I need some help in understanding and possibly mitigating an issue that I have seen in several locations. It seems that every time I have low fault current available from the utility, I always end up with a category 4 or worse on the secondary side / line side of the breaker downstream. I'm not really sure what causes this or how to mitigate the issue. Any help is appreciated.

Lower fault current often means longer clearing times of the OCPD. I fyou could be more specific maybe we can offer some mitigation solutions.

Hi Zog, I have a fault current available from the utility of only 349A 3-phase and 293A 1-phase. I have a 1000kVA xfmr, 12470/480V. It seems every time I have fault currents this low, the secondary of my xfmr is always category 4 or worse.

I assume those are fault currents for the 12470V side? Seems low either way, where are you getting those values?

Mitigation can be tough between the transformer and 480V main breaker, after that changing settings can be a good solution but your options may be limited for a small system like this. Remote operators may be your easiest solution.

What are you using for your max clearing time?

349A is barely enough current to energize the transformer?
Are you sure of your values?

Low short circuit currents look like inrush currents, so protective devices are usually in a 'time-delay' mode, resulting in long (i.e. multi seconds) clearing time.

Pay attention to your arcing fault current. Set your instantaneous trip below that value. If you don't then the trip times get excessive, and more importantly, you've disabled your instantaneous trip value in the first place. Note that this may wipe out coordination which is a separate (but interlinked) issue.

Any time you are on the secondary side of a transformer, as the only real protection comes from the upstream (primary side) device which is limited by the transformer itself, the protection is pretty weak at best.

Options to fix it:
1. Increase the impedance of the transformer, if possible. Note that without exercising this with a 480 VAC secondary, it is difficult at best to achieve <40 cal/cm^2 at or above around 2500 kVA.
2. Put a molded case circuit breaker either inside the transformer enclosure/air termination cabinet, or else bolted to it to minimize the affected area.
3. Put cable protectors on the transformer lugs. This is not what they are meant for, but they work. These are specialized fuses that are small enough that you can bolt them directly to the transformer lugs, reducing the unprotected high arc flash rated bus to just the transformer secondary coil and lugs themselves.
4. Give it up. Don't open the offending cabinetry without de-energizing first and do the de-energizing step on the transformer primary side.

Cable protectors

Could the previous poster please offer some more detail about the cable protectors? When I search, I find many protectors, designed for mechanical protection of cables. Would appreciate more detail about the electrical protectors described. Thanks.

I think they meant cable "limiters". These are like fuses that go on the individual cables making up large parallel runs. They are used a lot in utility underground networks. Let's say there are 6 underground cables per phase, rather than tripping the breaker off line in an underground network for a fault on just 1 conductor, the "limiter" takes just the one conductor out allowing the other 5 to maintain service continuity.

Some good ideas Paul!

Found the cable limiters

Hello, I found the "cable limiters" from 3 major USA suppliers, rated up to 600 V. Good for North American usage. The devices are interesting, not amp rated, rated by interrupt and cable size only.

I've requested a TCC so I better understand the behavior.

To any european readers, are similar devices available with 690V ratings?

Yes, they are rated according to cable size. They are not similar to fuses...they ARE fuses, with a definite application. But, IEEE 1584 is not sensitive to what specific overcurrent protection device you select.

As to 690 V ratings, I would try calling or emailing the fuse manufacturers and asking. Going between jurisdictions is always a challenge, but given that two of the manufacturers you probably located are Bussmann and Ferraz Shawmut that market to both U.S. and European customers, I'm sure you just have to ask for the "European rating sheets". Certain concepts just don't translate well in power distribution. For instance high resistance grounding and ring main busses are rare in the U.S. but common in many European countries.

Hi, I'm not sure if cable limiters will help you here. They are designed to disconnect large fault currents for parallel conductors to prevent cable damage. The problem is you do not have high fault currents in your example, because if you did the problem would solve itself. You need lots of current to make current operated devices work rapidly unfortunately.

The other thing about faults on cables in a parallel configuration is that the fault is likely to be to ground (earth) unless at the terminations. The most likely place for an arc flash will probably be the terminations and in line fuses actually will increase the chances of an arc flash initiation in my view because of the additional metal, joints etc.

From the very low utility currents mentioned it sounds as though the equipment that you install is in remote locations. Perhaps you install phone masts or something. I would love to know more as I have made a few assumptions in my answer.