Point-to-Point Short Circuit Current calculations w/ smaller neutral

[bjenks]

I have a question about using the Point to Point Short Circuit Current calculations for three situations and need to know how to address them when calculating the "f" factor

1) For Line-to-Neutral Faults most articles assume that the neutral conductor and the line conductor are the same size. Most of the time that is not true, so I am wonder how the "f" factor would change for a smaller size neutral. Is making the neutral the same saze as the phase conductor giving me a higher fault current calculation and thus why it is not considered?

2) Does anything change when you have a 3 phase delta high leg? especially for the high leg?

3) Can I use the Ia= Is x Mp formula from the Asymmetrical Factor table to get I asymmetrical where Is is derived from the point to point method?

I don't use any fancy software or spreadsheets and need to come up with a system that is accurate enought, but not complicated.

 

[rbulsara]

You are trying to follow an obscure and obsolete method. Forget the "f" factor method. Size of neutral have little to do with short circuit current calcs.

Refer to IEEE Buff book and Red book if you want to learn proper method of short circuit calculations.

Then at some point you must learn to use software, or you will be left behind the pack. A hand check is still required to verify that software is giving you a correct answer.

 

[bjenks]

I was under the impression from reading EC&M articles and Bussman Bulletins that the PU (IEEE method) and Ohmic have proven to be within range of the "newer" method of point-to-point method. In fact if you use bussmann's online calculator it says it is based on the point-to-point method.

http://www.bussmann.com/apen/software/s_circuit.asp

Also for service entrance fault current studies where there isn't a ground wire being ran, only phases and neutral, the neutral size would have a lot to do with the Phase to Ground fault current. Onces you are inside the building I understand your point as the ground wire is the main path back to the source.

Since I am a small firm that mainly does service entrance gear, meter packs, panel boards, with some motors I don't need a software package. as there just aren't that many items to effect the fault current. However, that being said I am upgrading to Revit Building Systems next year and fault currents calculations are part of the program...

 

[rbulsara]

Trust me, engineering is not something you want to learn from trade magazines or even manufactures technical bulletins. They are good to learn whats going on and get some general ideas, but stick to proper technical sources for proper knowledge. Only then you will be able to make use of information found elsewhere.

The method you are referring to is not new. If you own any kind of business however small, its success will depend on doing it right. Think of return on investment.

 

[bjenks]

Thank you for your input. Just out of curiosity, how do you feel about the KVA/MVA method? I should have mentioned that I use that method when motors or generators are use. Or since it is not IEEE, then it isn't any good? I use tables and estimates all the time and as long as it isn't too close to a thresh hold then I don't have a problem with using it. Especially when I use a method of Short Circuit calculations that might give me a 12kA result on a panel/breaker that is rated for 35kA.

having spent most of my career in the high availability data center arena I do appreciate making sure everything is calculated. Nothing like dropping a cabinet of servers (or an entire data center) because an engineer didn't design things right. I think in the end, both of my method are trustworthy as long as I consider asymmetrical faults and as long as the asymmetric is not within 25% of the equipment rating. I am interested in your feedback on this as I am always willing to change my ways.:-D

 

[davidbeach]

Asymmetry, per se, is not part of the rating standards, hasn't been for many years. Equipment is rated in symmetrical amps at a maximum X/R; maximum asymmetry is a function of X/R. If you stay below the amp rating and below the test X/R, no worries. If you find that the prospective fault has an X/R higher than the test X/R you have to calculate a new rating based on that X/R. The equation for that calculation is too hairy to try to type out here.

On the other hand, if you use the SKM package, which is the one I'm familiar with, you can use the equipment evaluation module and it will perform the necessary derating calculations for high X/R. For non-fused LVPCBs the test X/R is 6.591 (15% PF), for fused LVPCBs and all ICCB/MCCB >=20kA the text X/R is 4.899 (20% PF), for all MCCB between 10kA and 20kA, the X/R is 3.180 (30% PF) and for all MCCB rated <=10kA the test X/R is 1.732 (50% PF).

Hand calcs are useful for taking the PE exam and for spot checking the results of computer calculations. What I've seen looking at projects designed in the 50's through 70's is that the hand calcs resulted in systems with wildly conservative ratings if someone hadn't made too many bad assumptions. A bad assumption or two and the results could be wildly optimistic.

If I were checking someone's work, I wouldn't accept hand calcs for more than about two busses. If the data entry is correct the computer will make far fewer mistakes than will be found in hand calcs. If the initial data is wrong, the hand calcs can't make up for it either. To be very blunt about it, doing hand calcs to avoid use of software tools is bordering on negligent. Sure, the software tools cost money, but it is an investment in the business.

 

[bjenks]

OK, now that 2 people have really pushed the software and even though I feel comfortable with what I am doing, Image is everything! I now believe I should at least take a serious look at purchasing a small software package.

BTW: When I talk about Asymmetrical in the case above I am talking about the DC offset of the symetrical fault. This is important as if any system has a high X/R then their is energy stored in the system. This will show itself at the beginning of the fault, but decaying quickly and could raise the RMS current up to 150% over the symmetrical calculations. This why I personally have seen a blown-up breaker that if you look at the symmetrical calculations should have worked. Yes it is not part of the rating, but if my assymetrical current (DC offset symmetrical) is less than the equipment RMS symmetrical then I am fine. You are right that it could make me over design something that the customer didn't need though.

One thing about being cheap (such as not purchasing software), you have to really study what you are doing and look at all possiblilites. This is why I originally asked the question about Ph-N ground faults with different size neutrals. Most people using computers don't enter that data and thus don't question it. Thank you for both of your input as it has made me change course.

 

[rbulsara]

bjenks:

Please re-read what I said in my first post. Davidbeach is also right.

You said " people don't enter data for neutral and thus they don't question it!!!" Has it occured to you that if the software is not asking for the data, there may be a reason for it? Like it's not required?

As for education on X/R ratio and asymmetrical ratings etc., you have to read up and understand the two books I mentioned. You need them, even you if have software, so you can understand what the software may be doing. It is even more important if you insist on doing the calcs by hand.

All breakers are rated for symmetrical currents and they have buit-in ability to withstand asymmetrical currents as defined in applicable IEEE/ANSI standards. Usually they are suitbale for X/R ratios of 4 to 6 for LV systems. It's in the IEEE Red Book.

This forum is no substitute for formal education and learning. Please take some seminars on the topic. Look up classes offered by University of Wisconsin. (They do cost more than some softwares).

One benefit of using a good software is you need not know all the nuiances of formulas, only concetrate on inputting correct data. Assumptions, if any are fewer to make when using a software.

 

[mpparent]

I kinda like "A Practical Guide to Short Circuit Calculations" by Conrad St. Pierre. It should be in every Power Engineer's library.

Mike

 

[bjenks]

Well, I went and purchased SKM PowerTools (spent $5,500). At the end of the day I spent all that money on something that I really have little use for. Unless I was working on a multi-story building with multiple busses, parallelling switch gear and multiple large motors. So instead of my normal calculation of a fault current being 11.82kA it is more accurate at 11,131A. Glad I bought the program as it made no difference in the equipment selection. I then thought I could use it for circuit breaker coordination, but guess what, as you probably already guessed the breakers are all series rated and the MCB is thermal magnetic so there isn't anything to truely coordinate. Then I said to myself, I could use this to help me do my panel schedules, voltage drops and loading calculations. Then I found that this program does has some limited single-phase capabilities, but for the most part it is easier to use my Autocad Building Systems program and I still had to use my spreadsheets for the load calculations & VD. End of the day as I suspected it is NOT border line negligent to use a simple spreadsheet to do these calculations. If you only have 1-5 busses and if all the downstream breakers are series rated you don't have to purchase software and still be a good engineer. I don't dislike the software and will keep it for any large projects that might someday come my way. BTW: this all started when I just asked a simple question about reduced neutrals. I sent the question to Bussmann who responded back with the way to address it which is to do the calc for the phase and then add one for the neutral instead of multipling times 2 or 1.73. Oh, and I purchased all 13 of the IEEE color books, didn't realize the thousands of pages you get with it... Now I can say I have all the IEEE books and software.

 

[rbulsara]

Congratualtions! Now that the pond has arrived at the horse. Rest is upto the horse.

I would wait for your post here in the next year or two recommending the same to others.