Influence of Permanent Magnets With Network Cabling
By Paulo Marin
The objective of this paper is to offer a qualitative description of the effects and influence of magnetic fields of permanent magnets on network cabling. The term permanent magnet defines something that will always be a magnet such as a compass needle, which itself reacts to the permanent magnet in the earth's axis. In network cabling, these permanent magnets are typically used to maintain surface-mounting boxes attached to office furniture or similar surface where the Telecommunications Outlets are installed in the Work Area. The use of permanent magnets in structured cabling systems is outside of the scope of cabling standards and there are no TIA or IEC guidelines on the use of such hardware as it relates to structured cabling systems, to the best of our knowledge. In order to analyze the effects of magnetic fields on network cabling, a brief review of the concepts of electric and magnetic fields is offered. Interference effects caused by magnets are evaluated theoretically by considering them as small electric dipoles with the ability to carry electric current thus creating magnetic fields in the space around them. The level of influence of these magnetic fields on network cabling will depend upon the geometry of the magnet, the magnitude of its electric current, its proximity to the network cabling, and the variation of the magnetic field.
Structured cabling systems, electromagnetic interference, permanent magnets, noise coupling.
In order to discuss the effects of magnetic fields on network cabling it's necessary to review the concepts of electric and magnetic fields as well
Static electric charges set up an influence around them that is described as an electric field. At each point in space, an electric field value is assigned, referred to as electric field intensity E, measured in volts per meter (V/m). The amplitude of the electric field does not describe completely the influence of the electric charges. It is also necessary to assign a direction to the field at each point. The electric field is a vector quantity E, described fully when its amplitude and direction are given. The direction of E at each point is that in which a positive electric charge placed there would move. The electric field E(r), at a distance r away from a point charge q placed in the space is given by the formula
where, with q in coulomb, r is in meters, and ε0 is a constant. The electric permittivity of free space is ε0=8.8542 pF/m and then the electric field is obtained in V/m. The term point charge means a charge distribution occupying dimensions small compared to r and possessing spherical symmetry so as to look virtually like a charge concentrated at a point a distance r away from the point where the field is observed (see figure 1).
Figure 1. Electrical Field of Point Charge
When electric charges are moving with constant velocity to produce electric current, an additional influence is set up which is described as a magnetic field. At each point in space a magnetic field value is assigned, referred to as the magnetic flux density B, measured in tesla (T) or, equivalently, in Wb/m²(weber per square meter). The magnetic flux density is a vector quantity requiring in a full description both, an amplitude and a direction. Its value at a distance r away from a long straight conductor placed in the space and carrying a current I is
where μ0=4π.10-7 A/m is the magnetic permeability of a vacuum or, approximately air. The direction of B is perpendicular to the plane formed by the conductor and the observation point and is fully determined by the right-hand rule (see figure 2).
Figure 2. Right-Hand Rule
In summary, electric currents, which can be macroscopic currents in wires or microscopic currents inside permanent magnets, produce magnetic fields. These microscopic currents are associated with electrons in atomic orbits as in the case of permanent magnets. Magnetic field sources are essentially dipolar in nature, having a north and a south magnetic pole. The SI unit for magnetic field is the tesla. A smaller magnetic field unit is the gauss (1 tesla = 10,000 gauss). Figure 3 shows the magnetic field produced by a magnet bar.
Figure 3. Magnetic Field Produced by a Permanent Magnet Bar
The lines of magnetic field from a magnet bar are closed. By convention, the field direction is taken to be outward from the North pole and inward from the South pole of the magnet. Permanent magnets can be made from ferromagnetic materials¹.
The quantity known as the magnetic field vector or magnetic field intensity is denoted H and is related to the vector B defined by the force law through a constant of the medium known as the permeability, μ: B = μH.
¹ Ferromagnetic materials allow the microscopic ordering of the electron spin characteristic leading to the formation of the regions of magnetic alignment.
In SI units, force is in newtons (N). Current is in amperes (A), and magnetic flux intensity B is in tesla (T), which is a weber per square meter. Magnetic field H is in amperes per meter (A/m) and μ is in henrys per meter (H/m).
Magnetic Fields and The Geometry of Their Sources
The geometry of the magnetic source is a key-factor to determine its field as well as its intensity and the effects on devices or circuits placed in the space around the source. This papers' focus is on two classical elements: the wire loop and the solenoid.
Figure 4 depicts the configuration of the wire loop. Electric current in a circular loop creates a magnetic field that is more concentrated in the center of the loop than outside of the loop. Examining the direction of the magnetic field produced by a current-carrying segment of wire shows that all parts of the loop contribute magnetic field in the same direction inside of the loop. Stacking multiple loops concentrates the field even more into what is called a solenoid.
Figure 4. Wire Loop and its Magnetic Field
Figure 5 shows the configuration of a solenoid.
Figure 5. Solenoid and its Magnetic Field
A long straight coil of wire can be used to generate a nearly uniform magnetic field similar to that of a bar magnet (see figure 3). Such coils, called solenoids, have an enormous number of practical applications. The field can be greatly strengthened by the addition of an iron core. Such cores are typical in electromagnets. In solenoids, the magnetic field is concentrated into a nearly uniform field in the center of a long solenoid. The field outside is weak and divergent.
Interference From Magnetic Fields
Magnetic field lines can potentially interfere with several electric or telecommunications systems. An electric current may be induced into a wire, a pair of wires or a cable installed in close proximity to magnetic field lines. Likewise, voltage can be coupled into a conductor or pairs of conductors when they are placed within an environment with significant electromagnetic interference. There are three important laws of physics that explain the relation between electrical currents and magnetic fields and voltages and magnetic fields as well.
The source of a static magnetic field can be a permanent magnet, a linear magnetic field variant in time or a direct electric current. Consider an unbounded, homogeneous, isotropic medium with a small line element of length dl' carrying a current I' located at a point in space defined by the vector r' from an arbitrary origin as in Figure 6. The magnitude of the magnetic field at some point P in space defined by the vector r from the origin is:
Figure 6. Relation Between Magnetic Field and Electric Current — Biot-Savat Law
where R=[r—r'], the distance from the current element to the point of observation. The angle Φ is that between the direction of the current defined by dl' and the vector R = r - r' from the current element to the point of observation. The direction of the magnetic field at the point P is perpendicular to the plane containing the element of length of the conductor (or wire) and its intensity depends upon the distance from the conductor carrying the differential current element. Its sense is determined by the right-hand rule. Considering the current direction shown in figure 6, the sense of the magnetic field at point P is outward from the page.
In summary Biot-Savart Law presents the magnetic field intensity produced by a differential element of current. In some aspects, Biot-Savart Law shares some similarities with Coulomb Law, which is written for elements of charge (see equation 1). Both show a law of "inverse of squares" regarding the dependence of the distance and both show a linear relation between the source and its field.
Biot-Savart Law cannot be experimentally validated. This is because a differential current element does not exist in real life. This is only used as a theoretical tool to make the calculations easier. In practice, it's necessary to know the contribution of the total current that flows through a circuit and produces a magnetic field in the space in its proximity. Thus, Ampere's Law is used to calculate the total magnetic field intensity of the total current that flows along a current path or closed circuit, or mathematically.
In simple words, Ampere's Law allows us to determine the magnetic field intensity (H) at a point P in space, from the current flowing in a closed circuit. This magnetic field intensity can then be measured in practice.
Faraday's Law is quite complex and can be fully described by Maxwell's Equations. In simple words, Faraday's Law explains how a variable magnetic field produces an electromotive force (emf) that can generate a current into a closed electric circuit. An emf is then a voltage induced into the conductors that displace within a magnetic field or created by a variable magnetic field. Figure 7 shows the creation of an emf inside a closed circuit due to a variable magnetic field. In this case, the variation is obtained by the displacement of a permanent magnet inside a solenoid.
Figure 7. Creation of an emf Due to Variable Magnetic Fields
Likewise, a variable magnetic field can be created when a coil of wire is moving into a constant magnetic field in an environment as depicted in Figure 8.
Figure 8. Creation of an emf by Moving Wire Coils Into a Constant Magnetic Field
Magnetic fields in permanent magnets concentrated on the magnets North and South poles and are very weak and divergent elsewhere. It means that the influence of a magnet on network cabling will depend upon a number of factors like the proximity of the cable to the magnet, the angle formed between a given conductor and the magnetic field lines, and the variation of the magnetic field in the space where both interfering and interfered conductors or systems are placed in space.
EMC Standards for Conducted and Radiated Interference
In the U.S., the Federal Communications Commission (FCC) is responsible for radio, communications and interference. FCC regulations concern conducted and radiated interference and refer to two classes of equipment. Class A equipment are for use in commercial, industrial, and business premises, whereas Class B equipment are intended for residential use.
- Conducted interference covers frequencies ranging between 450 kHz and 30 MHz. Regulations aim at controlling interference current in power leads.
- Radiated interference limits are set on measurements taken using resonant dipoles at a distance of 3 m for Class B and 10 m for Class A from the equipment under test. Measurements of the electric field (in dBmV/m) are taken in an open-field site with antennas scanned in horizontal and vertical polarizations and with the equipment under test rotated to obtain the maximum emission.
Thus, emissions caused by permanent magnets are not limited or regulated by FCC. So, there is no normative reference to evaluate those emissions.
Permanent magnets are the most familiar sources of magnetic fields. A compass needle is a permanent magnet, which itself reacts to the permanent magnet in the earth's axis. Unfortunately, the fields of permanent magnets are very hard to calculate, and require an understanding of complex ferromagnetic phenomena, belonging as much to atomic theory as to electromagnetism. Here we will simply give a qualitative description of the effects of magnetic fields of permanent magnets on network cabling.
According to the several laws of physics presented and discussed here we can conclude that the influence of permanent magnets with network cabling is minimum and is not an issue of concern.
First of all, the influence of a magnetic field is strongly dependent on distance and its intensity is inversely proportional to the square of the distance between the interfered system and the interfering source.
Second, the distribution of magnetic fields in the space around a permanent magnet is very well defined being stronger and concentrated at its North and South poles and weak and divergent elsewhere. So, the position of the interfered element in regards to the magnetic fields produced by the magnet (interfering source) is critical.
Third and most important — in order to have interference between two elements due to a magnetic field it's necessary to have a variable magnetic field in the space where both, victim and source are placed, according to the Faraday's Law. In the case of permanent magnets and network cabling this condition is not met, as there is no variable magnetic field in the space where they are contained; both elements are kept in static and fixed positions after installation of the cabling system.
Principles and Techniques of Electromagnetic Compatibility, Christopoulos, Christos, CRC Press - 1995
Fields and Waves in Communication, Ramo, Simon; Whinnery, John R.; Van Duzer; Theodore, Electronics, Third Edition, John Wiley & Sons - 1994
Electromagnetism, 3rd Edition, William H. Hayt Jr., McGraw-Hill, Inc. 1981
About the Author
Paulo Marin serves as the Regional Technical Manager for the SIEMON CALA Office (Caribbean & Latin America), whose office is located in Miami, FL - USA. Marin is responsible for technical support and technical training programs, presentation in seminars, conferences and symposiums in that region. Marin has also supported the Siemon Iberia Office (Spain and Portugal). Paulo Marin holds an electronics engineer title, a post graduation title in telecommunications, a master degree in electrical engineering about signal propagation over balanced cables, and a doctor degree in telecommunications engineering with a study about interference in structured cabling systems.
Dr. Marin is an active IEEE member (Institute of Electrical and Electronics Engineers) in the U.S. and has taken place as speaker in a number of conferences about tranmsission impairments and EMI/EMC issues in The U.S., CALA, and Iberia in behalf of Siemon.
As a BICSI member for about seven years, Marin occupied the positions of BICSI Brazil District Secretary (1999), BICSI Brazil District Chair (2000), was member of the BICSI Brazil Steering Committee (2001-2002), and Regional Director (2003). Marin has been actively involved in the revisions and development of BICSI literature in the U.S. and other countries as well.
Dr. Marin was Regional Director of ABRAPI in Brazil (Brazilian Association for Intelligent Buildings & Automation) in 2001. Paulo Marin is a telecommunications cabling specialist engineer for about 16 years, and has worked with installations, design, consulting, and training in this field.
Paulo Marin has been invited to present in a number of conferences throughout the region about telecommunications cabling and EMI/EMC. Marin has written several technical articles for technical magazines in the region. Marin has been a columnist of RTI, an important Brazilian telecommunications magazine for about six years. Dr. Marin is currently the chair of the committee CE 03:046.05 (ABNT/COBEI) responsible for the development of the up-to-date Brazilian telecommunications cabling standard. ABNT/COBEI is a representative of ISO/IEC in Brazil.