I have been putting this discussion off until more people have had time to digest the new electrode configurations.

There seems to be confusion and uncertainty regarding which configuration to use as part of the arc flash study i.e.

VCB - Vertical Conductor in a Metal Box (also in 2002 edition)
VCBB - Vertical Conductor in a Metal Box Terminating in an Insulating Barrier (new)
HCB - Horizontal Conductors in a Metal Box (new)
VOA - Vertical Conductors in Open Air (also in 2002 edition)
HOA - Horizontal Conductors in Open Air (new)

2018 IEEE 1584 provides a couple of examples but guidance is limited.

One thing that I have been saying is it feels like 2002 all over again. What does that statement mean? Back in 2002 there was a lot of uncertainty regarding arc flash studies – the IEEE standard and software were new concepts. Do you enter actual gaps, do you really use the 2 second cut off (initially - many people did not) etc. etc. Then over time, people became more comfortable with their strategy. It seems we are facing that again and it will take time to sort out.

With that long intro, this week’s question is about addressing the many electrode configurations. In the wake of the initial uncertainty, some may stick with VCB/VOA for now which is what the 2002 edition was based on. Some may go for the configuration that provides the maximum incident energy – which may create protection issues when PPE suddenly goes from 8 or 12 cal/cm2 to something much greater. Some may analyze the actual equipment electrode and conductor configuration. Others may "wait and see"

One thing to note, the highest incident energy may not always be from the same type of configuration. i.e. there is not a single default configuration that always results in the highest incident energy. The arcing current changes among configurations and that could affect the protective device operation / arc duration.

Some software permits using multiple configurations and uses the maximum incident energy. Also, I understand some software defaults to parameters similar to the 2002 edition i.e. VCB/VOA

So, at this early stage (like 2002 all over again) this will take some time to sort out and get comfortable with.

Also, please provide your thoughts, opinions etc. – this is great time to mix it up!

Here is this week’s question.

How will you/consultant/company address electrode configurations
Use VCB/VOA for now - same as 2002 edition
Review actual equipment bus/conductor configuration and decide
Use the configuration that provides highest incident energy
Something else
Not sure yet

PLEASE provide comments, thoughts etc.

You forgot "Barrier" in your VCBB description.

In any case, I'm trying to use what I think is the proper configuration. Sometimes that is easy. Sometimes I'm just not sure. I do know this, it is taking me a bit longer to enter/edit this data along with dimensions. I'm not used to it yet but like anything else, one day it will become just how it's done and the "old way" will be forgotten.

Oops. Thanks, corrected it.

I still struggle with the concept of a "barrier". I've seen the tests. Most of them have basically a solid block of something (ceramic) clamped around the bus bars. I could be inclined to agree that this is representative for instance of terminating into the top of a breaker or a contactor. But then some of the tests on real equipment are simply clips holding vertical bus bars which in my mind is a fairly open structure. Still others would be for instance when you have glastic and/or rubber grommet type "seals" between compartments around either cable or bus bars. I'm even thinking for instance when it comes to CT's in medium voltage where trying to center the cables in the CT windows (or prevent tracking issues with unshielded cables) where the cables are inserted through glastic sheets with holes cut through them as both support and spacer for the cable. Some of these would be considered barriers and would cause the arc to stop and expand outward at that point while with others the arc passes through. Most importantly this determines though whether to use VCB or VCBB. So what constitutes a substantial enough barrier to trigger the increased incident energy in a VCBB configuration vs. the much less dangerous VCB configuration?

There is some guidance in IEEE 1584-2018, Section 6.6 and Annex C. It's not highly detailed, but gives you ways to think of various equipment. For instance, looking at an enclosed switch, it could be VCB (blades of the switch are normally vertical), VCBB (where the lugs meet the feeder conductors), or HCB (where fuse clips are). Annex C states VCBB is likely the best configuration for an enclosed switch.

Jim,

If you had to give a percentage, what % of the tests are done with actual equipment (panelboards, switchboards, disconnects, enclosed breakers etc.) and what are done with lab-created electrodes in a box?

Thanks.

We are creating a cheat sheet for our staff that includes various types (17 different types) of electrical equipment. We then list the typical worst case electrode configuration for each equipment type. We also include default dimensions in case the new equipment submittals do not list the dimensions. Our goal is to be as accurate as possible without spending a tremendous amount of time determining the electrode configuration or running multiple electrode configuration calculations.

Sounds logical and reasonable to me.

Robert,

Described approach looks reasonable.

Can you please list here what are the 17 different equipment type you have included in your list and assigned which worst case electrode configuration for each equipment type?

I am not sure this is being properly interpreted. My (poor) sketch shows what I believe is the test set up for VCBB on the top of the diagram. It appears that many believe the line side of a main circuit breaker is VCBB. For the test, the electrodes terminate on a flat surface with no obstruction in between the phases so the plasma of all three phases is in one plane.

For a circuit breaker there are barriers between the incoming lugs - bottom of sketch. Wouldn't the insulating barriers possibly stop the plasma of all three phases from connecting and escalating? I really question where VCBB conditions (as the test was performed) actually exists. It seems we are looking at two different things, a flat surface and a surface with barriers in between phases.

100%. None of the IEEE 1584 tests that are used for calibration of the model include actual equipment.

Verification and exploratory tests use actual equipment or mock ups. For instance barrier tests came into being specifically because of mock up/actual equipment tests. This article from Mersen summarizes the tests that they did, that were the basis for the reason that the barrier tests were done:
https://ep-ca.mersen.com/fileadmin/cata ... -Paper.pdf

One problem with using actual equipment is that the stuff in the cabinet changes the result, often quite dramatically. The example of a main breaker in an MCC with little to nothing else within the cabinet is one example where incident energy goes up compared to vertical electrodes hanging in an empty box (VCB). But by the time you include all the little parts that are stuffed into a typical MCC bucket they are energy absorbers and reflectors and decrease incident energy. In the case of MCC buckets in particular where the worst case incident energy is represented by arcing across the bus behind the buckets, it is quite a bit lower.

For comparison look at the free EPRI arc flash tests on their web site. They do actual equipment tests instead of lab models like IEEE 1584. The results are dramatically lower than that predcted by IEEE 1584. See for instance this article describing the overall results which is very different (drastically lower in most cases) from IEEE 1584 calculations:
http://distributionhandbook.com/papers/ ... t_2012.pdf

I can't find the article but one of the experiments that was published referring to various barrier configurations considered that in some switchgear there are very extensive sheets of glastic in place usually called phase barriers that all but make each individual phase it's own box. These aren't the shallow finger-safe boxes in your sketch but something much more substantial in size. The hypothesis was that this should prevent a 3 phase arc from forming and that we'd be looking at single phase cases, and that from a practical point of view it would be really easy to retrofit most equipment with phase barriers. Equipment with phase barriers is built specifically on the idea that it makes a big difference. Or at least that it would delay the onset of 3 phase arcing faults to the point where maybe we could try to stop a true 3 phase arcing fault from occurring and reap the rewards of reduced energy. At the time it was also believed that single phase arcs would have far less incident energy (divide by 3 was suggested in a lot of sources).

However the test results showed something far different: First off even when the test was modified so that only a single phase arc could occur, the incident energy was not much different from a 3 phase arcing fault. So the idea of single phase incident energy being dramatically lower is invalidated.

Second the phase barriers didn't make much difference at all. Once the super heated air expanded and spread throughout the enclosure area, a 3 phase arcing fault ensued. This occurred at roughly 1 to 2 cycles into an arcing fault both with and without phase barriers. They didn't make much difference at all in most cases.

So to answer your question and sorry that I can't give a reference but phase barriers don't make much difference at all. So the little short finger-safe type barriers you are referring to are even less of a barrier and won't make any difference at all.

Hopefully someone can chime in on the article I tried to find. I think it was early work done at Mersen or maybe Schneider but I can't find it right now.

The tests that were used for developing the equations/model were all done with the lab-created electrodes in a box/air.

I have attached the spreadsheet that lists different types of electrical equipment, the best type of IEEE-1584 Equipment Type, default gaps, electrode configuration, and box dimensions. This can be used with you have limited information about the equipment or can not remove equipment covers.

[quote="Robertefuhr"]I have attached the spreadsheet that lists different types of electrical equipment, the best type of IEEE-1584 Equipment Type, default gaps, electrode configuration, and box dimensions. This can be used with you have limited information about the equipment or can not remove equipment covers.[/quote]

Thank you I look forward to reviewing and comprehending.

thank you Robert, I was thinking about creating something similar.

I have a study re-validation coming up but we are not allowed to open any equipment and cannot verify configuration and cannot take measurements. How do you account for these situations?

We did a study comparing the different bus configurations and found the horizontal bus (HCB) is typically twice the incident energy of vertical bus. This sort of makes sense since the bus is pointed at you.

For most MCC's, it seems the horizontal bus configuration exists in nearly every bucket in one way or another.

So the question remains - use the default configuration?, Use the worst case?, or Open everything up and report exactly what is in the equipment?



PS. might as well measure the bus gap while you are there!? - bit of sarcasm but my point is most study's will not go to that level of detail.

What's is the electrode configuration for a MV Switchgear at the fuse holder? HCB? VCB? or VCBB?