Available Fault Current, PV Array, NFPA70E

[ControlThis]

I work in a small industrial facility in which I am working to organize an arc flash safety program. We had a visit from OSHA so we can no longer put this off. I've been looking through NFPA70E for the last 5 or 6 years and just took a course on it recently.
How I should consider the array in terms of available fault current to the building? We have grid tied inverters and no batteries.(150kw total capacity). I know that I need to consider the available fault current from the utility transformer and fault clearing times for our fault protection devices, but what do I do with a feed from the inverters that is parallel to this transformer? Is the AC output from the array to be considered a "transformer" for the purpose of this standard? Would I just add the available fault currents together?
As if NFPA70e isn’t already enough of a pain without an array hooked up, I’m just not sure yet how to deal with these two parallel feeds. Without the array, it seems I can keep the incident energy analysis fairly simple as even the worst case scenario for fault current at the utility's transformer puts us at level 1 PPE.
Any direction on this would be greatly appreciated. Even just a code section to refer to, or any information that could tell me what I am missing here.

 

[Jraef]

Your Available Fault Current has to be based on the worst case scenario, which is going to be the utility connection. No matter what, the PV array is never going to deliver more AFC than the utility can. So calculating for the purposes of issues relating to NFPA-70E becomes a moot point in my opinion.

 

[jaggedben]

For your purpose you could probably add the inverters' maximum output current to the available fault current from the utility. At 150kW the inverter output may be more than a rounding error.

You could also contact the inverter manufacturer and ask them what they think.

 

[mayanees]

PV Source

PV Source

I presume you're using a Power Systems Analysis software like SKM. Simply model the PV system through an inverter and put it in a scenario used in performing the incident-energy calculations.

 

[ron]

Your Available Fault Current has to be based on the worst case scenario, which is going to be the utility connection. No matter what, the PV array is never going to deliver more AFC than the utility can. So calculating for the purposes of issues relating to NFPA-70E becomes a moot point in my opinion.

Since the two sources (utility and PV) are in parallel, we would have to consider them both - additive.

 

[Jraef]

Since the two sources (utility and PV) are in parallel, we would have to consider them both - additive.

I was thinking it has a transfer switch, but now that I think about it some more, that's not right. Sorry.

 

[wbdvt]

Since the two sources (utility and PV) are in parallel, we would have to consider them both - additive.

Most likely the inverters have a rating for the short circuit that can be delivered. This is usually small (1.1 pu) and very fast tripping as they are trying to protect the electronics.

You do have to consider both sources and usually a software program such as EasyPower has a feature call integrated method for this. So what has to be done is to consider both sources as follows: (numbers are for illustrative purposes only)

PV has 800A fault current for 0.1 sec
Utility has 3000A fault current for 0.3sec

Therefore for the first 0.1 seconds the fault current is 3800A and you calculate the incident energy for that.
Then for the next 0.2 seconds the fault current is 3000A and you calculate the incident energy for that.

Add the two together for the incident energy value.

 

[ControlThis]

Thanks for the feedback.
I am not using any "Power Systems Analysis" software. Even if I had that available, I would want to learn how the back end works and what calculations are used in order to validate the output and also for professional development. I'm having a hard time finding standardized calculations that people might use because it seems everyone is using an app or software to give them the answers. Plus, some calcs I look at have been out for a number of years. Which ones are still good, which ones now have better replacements? I'm still in the early stages of my learning curve with this analysis but I am determined to learn it.
I looked at the skm software and had some sticker shock on the price tags. Considering that I will only use the software for the building I work in, I could probably hire someone to do a full analysis of the building for a similar price, but then I don't learn how to do it!
I may or may not buy a piece of software. I may or may not hire someone to help. Either way I want to learn. I've built a one-line in autocad over the past few years as I document the building, now it's time for filling in available fault currents and incident energy levels.

 

[wbdvt]

Thanks for the feedback.
I am not using any "Power Systems Analysis" software. Even if I had that available, I would want to learn how the back end works and what calculations are used in order to validate the output and also for professional development. I'm having a hard time finding standardized calculations that people might use because it seems everyone is using an app or software to give them the answers. Plus, some calcs I look at have been out for a number of years. Which ones are still good, which ones now have better replacements? I'm still in the early stages of my learning curve with this analysis but I am determined to learn it.
I looked at the skm software and had some sticker shock on the price tags. Considering that I will only use the software for the building I work in, I could probably hire someone to do a full analysis of the building for a similar price, but then I don't learn how to do it!
I may or may not buy a piece of software. I may or may not hire someone to help. Either way I want to learn. I've built a one-line in autocad over the past few years as I document the building, now it's time for filling in available fault currents and incident energy levels.

If you want to learn the back end of the calculations, there are several sources depending on what you are interested in:
1. If looking for basics on performing short circuit calculations there are many sources ranging from textbooks on power systems to tutorials on vendor websites such as Cooper Industries.
2. If looking for the background on arc flash calculations then you can purchase the IEEE 1584-2002 standard which explains how they arrived at the equations or NFPA70E-2015 has the IEEE 1584 equations in Annex D

Other values to performing the study using a software program such as EasyPower or SKM for example are:
1. Obtain the short circuit values at various locations in the facility which allows the determination if there is any equipment that is overdutied.
2. Coordination study - this is typically a part of an arc flash study and will show the coordination of the protective devices.
3. With the ability to easily shift time current curves, the impact of changing a protective setting to lower the incident energy can be seen. Many times lowering the instantaneous setting will lower the incident energy but the impact on coordination needs to be considered.
4. A one line model of the equipment included in the study. Many facilities do not have one lines or up to date one lines and this will provide them.
5. Changes to the facility are easily incorporated into the model to determine the incident energy in a timely manner.
6. Labels are generated from the study and will usually provide more information than is required by NFPA 70E. For example, the labels we create have the shock hazard, available bolted fault current at the device, where it is fed from (useful for LOTO).

In the big scheme of things, I think you have a greater ROI for having a consultant do the work for you. It will also be easier to update the study in 5 yrs as required by NFPA 70E although I have heard rumors that this will be going to a 3 yr cycle.

 

[ControlThis]

I think you basically answered all my questions!
-I am going to see what it would cost to get a copy of IEEE1584
-Also, I would like to investigate software packages. I get the sense that SKM is more widely used but wonder which package would be better for my application? Onto that research now!
Thanks again!

 

[wbdvt]

IEEE 1584 is roughly about $800. SKM or EasyPower software will run about $10-12K so you really have to decide if you will really use it or is it better to have a consultant do it. Depending on the size of your facility it may be more cost effective to have it done by a consultant.