers will highlight the consideration of the sym. RMS not the peak let through. But say a client will want to require the consideration be in "peak let thru" and even highlight SB4.3.3 as a reference. so clearly this is a battle of consideration between the one with "peak" and those with "rms" please advise. I am so confuse right now.

@@zidanetribal2343 Typically NFPA 70 (NEC) requires all components to be rated for the symmetrical fault current they may be exposed to. There are exceptions though such as manufacturer approved series rated devices. Other standards have different requirements and methodologies. NRTL's such as UL also approve manufactured assemblies where the assembly provides an integrated designed unit that has undergone independent testing and has specific operational guidelines. Ultimately it comes down to the details of the circumstance, engineering judgement and what the authority having jurisdiction will permit. With respect to safety and liability, one can never go wrong with a fully rated unit.

@@graceyj20 I see, so it goes down on where and what a scenario will require like in all standards, if say company A will require compliance with standard X then by all means we will comply with standard X and also if company B will require adherence to standard Y then we should follow. Thank you so much.

You should contact the manufacturer if their data sheet isn't properly marked.

There's a few things to consider, first most standard overcurrent protection device have to let through more current than it's rated for for a brief time to handle inrush current (motor, transformer, etc). Past that, yes it's ideal not to let through any fault current I'm not aware of any mass market protective devices that does this. "Instant" trips always have a minor melt time, latching time, etc so during that time there will be some level of fault current. Some fuses and circuit breakers specialize in minimizing let through current. Past that, the downstream devices need to be rated to carry whatever potential fault current they may be briefly exposed to. Protective devices should be rated to carry full fault current (not let through) unless they're in a manufacturer approved series combination.

That depends on the protective device characteristics and the short circuit current sources throughout the system. That said, if you're looking to reduce for a 10KAIC rated MCB you should confirm with the MCB manufacturer that the make/model of the fuse your planing to use is recognized for series rating with that specific MCB.

The conductor length/size/properties reduces the fault current from the source to the fuse due to it's impedance. Calculating the available fault current is not covered in this video.

@@graceyj20 I was binging and just put the comment on the current vid I was on. Thanks