short circuit method?
- Thread starter elicio
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: short circuit method?
This is the third thread you have started, and you have asked the same question in all three. After having re-read at all three, I still don?t understand what you are looking for.
Would the following three statements give you the answer you need?
(1) No. The NEC does not require the performance of a short circuit calculation.
(2) If you desired to perform a short circuit calculation, the NEC does not give any methods by which such a calculation could be performed.?
(3) Short circuit calculations are part of the design process, and are not addressed in the code.
This is the third thread you have started, and you have asked the same question in all three. After having re-read at all three, I still don?t understand what you are looking for.
Would the following three statements give you the answer you need?
(1) No. The NEC does not require the performance of a short circuit calculation.
(2) If you desired to perform a short circuit calculation, the NEC does not give any methods by which such a calculation could be performed.?
(3) Short circuit calculations are part of the design process, and are not addressed in the code.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: short circuit method?
That's very interesting. OK, then I'll ask Elicio, "Are you from the UK, and are you talking about a test or a calculation?"
That's very interesting. OK, then I'll ask Elicio, "Are you from the UK, and are you talking about a test or a calculation?"
Re: short circuit method?
After reading the profile of elicio I think you are correct Charlie. The gentleman is asking about short circuit calculations on transmission lines.
This calculation is a complex mathematical process. This is not a code issue, it is technical engineering for locating faults on distribution systems.
There is many power engineering books with the information on calculating the positive, negative, and zero sequence impedance of the source, for locating a fault.
After reading the profile of elicio I think you are correct Charlie. The gentleman is asking about short circuit calculations on transmission lines.
This calculation is a complex mathematical process. This is not a code issue, it is technical engineering for locating faults on distribution systems.
There is many power engineering books with the information on calculating the positive, negative, and zero sequence impedance of the source, for locating a fault.
Re: short circuit method?
Bennie
I have heard of this test but I am not familiar with it. What is it and why should we be doing it?
The way he is phrasing the question, I don't think he's from London.
If you look at his first post I think he want to know how to apply the withstand rating for #4 copper. He says he has about 40ka at the MCC.
[ September 24, 2003, 07:39 PM: Message edited by: bob ]
Bennie
I have heard of this test but I am not familiar with it. What is it and why should we be doing it?
The way he is phrasing the question, I don't think he's from London.
If you look at his first post I think he want to know how to apply the withstand rating for #4 copper. He says he has about 40ka at the MCC.
[ September 24, 2003, 07:39 PM: Message edited by: bob ]
Re: short circuit method?
elicio: The 7.5 HP motor will draw 11 amps. This would be a No.#8 cu. for a 3% voltage drop in the 380 meters. Motor branch circuit overcurrent size is 250% x 11 amps, or 27.5 = 30 amp inverse time circuit breaker.
I don't have the withstand curve for copper No. #8. Aluminum No.#4 will melt with 5 kA in 1 second.
Please anyone available check my math. It has been a long time since I have done these calcs.
[ September 25, 2003, 12:27 AM: Message edited by: bennie ]
elicio: The 7.5 HP motor will draw 11 amps. This would be a No.#8 cu. for a 3% voltage drop in the 380 meters. Motor branch circuit overcurrent size is 250% x 11 amps, or 27.5 = 30 amp inverse time circuit breaker.
I don't have the withstand curve for copper No. #8. Aluminum No.#4 will melt with 5 kA in 1 second.
Please anyone available check my math. It has been a long time since I have done these calcs.
[ September 25, 2003, 12:27 AM: Message edited by: bennie ]
Re: short circuit method?
Bob: The ground loop impedance test is to determine if the impedance will permit enough current to flow on a ground fault to activate the overcurrent device.
Should the ground loop impedance be low enough, then the short circuit impedance will also permit the protective device to activate. The short circuit voltage is double the voltage to ground on single phase 120/240 systems.
[ September 25, 2003, 12:27 AM: Message edited by: bennie ]
Bob: The ground loop impedance test is to determine if the impedance will permit enough current to flow on a ground fault to activate the overcurrent device.
Should the ground loop impedance be low enough, then the short circuit impedance will also permit the protective device to activate. The short circuit voltage is double the voltage to ground on single phase 120/240 systems.
[ September 25, 2003, 12:27 AM: Message edited by: bennie ]
Re: short circuit method?
Bennie,
The phase-to-ground voltage indirectly gives the touch voltage in case of a fault. If the grounding and phase conductor are of the same size, the touch voltage is half the phase-to-ground voltage.
According to NEMA,
ELECTRICAL INSTALLATION REQUIREMENTS - A Global Perspective
In 120 V circuits where the circuit conductor and the equipment grounding conductor are of equal size, the maximum voltage that could appear on an equipment or appliance enclosure during a ground fault condition would be 60 V. In a comparable 240 V circuit the touch voltage
would be 120 V under the same fault conditions. The potential of a higher touch voltage has influenced the IEC 60364 rules. Coupled with the higher voltage are the supply system earthing rules which would permit the higher touch voltages to exist for long periods of time, thus necessitating use of RCDs to limit the duration of the hazardous condition. In a 120 V system under the rules of the NEC, the lower touch voltage, supply system grounding rules, and requirements for effective grounding paths ensure that in circuits of practical lengths, the hazardous voltages under fault conditions are terminated in a short period of time.
This also means that the ground loop impedance is not considered to be critical by the NFPA.
Bennie,
...within a certain period of time. This time is determined by the relevant code section, but typically depends on voltage and what you are protecting (feeder, motor or a socket). The purpose of the ground loop impedance check is to ensure that the touch voltage in case of a ground fault doesn't persist long enough to be dangerous.
The phase-to-ground voltage indirectly gives the touch voltage in case of a fault. If the grounding and phase conductor are of the same size, the touch voltage is half the phase-to-ground voltage.
According to NEMA,
ELECTRICAL INSTALLATION REQUIREMENTS - A Global Perspective
In 120 V circuits where the circuit conductor and the equipment grounding conductor are of equal size, the maximum voltage that could appear on an equipment or appliance enclosure during a ground fault condition would be 60 V. In a comparable 240 V circuit the touch voltage
would be 120 V under the same fault conditions. The potential of a higher touch voltage has influenced the IEC 60364 rules. Coupled with the higher voltage are the supply system earthing rules which would permit the higher touch voltages to exist for long periods of time, thus necessitating use of RCDs to limit the duration of the hazardous condition. In a 120 V system under the rules of the NEC, the lower touch voltage, supply system grounding rules, and requirements for effective grounding paths ensure that in circuits of practical lengths, the hazardous voltages under fault conditions are terminated in a short period of time.
This also means that the ground loop impedance is not considered to be critical by the NFPA.
Re: short circuit method?
This decision is irresponsible from my viewpoint. A 120 volt ground fault that does not clear in a short time will ignite material surrounding the conductors.
A ground fault that clears by melting the equipment ground conductor will impress a full 120 volts on metal surfaces. This voltage can remain until someone is killed or injured.
If 60 to 120 volts is not lethal, lets discontinue 120 volt breakers.
This decision is irresponsible from my viewpoint. A 120 volt ground fault that does not clear in a short time will ignite material surrounding the conductors.
A ground fault that clears by melting the equipment ground conductor will impress a full 120 volts on metal surfaces. This voltage can remain until someone is killed or injured.
If 60 to 120 volts is not lethal, lets discontinue 120 volt breakers.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: short circuit method?
The original question of this thread is a duplicate of two others. I'm closing this one. Refer to the "short circuit calculation" thread to continue the original question.
If anyone wished to continue the sidebar discussion into which this thread has evolved, please start a new thread.
Charlie Beck
Moderator.
The original question of this thread is a duplicate of two others. I'm closing this one. Refer to the "short circuit calculation" thread to continue the original question.
If anyone wished to continue the sidebar discussion into which this thread has evolved, please start a new thread.
Charlie Beck
Moderator.
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