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EMC: MAGNETIC FIELD MEASUREMENT


❱❱ A combination of field meter


and magnetometer probe offers a straightforward approach to measuring magnetic fields


Probe measures up to magnetism


Measuring the magnetic fields generated by equipment can be an important part of compliance testing. Hugo Bibby of Link Microtek takes a closer look


force in 2016, they introduced, for the first time, mandatory limits on workers’ exposure to electromagnetic fields (EMFs). While the responsibility for ensuring workers’ safety rests with employers, equipment manufacturers also have a duty to ensure that their products are safe in the workplace and to provide sufficient information about the emissions from these products. Similarly, all consumer


W


products should at least comply with the general public reference levels laid down by ICNIRP (the International Commission on Non-Ionising Radiation Protection). The regulations place specific


limits on various types of EMFs over different frequency ranges up to 300GHz. At low frequencies – and especially where high levels of current are involved – the effects of magnetic fields are likely to be


hen the Control of Electromagnetic Fields at Work Regulations came into


the dominant concern, so it is incumbent on manufacturers to measure the fields generated by their equipment. As well as being a statutory


requirement, compliance with emissions limits helps to prevent hazards due to the indirect effects of strong magnetic fields, such as projectile risk or interference with active implanted or body-worn medical devices.


METER AND PROBE COMBINATIONS When it comes to measuring magnetic fields, one of the simplest and most effective methods is to use a combination of field meter and magnetometer probe. The NBM-550 meter and HP-01 magnetometer, which are both manufactured by Narda Safety Test Solutions, are examples of these. The two devices work in tandem and can be either attached directly to one another or connected via a choice of fibre-optic


cables. Suitable for compliance measurements in a test laboratory or on-site measurements in an


industrial environment, the NBM-550 has the capacity to


store up to 5,000 results, with each one automatically timestamped. It also includes a facility for recording voice comments, which are stored with the corresponding measured values. All data can be transferred to a computer and managed using the PC software supplied, making it easy to generate measurement reports. Attaching an HP-01


magnetometer probe to the meter provides the user with a complete system for magnetic-field measurements. An intelligent probe interface in the NBM-550 automatically retrieves calibration data from an EPROM in the probe, enabling the instrument to achieve its full calibrated accuracy without the user having to make any adjustments at all. The HP-01 probe itself offers a wide


measurement range of 10µT to 10T and is capable of measuring fields with frequencies from 0Hz (DC) to 1kHz. As an isotropic probe, its three mutually orthogonal Hall-effect sensors enable the field to be determined with a single measurement in any direction. Whenever safety measurements are being made, it is important that such isotropic probes are used rather than single-axis probes. Since the limit values in safety standards


are usually specified for static fields and time-varying fields with frequencies up to 1Hz, the magnetometer facilitates compliance measurements via a built-in FIR filter that captures only frequency components in the 0Hz to 1Hz range.


EV TESTING These capabilities make the HP-01 magnetometer particularly well suited to DC testing applications – for example, electric vehicles, which can generate strong magnetic fields if they are drawing high currents from the battery. If there is a requirement to measure the


magnetic fields associated with charging EVs, particularly with the transients involved, then another isotropic probe, the EHP-50F, would be used instead. With its frequency range of 1Hz to 400kHz, this device is ideal for measuring fields associated with both wired charging and low-frequency wireless power transfer (WPT). In fact, the NBM-550 can be fitted with a variety of different probes to cater for measurements of both magnetic and electric fields at frequencies all the way up to 90GHz, which makes it a versatile tool for use in the test environment. T&TH


April 2020 /// Testing & Test Houses /// 33


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