voltage drop?
- Thread starter brooksie
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charlie
Senior Member
- Location
- Indianapolis
Re: voltage drop?
I don't know what else needs to be said.
110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
110.10 Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.
I don't know what else needs to be said.
110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
110.10 Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.
iggy2
Senior Member
- Location
- NEw England
Re: voltage drop?
The distance is not about personnel safety, it's about the AIC rating of the equipment. The further from the fpl (assume FL Power & Light?) then the lower the short circuit available at the equipment. You need higher AIC equipment, or more cable between the transformer and equipment to reduce the short circuit availbale at the equipmment.
The distance is not about personnel safety, it's about the AIC rating of the equipment. The further from the fpl (assume FL Power & Light?) then the lower the short circuit available at the equipment. You need higher AIC equipment, or more cable between the transformer and equipment to reduce the short circuit availbale at the equipmment.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: voltage drop?
Charlie can you use fuses on the line side of the service equipment to meet this problem?
Charlie can you use fuses on the line side of the service equipment to meet this problem?
charlie
Senior Member
- Location
- Indianapolis
Re: voltage drop?
Not on a new install. If it is existing and the requirements of 240.86 are met in the 2005 Code, it can be done. It would be rare and expensive to take advantage of that new provision.
Not on a new install. If it is existing and the requirements of 240.86 are met in the 2005 Code, it can be done. It would be rare and expensive to take advantage of that new provision.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: voltage drop?
Hurk 27 and Charlie
Why can't you install current limiting fuses at the temp service pole as service disconnect protection? If properly chosen, they likely would limit currents at the load end of the fuses to less than 22ka. In my area, utility meter sockets are specified by the serving utility, and their rating is not part of my calculation.
Secondly Charlie points out correctly that, the fault current at the temp service pole would be reduced by the impedance of the service drop or lateral.
If you have a 100 amp 120/240 volt service, it won't take much #4 triplex to get you to 22ka, if you have an 800 amp 480 volt temp service, the service impedance will still be significant, but less.
If Brooksie will post a description of the service, voltage, size of conductors between xfmr and temp pole, etc. there is a simple calculation for determining downstream fault currents.
Jim T.
Hurk 27 and Charlie
Why can't you install current limiting fuses at the temp service pole as service disconnect protection? If properly chosen, they likely would limit currents at the load end of the fuses to less than 22ka. In my area, utility meter sockets are specified by the serving utility, and their rating is not part of my calculation.
Secondly Charlie points out correctly that, the fault current at the temp service pole would be reduced by the impedance of the service drop or lateral.
If you have a 100 amp 120/240 volt service, it won't take much #4 triplex to get you to 22ka, if you have an 800 amp 480 volt temp service, the service impedance will still be significant, but less.
If Brooksie will post a description of the service, voltage, size of conductors between xfmr and temp pole, etc. there is a simple calculation for determining downstream fault currents.
Jim T.
charlie
Senior Member
- Location
- Indianapolis
Re: voltage drop?
Jim, fuses are great by themselves, they just can't be used to protect downstream circuit breakers (I thought that was what Wayne was suggesting).
Jim, fuses are great by themselves, they just can't be used to protect downstream circuit breakers (I thought that was what Wayne was suggesting).
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: voltage drop?
Charlie
I am going to show my ignorance, but why can't a CL fuse be used to protect downstream devices like breakers?
Jim T
Charlie
I am going to show my ignorance, but why can't a CL fuse be used to protect downstream devices like breakers?
Jim T
charlie
Senior Member
- Location
- Indianapolis
Re: voltage drop?
Circuit breakers today are acting fast enough to put dynamic impedance into the circuit. In other words, the small circuit breakers are starting to operate in the current limiting range. When that happens, it can not be known how the fuse will work in conjunction with the circuit breaker without testing.
Circuit breakers today are acting fast enough to put dynamic impedance into the circuit. In other words, the small circuit breakers are starting to operate in the current limiting range. When that happens, it can not be known how the fuse will work in conjunction with the circuit breaker without testing.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: voltage drop?
Charlie
I'm still a bit confused. I thought the whole idea behind current limitation was to protect downstream lesser rated devices. I agree they might not provide positive coordination in the current limitation range, but in this application who cares.
Please post some more information.
Jim T
Charlie
I'm still a bit confused. I thought the whole idea behind current limitation was to protect downstream lesser rated devices. I agree they might not provide positive coordination in the current limitation range, but in this application who cares.
Please post some more information.
Jim T
charlie
Senior Member
- Location
- Indianapolis
Re: voltage drop?
If a fuse operates at a particular speed under certain fault conditions and all of a sudden a circuit breaker starts to open at the same time, the circuit breaker is now seeing all of the fault current. Now the fuse is going to open the circuit but will take longer to operate since the circuit breaker is adding dynamic (changing) impedance into the circuit. The problem is that the circuit breaker may blow apart before the fuse opens since the fuse is now working slower than it would have worked. The up-over-and-down method is valid as long as the parameters are not changed, that is what the dynamic impedance does to the circuit.
That is the best I can do, if someone else want to jump in, feel free.
If a fuse operates at a particular speed under certain fault conditions and all of a sudden a circuit breaker starts to open at the same time, the circuit breaker is now seeing all of the fault current. Now the fuse is going to open the circuit but will take longer to operate since the circuit breaker is adding dynamic (changing) impedance into the circuit. The problem is that the circuit breaker may blow apart before the fuse opens since the fuse is now working slower than it would have worked. The up-over-and-down method is valid as long as the parameters are not changed, that is what the dynamic impedance does to the circuit.
That is the best I can do, if someone else want to jump in, feel free.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: voltage drop?
sparkie001
Series rating is generally breaker to breaker within an individual manufacturers line, not fuse to breaker. The upstream breaker can operate quickly enough to avoid damage to the downstream underrated breaker. As you pointed out, they must be tested together.
I am anxious to get some more comment about current limitation whether breakers or fuses.
I often will spec CL fuses to reduce a 14,000 amp calculated fault current to less than 10,000 amps, and then allow 10 ka breakers in all downstream equipment.
Jim T
sparkie001
Series rating is generally breaker to breaker within an individual manufacturers line, not fuse to breaker. The upstream breaker can operate quickly enough to avoid damage to the downstream underrated breaker. As you pointed out, they must be tested together.
I am anxious to get some more comment about current limitation whether breakers or fuses.
I often will spec CL fuses to reduce a 14,000 amp calculated fault current to less than 10,000 amps, and then allow 10 ka breakers in all downstream equipment.
Jim T
Re: voltage drop?
Jtester
[ December 10, 2004, 10:50 AM: Message edited by: bob ]
Jtester
This method may not work. You can not be sure that the breaker will not unlatch and start to open during the interuption of the fault by the fuse. If this happens, then as Charlie said, the breaker introduces additional impedance in the circuit. The current is reduced thus increasing the time for the fuse to interrupt the fault. This is why the series rated devices must be tested prior to being rated for series operation.
[ December 10, 2004, 10:50 AM: Message edited by: bob ]
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