BS/EN 50647-2017 pdf download.Basic standard for the evaluation of workers’ exposure to electric and magnetic fields from equipment and installations for the production, transmission and distribution of electricity.
When assessing exposure in a particular area, it is normally sufficient to identify only the main EMF sources that affect that area. If the workers are very close to – or in contact with – one EMF source, any other sources located further away can generally be neglected (examples: HV live-line working, GIS inspection, etc.).
For working environments where different sources are at comparable distances to the workers (examples:
low voltage environments. where the three phases are close to each other), the simultaneous contribution of all such sources should be considered.
The worst case exposure of workers shall be determined. As the exposure may vary as a function of the operating conditions, it may be necessary to extrapolate the exposure to worst case exposure conditions. Magnetic fields are generated by currents in electric equipment, such as lines, cables, coils and windings. The current may vary significantly over time and thus so will the magnetic field. This means that extrapolations of measured or calculated values will often be relevant when considering magnetic fields. Electric fields are generated by non-shielded electric equipment, such as overhead lines or busbars in substations. The voltage does not vary significantly over time and neither does the electric field at a given point, provided it remains unperturbed i.e. not affected by any moving obecL Therefore, extrapolations of measured or calculated values are not normally relevant when considering power frequency electric fields. except under special circumstances such as a change in maximum operating voltage.
Waveforms of electric and magnetic fields at 50 Hz are not always purely sinusoidal but may contain significant harmonic components, which have to be taken into account in the exposure assessment when relevant. See 6.2.2 and 6.2.3.
6.2 Exposure assessment regarding external fields
6.2.1 General
External fields may be assessed by measurements and/or calculations.
Field meters shall be compliant with EN 61786-1.
This standard does not specify any specific measurement protocol, because of the diversity of the exposure situations expenenced by workers. The measurement protocol shall be compliant with IEC 6 1786-2.
For small distances to a magnetic field source, where the magnetic field is highly non uniform, the size of the measuring probe may introduce an averaging bias and therefore may underestimate the actual field. Therefore the size of the probe shall be appropriate to the spatial variation of the field. Guidance is given In the measurement standard IEC 61786-2.
Magnetic field measurements should not be performed for assessing compliance for dislarices to field sources smaller than the measuring probe.
Measurements and calculations of electric fields are much more complex than for magnetic fields, as the electric field is perturbed by all conducting objects, even poorly conducting objects, and particularly by metallic structures, This is known as the peak effect and It results in strong local enhancements of the electric held. The strongly non-uniform field resulting from the peak effect is not representative of the exposure of a worker. That is why, In order to measure the unperturbed field. IEC 61786-2 recommends:
measurements. IEC 61786-2 recommends a minimum distance of 1 m when possible. However, a measuring distance of 1 m is not apphcable to many work situations wtch involve contact to metallic structures, such as workers climbing In lattice steel electrical towers. In this case, a minimum distance of 20 cm shall be maintained between conducting objects and the measurement probe to reduce the peak effect measurement bias.
6.2.2 Harmonics of magnetic field
For transmission and distribution systems it is acceptable to perform a simplified assessment of harmonics, except around supplies for industry operating high power converters and rectifIers Annex A proposes different approaches for assessing harmonics.
For simplified assessment of harmonics, the total exposure index is allocated partly to the exposure index for 50 Hz and partly to the sum of exposure indexes of harmonics. A total exposure Index, including the fundamental and all harmonics, below I demonstrates compliance. Criteria for the 50 Hz exposure index, depending on the nominal voltage, are given in order to demonstrate that the total exposure index remains lower than 1. Alternative values may be used if based on specific assessment of harmonics.
Criterion for voltage range 60 kV
Based on available field measurements and current measurements, the harmonics of magnetic field at voltage levels higher than 60 kV can be neglected because the maximum harmonic content of currents in transmission and distribution systems Is limited during normal operations.
NOTE Circumstances where significant harmonics can our, such as resulting from saturation of transformers during energizrng or dunng geomagnetic storms, are too rare to need considenng further.
— Criterion for voltage range I kV to 60 kV
Based on available field measurements and current measurements from medium voltage distribution and transmission systems. the harmonics up to the 50th might be relevant, but the exposure is compliant if the exposure index for 50 Hz Is below 0,8 (see Annex A). The same criterion can be applied for broadband measurements.
— Criterion for low voltages (<1kV)
Based on available field measurements and current measurements from low voltage distribution systems, the harmonics up to the 50th might be relevant, but the exposure Is compliant if the exposure index for 50 Hz is below 0,7 (see Annex A). The same criterion can be applied for broadband measurements.
For power plants, assessment of harmonics may be limited to the excitation unit.
6.2.3 Harmonics of electric field
The harmonics of electric field at any voltage level can be neglected because the maximum harmonic content of voltage in transmission and distribution systems. which is limited by voltage quality requirements. usually increases the total exposure index only within the range of the expected uncertainty of assessment.BS/EN 50647-2017 pdf download.

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