Clearance of magnetic material from a high H field

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Zifkwong

Member
Does anyone know of any codes, standards, or common work practices for how far away magnetic material should be from a high current source.

I would like to determine how far away from a piece of carbon steel I have to be with approximately 40,000 Amps.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't think there is any requirement at all. In fact, enclosing circuit conductors inside a magnetic material such as conduit is the norm.

It may be that in specific cases you would not want to do so, but in general the answer is no code prohibits it.

Now for the kind of current you are talking about, the magnetic field would be substantial. That might introduce some design considerations you would need to keep in mind.
 

cpal

Senior Member
Location
MA
Zifkwong said:
I would like to determine how far away from a piece of carbon steel I have to be with approximately 40,000 Amps.


The field will diminsih expotentially for evey foot away from the conductors , some one will have the numbers I'm sure!

I would like to know what type of installation you are involved with 40kA is a significant installation, what is the operating voltage.????
Charlie
 

Zifkwong

Member
It is for an electric arc furnace.

I know that the B field is decreases in strength by r^2, r being distance from current source.

I also know that if you have a pair of channel-locks in your pocket and approach the cables to take a current reading....you get a nice brand through you FR pants.

Does anyone know what acceptable B and H field strengths are to avoid heating of carbon steel to high temperatures?
 

karl riley

Senior Member
I don't know of any standards, unless OSHA has them. Is this AC or DC amps? Must be AC if it induces current on the tool in your pocket. There would be a worker health standard for such high magnetic fields leading to heating of tissue. I would contact OSHA.

Karl
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
Zifkwong said:
..if you have a pair of channel-locks in your pocket and approach the cables to take a current reading....you get a nice brand through you FR pants.
If alternating current field Hysteresis is heating nearby tools, I wonder what it does to your red blood cells (iron), or the amalgam fillings in your mouth? Might make a good chemotherapy, or irradiation alternative for cancer or aids patients, or it might just cook you like a microwave, from the inside out.
Zifkwong said:
Does anyone know what acceptable B and H field strengths are to avoid heating of carbon steel to high temperatures?
Strapping some current xfmrs (CT) around that cable may feed a seperately derived system, with these kind of magnetics fields, and dampen the cable field where covered by the CT.

Otherwise, try locating some surge-arrestor specs. for distribution lines, to provide some guidelines for proximity of MV power lines. These bell shapped insulators on pwr poles & substation xfrms are stacked to various hights by voltage to provide specific distance from equip..
 

RayS

Senior Member
Location
Cincinnati
Zifkwong said:
Does anyone know what acceptable B and H field strengths are to avoid heating of carbon steel to high temperatures?

I'm guessing the actual steel temp. will have a lot to do with ambient, heatsink properties of steel etc. You may want to get a few "tempilstick" temperature crayons to mark the steel with- it is an easy way to verify that it has not exceeded the crayons melt temperature. They are available in a variety of temp. ratings. I'm thinking steel strength starts to go down at about 400deg. F.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
Yes, this issue may violate NEC 300.35 and 300.20 "heating surrounding metal by induction." If hysteresis fields are heating nearby equipment, what is it doing to the raceway?
 
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ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
If you wan't to sheild any equipment, special paint / barriors may be available.
"Sheilding EMI at high frequencies is relatively easy. We can use aluminum foil to shield the conductors and special paints are available for exclosures."
Loyd,"Electrical Raceways and Other Wiring Methods, 4th ed"., Delmar 2002, p.20.

The GEMI modeling application shows ferrous conduit or enclosures absorb all EMF's, and 40,0000 amps --not enclosed-- generates less than ~ 0.1 Telsa, 8 centimeters beyond any conductor surface.

The best EMF safety guide and qualified description of AC & DC field hazards may be The AAPM/RSNA Physics Tutorial for Residents, which describes the inductive heating of your tools under RF Burns, Time-varying Gradient Magnetic Fields, with the FDA guidelines shown in Fig.6.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
ramsy said:
If alternating current field Hysteresis is heating nearby tools, I wonder what it does to your red blood cells (iron), or the amalgam fillings in your mouth? Might make a good chemotherapy, or irradiation alternative for cancer or aids patients, or it might just cook you like a microwave, from the inside out.
One of the things that struck me when plowing through a lot of the NIHS EMF references and abstracts was, unlike many forms of pollution, denser, stronger EMF does not correlate with greater physiological effects. The studies, in general, showed that there are bands, or windows (if you will), of effect at certain combinations of electric field, magnetic field and frequency, and that, strikingly, the physiological effects at low field densities equaled or exceeded the effects at other higher windows of field density, while whole portions of EMF densities, as yet, produce no observed physiological effect.

Now, of course, microwave cooking is one of the exceptions to my generalization, here.

And think of MRI, magnetic resonant imaging. A technology that uses a magnetic field so intense that it excites the electrons of certain atoms in the body, notably potassium, to an elevated state that decays shortly after excitation. When the decay occurs, the released energy appears as a radio transmission that is then mapped in three dimensions, providing higher resolutions than other scans. As yet, I have not heard of side effects from MRIs.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
Zifkwong said:
There are no raceways nearby. It is open bus..
If people work on any piece of carbon steal near that 40,000A open bus, doesn't NFPA 70E limit personell proximity to energized equipment, without PPE, way beyond the point tools would encounter inductive heating?

OSHA 1910.333(c)(3)(ii) shows some strange proximities without PPE.
Code:
      TABLE S-5 - APPROACH DISTANCES FOR QUALIFIED
                  EMPLOYEES - ALTERNATING CURRENT
______________________________________________________________
                                  
   Voltage range (phase to phase) | Minimum approach distance
_____________________________________________________________
                                  
300V and less ....................| Avoid  Contact
Over 300V, not over 750V .........| 1 ft. 0 in. (30.5 cm).
Over 750V, not over 2kV ..........| 1 ft. 6 in. (46 cm).
Over 2kV, not over 15kV ..........| 2 ft. 0 in. (61 cm).
Over 15kV, not over 37kV .........| 3 ft. 0 in. (91 cm).
Over 37kV, not over 87.5kV .......| 3 ft. 6 in. (107 cm).
Over 87.5kV, not over 121kV ......| 4 ft. 0 in. (122 cm).
Over 121kV, not over 140kV .......| 4 ft. 6 in. (137 cm).
__________________________________|___________________________
These distances (perhaps intended for linemen working on overhead lines) seem awfully close. My copy of OSHA 1910 is dated 1990 & 1994, and may not include the more recent amendments referring to NFPA 70E.

My dated copy of 1910.333(c)(4-10) describes other restrictions not related to overhead lines. 1910.333(c)(6-9) specifically prohibits possession of conductive artifacts near energized parts if there is a "possibility of contact." This might apply if people are expected to maintain, handle, or perform "housekeeping duties" (9) on any piece of carbon steel near that 40,000A open bus.
 
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