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 Post subject: Falshtables
PostPosted: Wed Aug 20, 2014 6:06 pm 
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Joined: Mon Apr 14, 2008 7:10 am
Posts: 142
Have you ever heard about flashtables? Is it legitimate?


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 Post subject: Re: Falshtables
PostPosted: Wed Aug 20, 2014 10:32 pm 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
I do not know what you mean with flashtables. I have heard of a rough Dutch translation of the word.

On ships there are various operating conditions such as: Full speed ahead, standby in harbor and emergency. There are also many task specific conditions depending on the class of the ship such as: heavy towing, oil recovery, firefighting. All these conditions have different amounts of generated power, different loads and they can all be on the same ship. Then there is also a difference between summer and winter and bustiebreaker on main switchboard closed or opened.

The ‘flashtable’ I have heard about is that the incident energy and FPB are calculated for these conditions. So besides a sticker with the worst case incident energy they also use a table with the incident energy’s of the operating conditions. Full speed ahead, bustiebreaker closed is the worst case scenario if an arc flash was to happen. But why would an engineer want to know this energy if he performs work during the operating condition standby in harbor.

You do get ‘funny’ results were for example a condition with 6 generators online means highest incident energy but a condition with 1 generator online gives an higher incident energy then 3 generators online. I have had some fun conversations with people about this.


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 Post subject: Re: Falshtables
PostPosted: Thu Aug 21, 2014 6:00 am 
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Joined: Mon Apr 14, 2008 7:10 am
Posts: 142
Thanks for the reply, this is their website I have just recently found.

www.arcflashtables.com


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 Post subject: Re: Falshtables
PostPosted: Thu Aug 21, 2014 9:37 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Noah wrote:
Thanks for the reply, this is their website I have just recently found.

http://www.arcflashtables.com


The article seems to have disappeared since conversion to the new web site and I believe that Jim is going to put out a revised version, but this is probably similar to a simplified calculation procedure.

The IEEE 1584 incident energy calculation is driven by 4 variables:
1. System voltage.
2. Bolted fault current.
3. Arcing time.
4. Grounded or ungrounded.
5. The equipment type (switchgear, MCC, panelboard, etc). There are effectively 4 "classes" of equipment even though the table has more entries.
6. Sometimes, gap although this quite often is set by equipment type.

Out of these, we can safely say that factors 1, 4, 5, and 6 are generally "constants" or that we have a small number of standard cases. That leaves items 2 and 3 to work with.

Now, the standard approach is to calculate everything and then determine the incident energy and thus the PPE required.

Working it backwards, we start with PPE which givens an incident energy. Then if we produce a table for say "all 480 V MCC's", if we use some arbitrary maximum trip times such as 5/10/20/30/60/120 cyccles, we can back-calculate the maximum fault current possible for a given arcing time and incident energy. Or conversely we can set the maximum available fault current and calculate a maximum trip time.

This results in a considerably faster method for calculating incident energy, one that can easily be done "by hand". At least at first blush this appears to be the case. And getting a short circuit value, especially a conservative one, is extremely simple to do. For instance with a rough guess at X/R ratio and transformer kVA and %Z, one can assume wiring has zero impedance and calculate a worst case short circuit current. If there are motors involved, some small adjustments to the value are eeasily accomplished. This takes less than 5 minutes to do by hand and I have personally used this method (basically the ANSI short circuit method) for years.

But, there's a huge problem with this method. Quite often, incident energy INCREASES as available fault current (arcing current) DECREASES. I know that this sounds absolutely backwards. After all, incident energy is approximately related to power so a decrease in current would result in a decrease in power. However this misses the bigger picture issue. Inverse time devices (breakers, fuses) take longer to trip as current is reduced. This effect is definitely not linear in nature. So quite often the rate that the arcing power (arcing current) is decreasing is far less than the rate at which the arcing time is increasing.

Note also that this is not an absolute rule. There are times, especially with long cable lengths, that the expected thing happens and reductions in current result in reductions in incident energy. But the majority of the time, this is not true.

So notice the trap here. The simplified approach results in much higher arc currents and thus fast trip times and thus paradoxically, lower arc flash ratings.

However that being said, quite often I will use a simplified method anyways. If I use a simplified, conservative value for arcing current and calculate the incident energy with an arcing time of 2 seconds, I can immediately predict the maximum, worst case possible arc flash. Its just that I can't reliably assume anything about short circuit current with any degree of accuracy to the point that I could use a simplified approach beyond looking at incident energy from an extremely conservative point of view.

In another direction, I can do some extremely simple things with a given arc flash calculation. Assuming voltage, equipment design, and arcing current are fixed, and if I'm only making changes to a protective device, they are, I can then assume that time is linear (and it is) and simply do ratio analysis to figure out how to adjust the protective devices to achieve a given incident energy.


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 Post subject: Re: Falshtables
PostPosted: Fri Aug 22, 2014 5:14 am 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
Hey paul,

But isn’t it true that a ‘quick and dirty’ method is not in compliance with the IEEE 1584? And if a company demands an arc flash analyses that is in compliance with the IEEE 1584, this backwards method is a no-go?

I do not know what kind of options you have but we have to do an arc flash analyses in compliance with the IEEE 1584 (so no NFPA tables either).

However a table setup like you describe does sound interesting for our sales department. Since they will have to make an estimate of the arc flash energy and budget accordingly for it if changes have to be made. At this stage you simply do not know arcing times and short circuit currents but you can estimate.

But that is just how it impacts this company based here in the Netherlands. I can understand that it might be different in the states.


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 Post subject: Re: Falshtables
PostPosted: Sun Aug 24, 2014 5:41 pm 
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Location: Scottsdale, Arizona
I was looking for "flash tables" here but could not find it. I recall a promo post (promos are discouraged on the forum) was made but seems to have disappeared after it stirred up quite a bit of debate.

As far as complying with IEEE 1584, I don't believe there is anything regarding "compliance" with it. I know that type of language does not exist in the document. IEEE 1584 is a calculation method that can be used to comply with PPE selection requirements of other "Standards" such as NFPA 70E.


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 Post subject: Re: Falshtables
PostPosted: Thu Aug 28, 2014 12:02 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Luc wrote:
Hey paul,

But isn’t it true that a ‘quick and dirty’ method is not in compliance with the IEEE 1584? And if a company demands an arc flash analyses that is in compliance with the IEEE 1584, this backwards method is a no-go?


As Jim said, 1584 is a calculation method. It is based mostly on test data that simulates a given current in the lab. It does not specify how you get there. If I work the equations "backwards" to arrive at a set of inputs which yield a desired answer, and then work the equations "forwards" again, I arrive at the same number. How I get there is immaterial.

Quote:
I do not know what kind of options you have but we have to do an arc flash analyses in compliance with the IEEE 1584 (so no NFPA tables either).


IEEE 1584 is a calculation method. There are several out there. 1584 has its limits. It is basically useless below 300 V. It is useless above 15 kV. In the States at least OSHA has taken the stance with utilities to strongly recommend not using IEEE 1584 above 10 kV. There are strong reasons for ignoring IEEE 1584. Among them are:
1. If you have good test data that is more representative of a specific situation, this would be vastly preferable to an estimate, which is what IEEE 1584 is.
2. At the boundaries of IEEE 1584. At 10 kV or above as previously stated, OSHA in particular recommends not using IEEE 1584 and instead recommends ArcPro. Below 250 V, there is only a single test result and follow up work has shown that this is not very valid. As a result I would recommend using IEEE C2 (NESC) or EPRI reports (which IEEE C2 is based on) which provides actual test data rather than calculations which are at best questionable.

Quote:
However a table setup like you describe does sound interesting for our sales department. Since they will have to make an estimate of the arc flash energy and budget accordingly for it if changes have to be made. At this stage you simply do not know arcing times and short circuit currents but you can estimate.

But that is just how it impacts this company based here in the Netherlands. I can understand that it might be different in the states.


And this is where we get to the heart of the issue. I would recommend NOT using tables. The issue is that lots of engineering firms, end users, and so forth, somehow expect the vendor to do an arc flash calculation and supply labels on the equipment. After all, this is the way that it has been for decades with everything else. However this can't happen with arc flash because you don't have all the information. You need to know what the available fault current is and you need to know what the upstream protective device characteristics are. Thus vendors can't supply arc flash labels, simple as that. Your choices are:
1. Use the tables in 70E and then put lots of disclaimers in your literature on the validity of the tables (user must demonstrate compliance with the current and opening time requirements).
2. Offer to do an arc flash study. So basically you pay for an engineering firm to do an arc flash study and charge for the results plus a convenience fee.
3. State that you cannot and will not do arc flash labels and make it clear why you can't make any claims in this manner (labels are site specific for a reason).


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