Electromagnetic Shielding (Mu-Metal)
Mu-metal is a high-permeability nickel-iron alloy used to reduce static and low-frequency magnetic fields by guiding magnetic flux around a protected area. Its performance depends on the field, alloy condition, shield shape, seams, and openings. Start by measuring the field and setting a target; then design, install, and test the shield against that target.
If a magnetic-field reading is causing concern, a carefully planned shield can be a low-maintenance option: it has no moving parts, though it still needs inspection and performance checks. The key is to identify what kind of field you need to reduce. A design made for radio-frequency interference may not work for a static magnetic field, and a material name alone cannot guarantee a result.
I use a measured, testable plan rather than a generic promise of “shielding.” The steps below explain how to choose and install the alloy, avoid common design errors, and verify the finished assembly.
Define the field and the reduction target
A magnetic shield redirects flux, the path taken by a magnetic field. How much it reduces the field depends on the field’s strength, direction, and frequency, as well as the shield’s shape and condition. There is no universal sheet thickness or shielding factor that suits every installation.
First, decide what needs protection and where the field must be reduced. Measure at that point, not just beside the suspected source. Record the field’s magnitude, direction, and frequency when known. Use a suitable, calibrated magnetometer or gaussmeter, and note the instrument, settings, and conditions.
Set an acceptance target before designing. For example, specify the maximum allowed field at a sensor, or the required ratio between readings before and after installation. A target such as “reduce the field by half” is measurable; “remove interference” is not.
Also identify the type of problem. Static fields and low-frequency magnetic fields need a different approach from radio-frequency (RF) or electric-field interference. Mu-metal is not a universal fix for every kind of electromagnetic interference.
ASTM A753/A753M covers specified nickel-iron soft-magnetic alloys. It can help define procurement requirements, but citing the standard alone does not establish how a finished shield will perform. Specify the alloy grade, product form, thickness, required magnetic-property condition, and supplier certification.
Choose the material and plan the geometry
Geometry means the shield’s shape, panel layout, and openings. These features can matter as much as the sheet itself: gaps, doors, cable entries, and ventilation openings can let magnetic flux reach the protected area. Plan them around the field source and the required access.
Do not buy material based only on a listing that says “mu-metal.” Ask the supplier for the specific grade and product details, and for guidance on fabrication and any required final heat treatment. Magnetic properties can be affected by forming and other mechanical stress. Do not assume that a panel will retain its intended performance after it has been cut, bent, drilled, or welded.
There is no code-prescribed universal clearance between a shield and the equipment it protects. Establish spacing and overlap through a design review, modeling, or a prototype test. Keep the installation practical: do not block access to electrical equipment, required ventilation, egress, or service areas. Check locally adopted codes and the equipment maker’s instructions.
| Situation | Design point to check | Practical next step |
|---|---|---|
| Static or low-frequency magnetic field | Source direction, field at the protected point, and shield shape | Measure first; test a proposed design |
| RF interference | Frequency range, seams, and openings | Select a design and test method suited to RF |
| Enclosure with a door | Door edges and repeatable closure | Measure with the door in its normal position |
| Shield near powered equipment | Heat removal and service access | Check operating temperature and clearances |
Ordinary steel, conductive foil, and a grounded sheet-metal box are not guaranteed substitutes for a designed high-permeability magnetic shield. They may help in some settings, but do not assume they will meet a low-frequency magnetic-field target.
Fabricate and install without harming the alloy
Fabrication is the process of cutting and shaping material into the planned assembly. For high-permeability alloys, the supplier’s instructions matter: mechanical stress and unsuitable processing can change magnetic performance. Plan cuts, holes, seams, and any final treatment before installation.
A practical sequence is:
- Survey: Map the source and protected area. Record baseline readings and identify whether the issue is static, low-frequency magnetic, RF, or electric-field interference.
- Design: Set the target and plan panels, seams, access, and penetrations. Model the design or build a test section if the result is uncertain.
- Specify: Purchase the alloy with a clear grade, product form, thickness, magnetic-property condition, and supplier certification. Agree on fabrication and any required final anneal with the supplier.
- Fabricate: Minimize forming, drilling, welding, and other stress after final magnetic treatment. Do not substitute a generic annealing temperature or process.
- Install: Use the overlap and seam design that was tested or approved. Treat doors, fasteners, cable openings, and vents as possible leakage paths.
- Finish and document: Use a corrosion-protection finish compatible with the alloy and its treatment. Keep material certificates, fabrication records, and installation drawings.
Grounding does not replace magnetic shielding. If the assembly is an electrical enclosure or has parts that require bonding, apply the relevant electrical rules to that installation. For example, NFPA 70 (the National Electrical Code), Section 250.96(A), addresses bonding metal raceways and enclosures. Do not assume every magnetic shield must be grounded simply to improve low-frequency attenuation.
Learn from two design examples
These examples show a method, not guaranteed performance. Actual results depend on the field, material, geometry, and installation. I use examples like these to show why measurements and a clear acceptance target are more useful than relying on a material label.
Sensor beside a magnetic source: Suppose a sensitive sensor has a measurable field at its normal operating point. The team records the baseline, maps the field direction, and sets a maximum field for the sensor. It then tests a proposed shield with the sensor in the same location. If the target is missed, the design is reviewed for geometry and leakage paths before changing thickness or adding panels.
Enclosure with access and ventilation needs: Suppose equipment needs a shield but also needs airflow and regular service. The design team tests the enclosure with the door closed and includes the planned vents and cable entries. If the magnetic result is acceptable, it still checks equipment temperature and confirms that access remains safe. A shield that reduces the field but causes overheating is not a successful installation.
In both cases, the important point is to test the complete assembly. A flat sample of material cannot show how a door seam or cable opening will affect the finished shield.
Verify the installed shield
Verification means repeating the field measurement after installation and comparing it with the baseline. Use the same measurement points, instrument orientation, source condition, and instrument settings. Include doors and access panels in their normal operating state.
For magnetic-field amplitude, calculate reduction in decibels (dB) as:
Reduction (dB) = 20 log₁₀(Bbefore / Bafter)
Here, Bbefore is the baseline field reading and Bafter is the reading after installation. For example, a ratio of 2 corresponds to about 6 dB of reduction. This calculation describes the measured ratio; it does not mean the result will be the same at other points or frequencies.
Record the instrument model and calibration status, measurement conditions, readings, and whether the target was met. If the shield has seams, penetrations, or vents, include them in the test. A reading taken only at an easy-to-reach spot may miss a local leak.
IEEE 299-2006 provides test methods for shielding effectiveness of enclosures over 9 kHz to 18 GHz. That range does not establish performance for static fields or frequencies below 9 kHz. Select a measurement method suited to the field you are testing; do not use an RF result as proof of static-field performance.
Common questions about high-permeability magnetic shields
These answers cover common choices and checks for a magnetic shield. The right design still depends on the measured field and the protected location. Where the installation involves electrical equipment, confirm applicable code and equipment requirements with qualified professionals.
What does mu-metal do?
It guides magnetic flux around a protected area, which can reduce the magnetic field inside. The amount depends on the alloy, field, shape, and openings.
Does it block all electromagnetic interference?
No. It is used for magnetic-field shielding, but it is not a universal solution for RF, electric-field, or every other interference problem.
What thickness should I use?
There is no universal thickness. Choose it through a design based on measured conditions, geometry, material data, and testing.
Does the shield need to be grounded?
Not solely to improve low-frequency magnetic attenuation. Grounding or bonding may be required for electrical safety, depending on the equipment and installation.
Will ordinary steel or foil work instead?
Do not treat them as guaranteed substitutes. Their usefulness depends on the field and design, so test the actual assembly against a defined target.
Can I cut or bend treated material on site?
Do not assume its performance will remain unchanged. Ask the supplier about approved processing, and verify the finished assembly.
Do seams and doors matter?
Yes. Joints and openings can affect the field inside. Include them in the design and test with doors and panels in normal use.
How do I calculate reduction in dB?
Use 20 log₁₀ of the baseline field divided by the field after installation. Compare readings made under the same conditions.
Does IEEE 299-2006 cover static magnetic fields?
Its stated enclosure test range is 9 kHz to 18 GHz. It does not establish performance for static fields or frequencies below 9 kHz.
What records should I keep?
Keep material certificates, fabrication and treatment details, drawings, instrument and calibration information, field readings, and the acceptance result.
A reliable shield begins with a clear field measurement and a specific target. Specify the alloy fully, protect its magnetic condition during fabrication, and test the completed installation at the protected point. That process makes performance easier to assess and maintenance easier to plan.
(This article was written by one of our staff writers, Steven Langford. Visit our Meet the Team page.)
