Fuses and Let-Thru Current 2

[unclebob]

Hi all!

I have been asked to do short-circuit and Arc Flash studies for an hosptital...But, the consultant does't want your ordinary short-circuit calculations. He wants the study to show the current-limiting effect of the fuses on the distribution grid. So I had to find the Peak Let-Thru current, at the RMS fault current calculated, for all the fuses(the Op-and-Over method). And then, apply these values in calculations to account for the current-limiting effect.

Fuses are A4BQ's, and AJT's.

Of course, SKM software calculates fault current, but does not account for the current-limiting effect. There must be a reason for this?

I'm wondering if this is a good thing or not.

Thanks

 

[davidbeach]

There is a really good reason that SKM doesn't account for the current limiting effects. Those effects do not apply in the presence of circuit breakers in the circuit. As a circuit breaker starts to open, and than can happen within 1 or 2 ms of the initial fault, the fault impedance as seen by the fuse changes dynamically and that changes the operating time characteristics of the fuse.

Don't go there. But what you can do with SKM is add information about series ratings based on the fuse/breaker combinations. Tables of series ratings are available from all of the breaker manufacturers. With the series rating information included, the equipment rating module will include that information in its report.

 

[Skogsgurra]

That is fast! It was ment to be 1 to 2 cycles? But then, a current limiting fuse should already have tripped and arcing is probably also over.

Are there breakers that actually can break in 1 or 2 ms from fault start?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

Yes, the motion begins that soon and with the motion the dynamic change in circuit impedance. Opening may not occur for a while longer. Current limiting circuit breakers utilize a magnetic "blow-apart" design and will open and clear in less than 4 ms and nearly all non-current limiting breakers begin to have contact motion as soon as the current exceeds the operating current of the magnetic instantaneous element. That is why it is impossible to calculate meaningful series ratings but rather they must be determined by testing.

 

[Skogsgurra]

Thanks! Yes I remember that picture in the AEG handbook. I think they were the people that pioneered this kind of breakers. At least, that is what they say.

But we all know it was the Russians

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

I agree with David Beach's comment. Just want to add that in IEEE 1584 standard on arc-flash calculations, they do have methods for taking CLF into account for arc-flash calcs. I believe EasyPower has an option for this - not sure about SKM. But this only applies to arc-flash calcs, not general short circuit calcs, snd only if the fuse is in the current-limiting range.

Another important thing to bear in mind is that the CLF has to clear the fault within 1/2 cycle to provide any current-limiting. If the fault clears more slowly than that, based on the fuse curve, it is NOT current-limiting.

 

[davidbeach]

SKM does use the high speed clearing of CL fuses for arc-flash calculations, but that does not affect the fault current, only the clearing time.

 

[DanDel]

Per IEEE 242-1986, current-limiting devices should not be used in short circuit calculations.

The actual wording, in Section 2.3.6(page 78) states: "In industrial and commercial power systems, it is common practice to use current-limiting devices to protect other devices against high values of short circuit current, which exceed their short-circuit ratings. Short-circuit calculations should be made without considering the effects of current-limiting devices to determine the “prospective” short circuit current, or that which would flow if no current-limiting device were present. This “prospective” short circuit current is used in applying the devices. The manufacturer of the device to be protected should be consulted to determine the suitability of the application for the level of short-circuit current available at the line-side terminals of the protected device. At this time the best practice is to determine the suitability of various applications by performance testing in accordance with recognized test procedures of qualifying organizations, such as Underwriters Laboratories. No general agreement has yet been reached on how to use the I2t and peak current let-through values of a current-limiting device to establish its capability to protect another device.
It should be noted that a current-limiting device cannot be used to protect another current limiting device. Also, current-limiting devices in series do not work together to compound the current-limiting effect. The faster acting of the two, which is usually the one to be protected, will try to interrupt the full short-circuit current before the other device is able to operate.
".

 

[advidana]

I may be wrong But series rated devices just began to come out after 1986.