Overdutied Equipment

[wbd]

I have run into a situation where some power correction units are overdutied. The manufacturer has rated them at 10kA whereas the available fault current is around 29kA at 480V. I am thinking that I may be able to continue using these if current limiting fuses are installed upstream.

Looking for thoughts/opinions.

Thank you in advance.

 

[magoo2]

Power correction units? I'm guessing you mean shunt capacitors. Yes?

If you're fusing at the cap bank location then a CLF should take care of the high fault current concern.

If you are applying the fuses upstream, then you have to pick a rating that will handle the total load. In this case, the CLF rating may be too large to protect a downstream capacitor bank.

 

[wbd]

This is active power correction so it contains more than capacitors.

 

[dpc]

It will depend on the size of the fuse. From a strict code compliance viewpoint, I don't think it would be acceptable unless the PFC manufacturer had tested their equipment with an upstream CLF. Is the thing UL labeled? At the least, you would be assuming all of the responsibility/liability if you recommend this if not blessed by the PFC supplier.

 

[X49]

I can't speak to whether or not current limiting fuses are appropriate in this situation, but you could consider installing reactors upstream of the PFC units to limit the short circuit current.

Some automatic power factor correction units come with reactors that are intended to reduce harmonics, so I wouldn't expect that adding reactors upstream of your PFC units would affect their performance.

I would also check with the manufacturer to see if there are any solutions they would recommend.

 

[krisys]

From the description, it appears that the subject equipment is quite small. Try to increase the feeder cable length. You should be able to reduce the short circuit level to a safe level at 480V.

 

[wbd]

krisys - I would have to model it but I think it would have to be quite a bit of conductor added as the devices are rated at 10kA and available is near 29kA. Ok, did a quick model change and I would have to add about 400 feet of conductor to drop it close to 10kA using the present size of conductor. I don't think that will work.

 

[waross]

Two suggestions;
Did you use the resistance of the cable or the impedance tables? The voltage drop or impedance tables include cable reactance and will yield a shorter length.
Have you looked at the impedance of a higher voltage rated cable? Higher voltage cables tend to have a higher reactance per foot due to the greater conductor spacing as a result of the thicker insulation.
This probably won't get you down enough, but if a standard rating reactor needs just a little help, you may get if from longer cables.

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