High impedance wires

Tainted

Tainted

Senior Member
Location
New York
Occupation
Engineer (PE)
I am trying to reduce the available fault current at a new piece of equipment. In order to do this I need to run like 40 feet of copper wires when the equipment is literally just 10 feet away. It just doesn't seem practical.

Is there any high impedance wire out there that can reduce the length of the run?
 
kwired said:
Loop 40 feet or conductor in a large enough pull box?

Install a line reactor?
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no space in the room for line reactor and I haven't seen that done in any of the buildings. Out of curiosity, would that require ventilation in the room?

Looping 2 sets of 500KCMIL is gonna be super tough?
 
High impedance wire is what we usually call heating cable and likely will be too much impedance and will drop voltage to the load without very careful calculating and application. Plus would not be so readily available for something that carries the kind of current you are using 2 sets of 500 kCMIL for
 
Kind of a crazy idea, but if the location is at or below grade and near an outside wall, then a loop going out and back could be made underground in an isophase arrangement. Depending on the spacing of nonmetallic conduits, the inductance from the isophase arrangement could increase the impedance per unit length by around 2 to 3X.
It would get you the "high impedance wire" you asked for.

This has some info on wire impedance vs. spacing:
https://www.anixter.com/content/dam/Anixter/Guide/7H0011X0_W&C_Tech_Handbook_Sec_07.pdf
 
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Is this at 208V? If you are that close to the rating, I would verify the available fault current and exactly where in the system it was calculated. My first preference would be to get adequately rated equipment. Current-limiting reactors might be feasible if you are close to the rating.
 
David Castor said:
Is this at 208V? If you are that close to the rating, I would verify the available fault current and exactly where in the system it was calculated. My first preference would be to get adequately rated equipment. Current-limiting reactors might be feasible if you are close to the rating.
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He didn't think he had room for such a thing when I suggested it.
 
I've been wondering if a string of ferrite beads on the conductors in some sort of wire trough would add the necessary impedance in a way that fits the space.

Jon
 
winnie said:
I've been wondering if a string of ferrite beads on the conductors in some sort of wire trough would add the necessary impedance in a way that fits the space.

Jon
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I'm patenting that, thank you. Just kidding.

I'm sure someone thought of it but when they made it, it turned out to be a bad idea. But who knows lol
 
Tainted said:
I'm patenting that, thank you. Just kidding.

I'm sure someone thought of it but when they made it, it turned out to be a bad idea. But who knows lol
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I think the basic problem is that the inductance of a practical sized ferrite bead introduces a very small impedance at 60 Hz compared to the impedance at RF frequencies.
It could soften the leading edge of the current into a bolted fault, but will do almost nothing for the current over a full cycle.
 
winnie said:
I've been wondering if a string of ferrite beads on the conductors in some sort of wire trough would add the necessary impedance in a way that fits the space.

Jon
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How about put each current carrying conductor in it's own ferrous raceway? NEC of course doesn't allow this but would likely reduce the available fault current better than ferrite beads would.
 
kwired said:
How about put each current carrying conductor in it's own ferrous raceway? NEC of course doesn't allow this but would likely reduce the available fault current better than ferrite beads would.
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If they were used as sleeves rather than raceways, I think the NEC would be silent on the issue. For example if the 'wire in pipe' assemblies were all together in a trough.

You would need to insure that you didn't have current loops that would act as shorted transformer turns.

The biggest problem, however, is that the ferrous conduit has no specification for magnetic qualities. You have no way (other than experiment) to determine how much pipe you would need for the required choke effect, how much heating to expect, etc.

What ferrite brings to the table is that it is magnetic while being an electrical insulator, along with magnetic specifications. But it is optimized for much higher frequency use, expensive, and has poorer permeability than common steel.

I'm guessing that some form of tape wound transformer steel tubed would be optimal for 'bead chokes' to reduce fault current.

Jon
 
winnie said:
... What ferrite brings to the table is that it is magnetic while being an electrical insulator, along with magnetic specifications. But it is optimized for much higher frequency use, expensive, and has poorer permeability than common steel.
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Yes, a steel conduit is conductive and it's going to have significant eddy current losses (and therefore heating) since there will be nothing like the laminations in a transformer to break up circulating eddy currents.

winnie said:
I'm guessing that some form of tape wound transformer steel tubed would be optimal for 'bead chokes' to reduce fault current.
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Also perhaps cut cores (such a C cores) could be used so that you could put in an air gap to prevent possible saturation during a fault. The thing is that you'd end up building a reactor in a different way, and so I'm not sure how much if any would be gained rather than using a reactor that's designed specifically for the purpose of current limiting.
 
Early Minuteman EMP protection filters needed to limit the few HUNDRED kA that would result from nearby nuclear strikes (aka SREMP) to a few kA to be able to not blow surge arrestors apart.

Used 40 turn air core inductor of solid busbar with heat sinking, the operating inductor current density was 2500 amps per square inch, potted in thermal potting compound on heat sink with high strengh restraint enclosure to keep from blowing apart from magnetic forces due to the hundreds of kA.

Could not use C core or other ferro material due to air gap and core size became same size as air core inductor due to saturation issues.

One alternative studied was use of iron wire on poles.

The major DC interties have the problem of how to dissipate the i^2*L energy of hte line due to an open. One solution used is a field of EHV towers strung with iron wire.
 
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synchro said:
Yes, a steel conduit is conductive and it's going to have significant eddy current losses (and therefore heating) since there will be nothing like the laminations in a transformer to break up circulating eddy currents.



Also perhaps cut cores (such a C cores) could be used so that you could put in an air gap to prevent possible saturation during a fault. The thing is that you'd end up building a reactor in a different way, and so I'm not sure how much if any would be gained rather than using a reactor that's designed specifically for the purpose of current limiting.
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OP stated he has no space for a line reactor earlier in thread, so next comes rather unconventional and possibly unproven methods of dealing with his problem ;)
 
Here's an idea:. What about sizing the conductors at their 90 degree ampacity, which would make them smaller. You would need to transition to the 75 degree sized conductors on each end for termination.

Another idea if this is a continuous load: use of a 100% rated breaker would result in smaller conductors as well.
 
electrofelon said:
Can you "re-evaluate" the wire size and maybe go a bit smaller? How was the two sets of 500 cu chosen? What is the equipment exactly? Is this for SCCR purposes?
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it's for a 800 amp meter bank with circuit breakers. So i need 2 sets of 500s for that
 
electrofelon said:
Here's an idea:. What about sizing the conductors at their 90 degree ampacity, which would make them smaller. You would need to transition to the 75 degree sized conductors on each end for termination.

Another idea if this is a continuous load: use of a 100% rated breaker would result in smaller conductors as well.
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not even sure if the terminals are rated 90 degree but good idea... would need 2 sets of 400s if this is the case.

We need main fuses to feed the meter bank
 
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