Reducing available short circuit current

Hello all.

I recently discovered this forum and for the past few weeks been reading a lot of posts that pertain to arc flash that my company has started to get involved with. I have learned quite a bit from just reading the questions and answers from all you folks and find this a valuable place to gain more knowledge as we move forward with this work.

Starting off in electrical construction 30 years ago, moving into testing and maintenance 12 years ago, and now adding arc flash studies to our services, I have some catching up to do.

Lately we have been contracting out the engineering portion of the studies. We are gathering the information, the engineering firm is performing the modeling and reports, and we return to label the equipment,review the report with the customer, and assist them with instituting a safety plan including energized work permitting, etc.

I have been asked to consult and review the final product from the engineering firm. What I have noticed recently is a lack of close attention to reducing arc flash IE or any attempts at reducing the available SCC. It almost appears that these studies are being reported with "as found" conditions with some recommendations to coordinate certain areas of the distribution system, but no suggestions to reduce arc flash.

This is where I will be getting more involved. I understand that this follow up would be outside of our original scope with the engineering firms and it may also be that the firm is leaning excessively on the side of safety and system reliability. I have not had the opportunity to jump in as of yet but I am getting prepared.

A study was just completed last week for a regular customer of ours for 12 years that has recommedations to replace a substantial amount panel boards that do not meet the calculated available SCC. This facility is approximetely 15 years old and was originally engineered by a well known firm. No major electrical changes have taken place to my knowledge.

I do not want to point fingers at anyone but I feel something was done wrong and I am hoping with some help from the forum I can either validate or mitigate.

Thanks and I look forward to reading more posts. Great stuff here!

First, you need to find out how the present SCA was calculated.

Did they have new design criteria from the power company (many POCOs require equipment to be selected based on a worst, or design, case)?

Are there a large amount of motors in the facility that may be contributing additional fault current?

Was an actual short circuit study ever completed during the initial design, or where the equipment AIC levels chosen based on "this is what we always done'?

Remember that arc flash incident energy depends on both current and time, therefore lowering SCA can actually result in an increase in cal/cm².

The building is an 8 story office building. 480/277V bus riser up through stacked closets for power and lighting. Seperate bus riser for mechanical penthouse.

Typical mechanical penthouse, gas boiler, chiller unit with cooling tower and pumps.

We inventoried the job from scratch starting at the utility vault, transformers, switchboards, d-panels, branch panels, MCC's and motors. Generator and ATS's. Basically a complete inventory. Utility contributions were obtained.

The job was modeled using SKM.

I am unaware if an original SC study was done, I will look into that.

I am assuming the floors have Fan boxes with motors.

Thanks for the tip about short circuit vs incident energy. By this are you saying the less available current the longer an arc can sustain?

Performing the calcs from gathered data is the easy part. Knowing how to reduce IE is the hard part. This is usually going to mean increasing sensitivity of trip devices, so you have to know the facility well.

If the equipment is really old it may pay to upgrade it. If you want to keep it but reduce IE, look at installing:
Current limiting fuses;
In line reactors;
Changing out transformer to one with higher impedance.

You could contact the original engineering firm. They may have copies of a fault study that you could review. To review the current engineer's results, you would need to find out what program they used for the calculations, get their input data and compare it to your field gathered data. If the engineer used a computer program for the analysis, you can't really check the calculations, just the input. You might have to do you own analysis.

The problem with the panelboards not meeting the calculated SCC is not related to the arc flash hazard.

Lower fault currents can result in higher IE if the lower current results in longer interrupting times.

I would echo this sentiment. Just because it was originally designed by a "large well known firm" does not mean that they performed a full study at the time. They were very likely to just rely on experience in that area and go by other designs and "what we have always done."

jghrist, I know you know, but just to clearify for the OP.
If the equipment does have a lower rating then the available short circuit current, this could pose an increased hazard and make the likelyhood of an arc flash event occuring increase. I don't think that it would increase the arc flash energy, however, although it may increase the hazard due to flying debris as the breaker blows.

The thing to remember, as many have already pointed out, is that just because you lower the available current, does not mean that you lower the arc flash energy. Arc flash energy is proportional to available current times time, so you must look at both.

We see a LOT of this. We have done Arc flash surveys where most of the CB's in the facility are underrated for AIC. Perhaps the original contractor simply bought what was cheaper and was not aware of the consequences.
In any case, you can:
1) Replace a lot of CB's/Panelboards, very expensive but the best solution.
2) You can look into "Series Ratings". This works but I am not a big fan of this method because series ratings are easily modified out of existence.
3) You can look into re-feeding. For example, if you have a 400A panel that is only drawing say 90A. You could re-feed this at 125A with #1 copper. This might lower the SC value to an acceptable level but you would lose capacity that might be needed for future use.
4) Isolation transformers will work. However, you may reduce fault currents too much for good arc flash results.
5) Using SKM I have modeled line-reactors and the results look good. You can size the reactor to get the fault current below the AIC of the CB's but high enough to ensure fast trip times.
Utilities use reactors for this purpose and they are used to protect VFD's as well. However, I have never seen them used to reduce fault current for general power distribution and have not contacted any manufacturers to see if they recommend their products for this purpose.

Indeed, this is a problem with no easy fix.

Thanks all for the insight and suggestions for solutions. I will be working on this next week and the first order will be to try and get in touch with the previous engineer for the project to find out if a short circuit study was ever performed.

I will be back.