Bus Protection
- Thread starter danilo
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charlie
Senior Member
- Location
- Indianapolis
Re: Bus Protection
It depends on how you have gotten your information to do your fault current calculations. If you used the actual impedance of the transformer and gotten the actual available fault current to the primary of the transformer, my answer would be no. If you used the possible impedance of the transformer that the electric utility gave you as a worst cast and assumed an infinite bus to the primary of the transformer, my answer would be no problem since you are looking at extremes.
It depends on how you have gotten your information to do your fault current calculations. If you used the actual impedance of the transformer and gotten the actual available fault current to the primary of the transformer, my answer would be no. If you used the possible impedance of the transformer that the electric utility gave you as a worst cast and assumed an infinite bus to the primary of the transformer, my answer would be no problem since you are looking at extremes.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Bus Protection
Sorry, Charlie, but I have to disagree on this one.
I would say "no" in any event. Your calculation is your calculation. If it shows more than 42 KAIC, then you have to have equipment rated for more than 42 KAIC. You have to protect the equipment against any fault that you believe is possible. You don't get the option of saying the calculation is conservative and the the result is "close enough."
That said, you do have an option of re-thinking your calculation. If you can justify a revision to the assumptions, and can obtain thereby a defensible result of 42 KAIC or under, then you can use that revised calculation as a basis for selecting the 42 KAIC bus.
Sorry, Charlie, but I have to disagree on this one.
I would say "no" in any event. Your calculation is your calculation. If it shows more than 42 KAIC, then you have to have equipment rated for more than 42 KAIC. You have to protect the equipment against any fault that you believe is possible. You don't get the option of saying the calculation is conservative and the the result is "close enough."
That said, you do have an option of re-thinking your calculation. If you can justify a revision to the assumptions, and can obtain thereby a defensible result of 42 KAIC or under, then you can use that revised calculation as a basis for selecting the 42 KAIC bus.
charlie
Senior Member
- Location
- Indianapolis
Re: Bus Protection
Charlie, as usual, your logic and methodology is impeccable.
Charlie, as usual, your logic and methodology is impeccable.
- Location
- Bremerton, Washington
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- Master Electrician
Re: Bus Protection
What would be the accuracy of a fault current calculation?
If its +/- 10% then it would be clear that gear rated above Kaic would be required.
What would be the accuracy of a fault current calculation?
If its +/- 10% then it would be clear that gear rated above Kaic would be required.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Bus Protection
Again, as mentioned, it depends on how you calculated the # you did. Very often impedances of terminations, interconnections of busway, contact impedance are not included but are there. You can use those as "justify a revision to the assumptions, and can obtain thereby a defensible result".
Again, as mentioned, it depends on how you calculated the # you did. Very often impedances of terminations, interconnections of busway, contact impedance are not included but are there. You can use those as "justify a revision to the assumptions, and can obtain thereby a defensible result".
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Bus Protection
Here's where it gets tricky. If you are interested in voltage drop, you pick an "assumed length" that you are certain is as long or longer than the real length. That is because a longer wire has more resistance, and give a worse voltage drop than a shorter wire. But if you are interested in fault current, you pick an "assumed length" that you are certain is as short or shorter than the real length. That is because a shorter wire has less resistance, and give a worse fault current than a longer wire.
All of this is part of the calculation. If there are any inaccuracies, they arise from an imperfect knowledge of the actual configuration. These inaccuracies are taken into account by the use of conservative assumptions. But once you get an answer, you can use that answer as though it represented absolute truth. You don't need to add another "fudge factor," such as the + or ? 10% you suggested. That is because conservatism is built into the calculation process.
There is no such thing! A calculation is a calculation, and its results are its results. It will require input information, some of which is knowable, and some of which must be assumed. For example, you may know the size and type of wire that is used. But you will not know the exact length, since there may be more than one way to route the wire from one point to another. So you assume a length.
Here's where it gets tricky. If you are interested in voltage drop, you pick an "assumed length" that you are certain is as long or longer than the real length. That is because a longer wire has more resistance, and give a worse voltage drop than a shorter wire. But if you are interested in fault current, you pick an "assumed length" that you are certain is as short or shorter than the real length. That is because a shorter wire has less resistance, and give a worse fault current than a longer wire.
All of this is part of the calculation. If there are any inaccuracies, they arise from an imperfect knowledge of the actual configuration. These inaccuracies are taken into account by the use of conservative assumptions. But once you get an answer, you can use that answer as though it represented absolute truth. You don't need to add another "fudge factor," such as the + or ? 10% you suggested. That is because conservatism is built into the calculation process.
Re: Bus Protection
thanks guys for your great response..
actually,i got the impedance from the transformer nameplate itself and not from the table,however, i just presumed that this value (43kaic) can be rounded off because i didnt have any X/R for my calculation so i guess to use 42kaic is a little bit conservative and a real factor in reducing the investment cost.
am i missing something here?
regards,
Danilo
thanks guys for your great response..
actually,i got the impedance from the transformer nameplate itself and not from the table,however, i just presumed that this value (43kaic) can be rounded off because i didnt have any X/R for my calculation so i guess to use 42kaic is a little bit conservative and a real factor in reducing the investment cost.
am i missing something here?
regards,
Danilo
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