Arc Flash & Multiple Sources 1

[wbd]

Hello,

I am currently working on a arc flash study on a system having multiple sources operating in parallel. This is providing some interesting results and I would welcome any comments.

Faulting a common bus, with both sources active, results in an incident energy of 268.7 cal/cm2. Faulting the same bus but only one source at time, results in 7.6 cal/cm2 for one source and 21.5 cal/cm2 for the other. I don't how the program arrived at the 268 number unless it took the higher arcing current but used the longer time time figure.

From the individual results, the worst case is 21.5 cal/cm2, so would it be correct that this would be what should be protected to?

 

[dpc]

If you're using EasyPower, it does handle multiple sources that way - it takes the total fault current for the longest duration of any of the sources.

In the latest version, there is an "Integrated" mode that considers each source individually and integrates the arc energy over time. That should give a more realistic result.

 

[umrpwr]

Hi,

I commonly perform short-circuit, coordination and arc-flash studies for systems that have multiple sources. In fact almost all of the systems I do analysis on have multiple sources operating in parallel. There are a couple of things that you need to consider when performing arc flash analysis on these types of systems. The first is that with all of the sources in parallel that may or may not be the worse case arc flash incident energy. Higher fault currents will allow the protective devices to operate more quickly especially if there are instantaneous elements which may lead to lower arc flash values. Where as a low magnitude fault may not be cleared as quickly by the protective device resulting in higher arc flash incident energies.

Secondly I would suggest a careful reading of the IEEE Std 1584. The standard provides some guidelines on what the maximum time duration for calculation of arc flash values should be. Otherwise you can end up with some extremely high numbers.

Lastly if you are doing arc flash analysis on a system with multiple sources you will have to run multiple scenarios to determine what the worst case arc flash incident energies will be at each bus. You can then do a filter and sort to find these values for each bus. This is more time consuming, but the only way to do it correctly. This will involve running cases with the sources in parallel and also with each source separate. In addition it would also be recommended to look at running the arc flash analysis for all of the scenarios above for faults on both the line side and also load side of the buses. This is especially true if you have large motor loads for the same reasons as having multiple sources.

 

[wbd]

dpc -

I am running EasyPower version 8.0.2.204. This is the latest version and I have looked around for this "Integrate" feature. Where is it located?

 

[wbd]

dpc,

I did find where that option is located. Integrated did not show up in the Help Index but did when I used the Help Search feature.

thanks for the tip.

 

[dpc]

The Integrated method also drops out the motor and generator contributions per ANSI C37 fault calc methods. Still conservative, but more realistic than no decay.

When using the Integrated mode, the arc-flash reports shows only the LAST fault contribution current and time, so it can be a little deceptive sometimes.

 

[veritas]

wbd

I do not know Easyower at all as I use the Power Tools for Windows (PTW) package. What I suggest you do consider is that the case of both sources in parallel could very well be the worst case. This is because each source sees less current than what it would have if it was the only source.

This is based on the basic concept that a source contributes maximum current to a fault if it is the only source in service. The moment other sources are in service as well, the total fault current increases but the individual source see less, thus the protective devices take longer to operate and the incident energy is higher.

Your results are possible if one or both sources are moving say from an instantaneous part of the curve to an inverse time part which has longer operating time.

A bit more complex is the fact that one source trips off before the other. In PTW one can set the software to consider the fault cleared if say 80% of the original arcing current has been cleared within a maximum time (usually set to 2s). These calcs are all base on IEEE 1584.

Regards.

 

[rbulsara]

wbd:

I am not sure what you meant by "this is what should be protected to". The primary purpose of the arc flash is to come up with PPE category to protect the personnel not the equipment.

Imho, intent of the arc flash study is not to find the max. hazard but the min. hazard that can be obtained so when absolutely necessary, a live work can be performed with least danger. For example, it may be less hazardous to perform a live work when a generator feeds the equipment than when it is on utility source.

I would recommend that the labeling method should include adequate warning for presence of arc flash hazard and then lead to a written and readily available maintenance procedures that includes as to in what state a live work, if at all any, could be performed on the equipment.

It certainly would not be sufficient to slap only a worst-case scenario label on an equipment fed by multiple sources.

 

[wbd]

rbulsara-

I was meaning what the level of PPE would be for proper protection. While I agree that there are methods to reduce the afh, I believe that the labeling should represent the worst case hazard.

For example, existing settings on a breaker for proper cordination might result in a afh of a Cat #3. But by lowering the instaneous setting, the afh may end up reduced to Cat #1. The label should reflect the Cat #3 hazard which is the normal operating configuration. Imho, this is the safest approach.

And yes, a afh analysis study should look at methods, such as temporarily changing breaker settings, that would allow live work to be performed in the safest manner possible. However, since this would require procedures or an active effort, the label should not reflect this. This would come out of the live work permit that would be required.

While it "certainly would not be sufficient to slap only a worst-case scenario label on an equipment fed by mutliple sources", it certainly would be the safest. That is the bottom line of what we are all trying to achieve in performing afh analyzes.

 

[Zogzog]

What about using arc flash reduction switches? We have retrofitted these and provided labels with 2 Ei's, depending on what mode you are in

 

[rbulsara]

wbd:

The "safest" method can't be misleading either as if there is no alternative! I would think if the hazard can be minimized by chaning the source, the owner and electrician has the right to know that and consultant owe that information to the Owner in no ambiguous terms.

As you stated all work related to arc flash must come out of the live work permit/ticket and the lock out tag out procedures. The label is part of that. It can't be misleading. You may need to place more than one lables, if needed, as zogzog also mentioned.

But again, all sites have different methods of controlling their work safety.

 

[Homerjs78]

Wbd:

The integrated method is described in the Easy Power help if you open the help from the arc flash options page. There is a help button near the lower right corner. Click here and scroll down. It’s a small blurb but it does describe how it works. If you take the advise above and try a demo copy, try asking for a copy of their manual (if they will give you one). There might be more on the subject.