Good day all and thanks for welcoming me to the forum.

Our company recently got a copy of the new IEEE 1584 2018 and we are required to do an arc flash study for an oil refinery.

One of the important parameters when doing an arc flash study is the enclosure size of the switching units.But most MCC Panels have motor drives with different sizes, which means the motor starter buckets are in differently sized enclosures on the same panel.How would you factor in enclosure size in such a case? Would you work out the arc flash based on the largest enclosure or do calculations on all enclosure sizes/each bucket and see worst case scenario ?

Please assist as I am a bit confused here.Your help is greatly appreciated.

Regards

Ash

Welcome to the Forum!

First off, since this is your 1st arc flash study it would best to have someone experienced mentor you on it or better yet just do the study since it mostly will be a one time study with periodic updates. Do you have any software to do the study with as the software, training on it and annual maintenance fees add up and would be better put towards having a consultant do the study?

Your question shows your inexperience in performing a study and it is good you are asking questions but there are many nuances to performing a study. From your question, it appears that you are under the impression that you need to analyze each MCC bucket. This is not the case. The MCC bucket will be the same incident energy value as the MCC.

Thanks a lot for responding.My question came from the fact that there is a stage , according to the IEEE 1584,where you have to compute the enclosure size correction factor and you need to have the actual dimensions of the enclosure.So since an MCC panel may have different enclosures with different sizes, which one then would you use? Apologies if I might be going off a tangent

Since the incident energy is going to be the same for the entire MCC the enclosure size would be the MCC dimensions.

Do you understand why you do not need to analyze each cubicle?

What software are you using to model and analyze your system?

I understand now.I'm busy putting together excell spreadsheets based on the IEEE 1584- 2018 equations.Then later on ,I plan to model the system in ETAP.

Why are you using spreadsheets if you have ETAP? You are just making more work for yourself as you will need the specific fault currents to input into the spreadsheets which would have to be calculated.

Did you obtain the available fault current from the utility as well as primary protective device, riser cable, transformer data? Be careful as most likely the first time you contact them it will a customer service person providing bolted fault current on txf sec based on infinite bus.

All that data we have , network fault currents , cable sizes, transformer data , As-built single line diagrams , protection settings of switch gear (including characteristic current curves) etc.Those were the first things we requested from the client.We have an old version of ETAP (2016) ,which doesn't have the latest 2018 arc flash model.

I would make sure that you were provided with the latest available utility fault current. I contact the utility directly for that information.

It appears you are a consultant but this is your first arc flash study?

We got the fault levels from the municipal supply authority.I am a consultant , junior/mid level.The senior engineer will look at it.Im just giving myself a go while he's still away (on a project site out of the country).

However, I have recommended the company to get the latest version of ETAP as this will make our work very easier

I had a conversation with another engineer just now. Obviously, one of the most important steps before doing an arc flash study is getting the network fault levels from the energy utility.This engineer also mentioned that where the network fault levels are unknown, you can use the transformer rating and impedance to work out the max worst case 3 phase fault level.But I mentioned that this can result in 'over design/specification.'

Also on the other hand, its good to use the actual fault levels from the utility but the fault levels can change when the utility changes/upgrades their infrastructure e.g HV/MV transformers.What's your take on this?

My opinion is that it is best to use the available fault currents and other information from the utility. It has been my experience (and I worked in utilities for 32 yrs) that the systems do not change as much as they will lead you to believe in the legal disclaimer with the fault current levels. But this is one reason why the study needs to be reviewed every 5 years as you will request new fault current levels from the utility.

Many many thanks Barry for such valuable insight.

At my utility, the upgrade planning is constant. You don't need to wait for an upgrade, though, to see source impedance change. System reconfigurations occur daily to deal with outages and maintenance. Redundancy is built in for reliability, but ensures fault current levels are variable.

I see.Will do the arc flash study as per the existing network fault levels

Spreadsheets are not of much use, except as checklists.

You can build your model in your old ETAP and then open it in the newer version. Yes, you may have to make adjustments to the electrode type, and maybe enclosure size, but that is much easier to do than entering all of your data into two different software packages.

I might have misunderstood a previous response regarding the sizes of the MCC cubicles and that you do not need to calculate the AFIE on each of the. I disagree.

I measure the size of EACH cubicle, including the income section and main breaker. I get the electrode configuration of each of them. Then I calculate the AFIE at each of these cubicles and I choose the worst case incident of all of them for the MCC label.

If you believe I am wrong, can you please explain why?

We calculate the AF energy of a MCC using the smallest cubicle (bucket). We will then label the entire MCC using this energy value. The smaller the bucket, the higher the energy level.

What about the electrode configuration. It could be that a larger cubicle with a different electrode configuration than the smaller has higher AFIE.

For most LV MCCs, we have defaulted to use VCCB for the electrode configuration. FOr MV MCCs, we default to HCB.

Also, AF studies are extremely expensive to perform. The majority of our clients do not want to pay for use to calculate the AF energy for each cubicle.

Generally I would assume the intent is to ensure the calculation is reasonably conservative.

For buckets I would suggest VCBB. Protection afforded by the OCPD in the bucket is of little importance as the higher energy will, in theory, occur at the line side of the OCPD which is why the VCBB calculation.

I would suggest the smallest enclosure to ensure you have all enclosures covered. As was mentioned previously, I do nto see value in calculating for each bucket size.

At the main or incoming section you need to ensure there is no HCB exposure which would generate higher Ei and hence would need a dedicated calculation..

And if the MCC has a main and a remote feeder with different settings then you may need to calculate the lien side of the main. However, that is rare in my experience.

The net effect is that when a worker opens a panel, the Ei he is prepared for is the highest he is exposed to. Usually that is at the line side of whatever OCPD is in front or near the worker.

As far as fault current values to use... I am sure this is not popular to those that must do studies for a fixed fee, but in my opinion one should consider a range of fault currents, from highest possible too lowest possible. Interestingly, Etap is launching, or has launched a module that allows calculating Ei over a range of variables, i.e. input variables with tolerance. I think this may prove to be particularly valuable!

And, last, if you are not sure about gap, use a larger one. A larger gap is conservative from the perspective on Iarc (lower) and Ei (higher).