There has been some discussion regarding maximum available fault currents from the electric utility on low voltage urban networks i.e. 208, 400, 480 volts (other locations may have this too).

Currents can reach upwards of 200,000 Amps due to systems with spot networks and paralleled transformers.

This week’s question:

What is the maximum fault current you/company/client have encountered?

Less than 100 kA
100 to 150 kA
150 to 200 kA
>200 kA

But wow...most equipment AIC's rarely get above 100 kA. Maybe you can do something with line reactors but every time I look at that scenario it seems like a loser because it also causes voltage instability. What good does it do to have that much fault current?

It is mostly a "side effect" of utility underground network system's in urban areas. It doesn't even need to be Los Angeles or New York. Back in my utility days, the underground network for Akron, Ohio was somewhere in the neighborhood of 150 KA.

Downtown networks have "spot networks" where there may be 3 or 4 matched transformers connected in parallel to a common secondary bus (collector bus). Each primary normally comes from a different source. That way if there is a loss of one transformer or one of the primary sources, the network protectors trip that line/transformer out and the remaining transformers continue to carry the load.

The main issue is if there are let's say, 4 - 1500 kVA transformers in parallel on a collector, the secondary short circuit current for each might be around 30 to 35 kA. Add 4 together and the secondary fault current easily goes beyond 100 kA (although it does not directly but will be close) I had one case years ago in downtown Washington, DC where there were 4 - 2000 kVA transformers and the fault current was around 150-160 kA

Fusible switchboards are often used in these cases with fuse/equipment ratings reaching 200 kA for the service equipment. Then if properly selected series ratings are used with fuses and circuit breakers, the circuit breaker panels etc. downstream can have a more modest (normal) rating as they are protected by the upstream current limiting fuses.

Although more reliable, downtown networks can often cause interesting issues.

Have never had more than 56 K of available fault current. That was on a 208 volt system with a single utility pad mount transformer.

What is the physical separation distance of the transformers? Cabling between them will limit AIC substantially.

In the case of a spot network it is negligible. The 3 or 4 transformers are connected directly to the collector bus for an "electrically tight" network.

Los Angeles LADWP standard for 3750kVA xfmr is 62,500
Sadly this kills 65KAIC withstand rating by the time building contribution is added!

Is there a test lab that even gets above 100 kA? Not sure how anyone would even be able to evaluate equipment built to 200 kA AIC for certification purposes. As I recall one of the big limitations on AIC is actually being able to test it.

That is a tough one. I just had a conversation about an hour ago about that very question. It seem some "modeling" is involved with this. I didn't probe any further - reading between the lines. About the only thing I know above 100 kA are current limiting devices. They don't actually interrupt 200 kA, instead they are located where then can be a prospective fault current of 200 kA and then they operate between 1/4 and 1/2 cycle so the 200 kA never really materialized.

Many of the labs where we used to be able to head up to 100 kA have throttled back due to some damage they have encountered with the generators at higher fault currents.

OK...so we know what the fuse does so we just scale up the current limiting. I have worked with some Powell 600 V class breakers that had 100 kA ratings before by using backing fuses to raise their 65 kA rating to 100 kA. But I knew that 100 kA is pretty much the limit from any lab out there. The good news is you can say 200 kA and nobody can actually verify!