A modern megacity — is not only glass and concrete high-rises, but also a complex network of engineering systems. One of the city’s «invisible residents» has become electromagnetic fields. They cannot be seen or heard, which is exactly why they are especially insidious: exposure is long-term, cumulative, and often exceeds allowable sanitary norms.
A magnetic field of industrial frequency (50–60 Hz) is especially dangerous. Even weak but constant exposure can cause negative changes in the human body.
Every city resident daily enters the zone of action of many electromagnetic field sources:
high-voltage power lines;
cellular base stations;
TV and radio towers;
radar stations and high-power radio transmitters.
In other words, an invisible «network» surrounds us everywhere — from the entrance hall to the workplace.
In residential areas the main sources are:
overhead and cable power lines;
built-in or nearby transformer substations (TS).
The problem is especially acute in multi-story buildings where transformer substations are placed on the lower floors. In such cases the distance from the magnetic radiation source to living premises may be only a few meters.
A transformer substation — is a powerful anthropogenic source of electromagnetic radiation. In modern high-rises it supplies dozens of floors with electrical energy with minimal losses, but at the same time creates an elevated magnetic field level.
Studies show: the field of built-in substations rated 400–800 kV can exceed sanitary norms in neighboring apartments by 3–10 times. The main culprit — current-carrying conductors. Even if magnetic scattering from the transformer itself noticeably decreases at a distance of two meters, that is not enough for residents of the nearest apartments.
Reducing radiation by changing substation design is difficult, so the most accessible and effective solution becomes electromagnetic shielding.
An electromagnetic screen works as a shield and solves several tasks at once:
reduces radiation level to safe values;
protects equipment from interference and failures.
Among the consequences of magnetic field exposure:
headaches, insomnia, fatigue;
blood pressure swings, heart function disorders;
reduced immunity, chronic fatigue;
depression, irritability, memory decline.
Different countries have their own standards, but many of them rely on recommendations of the International Commission on Non-Ionizing Radiation Protection (ICNIRP).
For magnetic fields of industrial frequency (50–60 Hz), population limit levels in Europe — about 100 µT, while higher values are allowed for occupational exposure. In Russia and CIS countries sanitary norms are stricter — for residential premises the limit level is often restricted to 0.5–1 µT.
Shielding basics are based on the theory of electric and magnetic field propagation. Radiated energy is carried by the electromagnetic field. When it changes in time, the electric (E) and magnetic (B) components exist simultaneously, but their contribution may be unequal:
Near the source (within a small fraction of the wavelength) the field has a pronounced character: near current-carrying buses and cables the magnetic component dominates.
At large distances in free space the energies of E and B equalize, and the field behaves as a traveling wave.
There is another nuance of the urban environment: any conductor in a field not only absorbs but also re-radiates energy. Therefore near structural elements (rebar, cable channels, equipment housings) field distribution differs from «ideal» free space — spikes, local amplifications, «leaks».
The screen idea is simple: create a shell into which magnetic flux readily «goes» instead of penetrating living space. Ferromagnetic materials have high magnetic permeability: their resistance to magnetic flux passage is lower than that of air space. Thanks to this, flux closes within the screen itself (metal) rather than entering nearby apartments.
However, the degree of protection depends not only on the material but also on the design:
presence of doors, ventilation, or window openings can reduce effectiveness;
screen shape plays an important role for uniform magnetic flux distribution.
Interestingly, screens were first used not in residential buildings at all, but to protect military radio equipment from interference.
The most common protection method — passive shielding. It consists in installing screens of ferromagnetic materials in walls, partitions, and ceilings. Multilayer structures are sometimes used: the inner layer collects magnetic flux, the outer one — due to eddy currents additionally reduces the field and shields the electric component.
For effective protection the shielding shell must form a closed magnetic circuit without long gaps:
doors — with labyrinth or ferromagnetic seals;
ventilation — through honeycomb/labyrinth panels;
grounding — short, wide, with minimal inductance;
cable entries — through minimal openings or ferromagnetic feedthrough modules:
cables — balanced three-phase routes, laying «phase-phase-phase» as close as possible for mutual field compensation.
Recommended shapes for a passive screen — a box, cylinder, «bell», where flux density is distributed more evenly and local saturability is lower.
Active shielding (compensation) — a modern method in which additional coils or devices creating an opposing magnetic field are used.
Active protection consists of:
field sensors placed at control points of the protected zone;
compensating coils located around the room, along walls/ceiling, or as «frames» near main sources;
a controller that automatically maintains specified parameters.
The prospects of the latter approach are growing, yet in practice the correct choice of screen materials remains of primary importance. After all, in a typical building with a built-in substation the basic protection — is a passive ferromagnetic wall/partition screen with careful detailing of joints and penetrations.
In selecting materials for shielding, key parameters are:
magnetic permeability (initial and maximum);
saturation induction;
coercive force;
magnetostriction.
Technological characteristics are also important: machinability, weldability, corrosion resistance.
Permalloys — iron–nickel alloys with high magnetic permeability. Produced with different nickel contents; often alloyed with molybdenum, chromium, etc.
Key characteristics:
the main indicator of magnetic properties — initial and maximum magnetic permeability — the higher they are, the better the flux «collection» in weak magnetic fields;
coercive force — the lower it is, the smaller the hysteresis losses — losses associated with remagnetization of the material;
saturation magnetization should be as high as possible.
Permalloys work well under cold pressure processing; after mechanical deformation annealing is required to restore magnetic properties. They work excellently in both alternating and constant magnetic fields.
Application areas:
magnetic screens operating in weak constant magnetic fields — low-nickel permalloys;
cores of sensitive apparatus — alloys with the highest magnetic permeability in weak magnetic fields and saturation induction of 0.5ー0.75 T.
Permalloys have found use not only in transformer substation assemblies, where the source does not create ultra-high induction, but also in contactless relays, magnetic amplifiers, and other equipment requiring sensitivity to weak magnetic fields.
A high-quality iron–nickel alloy with high initial permeability and low coercive force. Suitable for the most sensitive systems and minimal magnetic fields.
Main physicomechanical properties:
low coercive force — magnetizes and demagnetizes easily;
low magnetostriction — minimal deformations under alternating magnetic fields;
high corrosion resistance — durability in service;
elevated magnetic permeability in weak magnetic fields at saturation induction from 0.65 to 0.75 T;
relative initial permeability of at least 100,000 at magnetic field strength ≈ 0.1 A/m — high sensitivity in weak fields;
spot welding during assembly of structures makes it possible to retain magnetic properties in service up to +200°C.
These qualities make supermalloy indispensable in manufacturing:
laboratory screens for low and very low fields;
precision sensors and instruments;
sensitive electronics where stability and long service life are important.
Supermalloy is expensive and demanding of assembly technology — every mechanical or thermal effect can reduce magnetic permeability, so production requires correct heat treatment and gentle joining methods.
Alloy 50N — a permalloy modification aimed at combining high magnetic permeability and elevated saturation induction.
Among the physicomechanical characteristics of the alloy, the following stand out:
high magnetic permeability — ensures good magnetic flux conductivity in weak fields:
initial permeability depends on material thickness; for example, for cold-rolled strip 0.35–1.0 mm thick it is ≥ 5 mH/m, and maximum ≈ 65 mH/m;
high saturation induction — up to 1.5 T (one of the highest among iron–nickel alloys), making it possible to use 50N in devices operating at high induction levels without magnetization bias or with slight bias;
low coercive force — low hysteresis losses, easy magnetization and demagnetization of the material;
saturation magnetostriction coefficient 25 W/(m·°C) — makes it possible to use the alloy where minimal linear dimensional change under a magnetic field is required.
Alloy 50N is suitable for high-induction conditions. Shield elements next to powerful transformers and current conductors are made from it.
Protecting people from electromagnetic fields — is not only care for comfort, but also a matter of the health of future generations. Modern residential complexes cannot be imagined without transformer substations, and therefore without effective shielding solutions.
Using alloys with high magnetic permeability — permalloys, supermalloy — remains today the most reliable way to combat unwanted radiation.
Materials that combine high physical performance and durability are already available on the market. For example, at PZPS you can buy cold-rolled strip of permalloys and supermalloy, which opens the possibility of introducing shielding technologies into mass construction.
Shielding — is not a luxury, but a real necessity for megacities, where every meter of space is worth its weight in gold, and care for health — is the top priority.
