Environmental Exposures to ELF Georg Neubauer

Transcription

1 Environmental Exposures to ELF Georg Neubauer 1

2 Table of Content Environmental exposures to ELF sources Measurement methods Requirements on measurements Conclusions 2

3 Environmental exposure to ELF sources 3

4 Sources of electrostatic fields: Natural sources: Electrostatic field of the earth Ionosphere charged positively (More positive carriers of charge) Potential difference between ionosphere and earth about kv Field strength close to ground 100 V/m V/m (depending on weather, vegetation) During thunderstorms considerably higher for short periods Technical sources: All DC Applications e.g. underground, tramway Electrostatic fields can be easily shielded by conducting materials 4

5 Sources of static magnetic fields (1): Natural sources: Static magnetic field of the earth At the poles 62 T, at the equator 31 T, average latitudes 50 T Occurrence of iron in the ground can lead to local changes of the distribution of the field Lightning Strong magnetic fields for short periods Peak of lightning current up to 200 ka No static fields Harmonics up to the MHz range Distance B[μT] e.g. peak current of 100 ka 50 m 100 m 500 m 1000 m

6 Static magnetic fields (2) : Technical sources: All DC current sources MRI (typically 1,5 T in centre, μt in the close vicinity (Treatment - Room) E.g. tramway: not constant current (Start up current, load variation) in principle not a static field, variation versus time, but no change of vector orientation 6

7 ELF fields: Traction supply 16,67 Hz : Austria, Germany, Switzerland: 15 kv overhead contact wire voltage, about 100 A (about 1000 A start up current) Picture from: Elektromagnetische Felder und Wellen (EMF), Interaktive Lernsoftware CD-ROM, Technische Universität Graz, 7

8 Power supply 50 Hz (1) : 3 Voltage - Arrays: High voltage (110 kv -400 kv), medium voltage (10 kv-30kv), low voltage 400V (house connection) High and medium voltage: 3-phase system, low voltage 3 phase + neutral Power plant G 3~ Transmission lines (in urban areas long distance cable possible) Medium voltage transmission lines (in urban areas cables common) kv 110 kv kv 10 kv - 30 kv 400 V Low voltage distribution lines (cables common) 8

9 Power supply 50 Hz (3) : 3 phase system Not only one conductor, but in addition phase shift (time shift) of the currents on the conductors Resulting vector of magnetic field is rotating Rotary field Taken from: Elektromagnetische Felder und Wellen (EMF), Interaktive Lernsoftware CD-ROM, Technische Universität Graz, 9

10 Power supply 50 Hz (5) : Taken from: Elektromagnetische Felder und Wellen (EMF), Interaktive Lernsoftware CD-ROM, Technische Universität Graz, 10

11 Power supply 50 Hz (6) : Taken from: Elektromagnetische Felder und Wellen (EMF), Interaktive Lernsoftware CD-ROM, Technische Universität Graz, 11

12 Household Appliances (6) : TV-Set (Device with magnetic deflecting system) Directly nearby screen magnetic fields above limits possible Taken from: Elektromagnetische Felder und Wellen (EMF), Interaktive Lernsoftware CD-ROM, Technische Universität Graz, 12

13 Household appliances (7) : Taken from: Elektromagnetische Felder und Wellen (EMF), Interaktive Lernsoftware CD-ROM, Technische Universität Graz, 13

14 Household appliances (hair dryer & razor) (9) : Taken from: Elektromagnetische Felder und Wellen (EMF), Interaktive Lernsoftware CD-ROM, Technische Universität Graz, 14

15 Welding Applications Arc and resistance welding uses high electric currents (up to several thousands Amperes in case of resistance welding) These currents are provided by the welding power source, flowing through the torch cable and the torch to the welding arc or electrodes into the work piece and back via the ground cable Very strong magnetic fields can be produced close to the worker causing induced currents in the body of the workers 15

16 Welding Applications (2) Very wide range of current waveforms in welding technologies including DC, sinusoidal signals but also complex pulsed signals with a wide spectrum up to several khz A wide exposure spectrum requires frequency selective exposure evaluation (according to ICNIRP) A system which is able to make frequency selective exposure evaluation of transient signals is required 16

17 Measurement methods 17

18 ELF - Range Assessment of undisturbed field strengths (for comparison with reference values) Assessment of induced current densities (for comparisons with basic restrictions) Measurement Numerical Calculations Measurement only currents to ground, not for eddy current densities Numerical Calculations Broadband Frequency selective 18

19 Approaches based on ICNIRP guidelines Worst case method: additive without consideration of phase Consideration of phase between different spectral component 19

20 Hallsensor for measurement of ELF magnetic fields 20

21 Induction coils for measurement of ELF magnetic fields U IND = N A B t 21

22 Induction coils for measurement of ELF magnetic fields 22

23 Data Acquisition and Storage 23

24 Evaluation without phase content 24

25 Requirements on measurements 25

26 Requirements (1) 1. Preparation of measurement: selection & check of equipment (adequate dynamic & frequency range, adequate sampling rate & time response, plausibility test of equipment, check calibration, adequacy of measurement system for signal shape ) 2. Documentation before measurement: technical data of DUT (frequency, power, modulation, pulse shape, type of device, manufacturer, serial number, location), adjacent EMF sources, location and/or working area of workers, stage of operation.) 3. Check if environmental conditions are in line with specifications of manufacturer, e.g. temperature, humidity,.. 4. Identification of field zone to identify the need to measure both electric and magnetic fields 5. Start of measurement: prudent approach to source with non sensitive probe using the less sensitive range, start of measurement typically several meters away from source 26

27 Requirements (2) 6. Consideration of the impact of the engineers body on the measurement, avoidance of field distortions: the probe shall be located between the source and the technician or a tripod shall be used 7. Adequate distance between source and measurement system (e.g. RF sources: antennas can change their properties, i.e. impedance and antenna pattern due to the measurement system), avoiding of positioning of probes in the reactive near field 8. Adequate distance between measurement system and objects located nearby, e.g. wall, typically 1 m or more 9. Consideration of possible out of band responses of measurement system 27

28 Broadband measurement Set Up measurement position Field probe wooden tripod fibre optic link notebook shows field strength 28

29 Frequency selective Approach 29

30 Requirements (3) 10. Post processing: make uncertainty budget and include uncertainty in documentation (e.g. see pren 50413) 11. Verification of results by calculations and/or experience 12. The selected assessment methodology should be documented in the report/expertise including a rationale for the selected method 13. Documentation: identification of report, date and time of measurement, technician, location of the measurement (address, room, ), identification of DUT and applied measurement equipment, operating condition of DUT, environmental conditions, locations of measurements, uncertainty, results, date of calibration, 30

31 Example of an uncertainty budget 31

32 Validation of measurements by calculation 32

33 Conclusions Broad range of ELF sources with different complexity of signals exist in the working environment In most cases exposure well below the limits, however exceptions exist Local peak values should not be mistaken with averaged or median values, e.g. 1.4% of households below median of 0.2 T (Schüz et al 2000) To fulfil the requirements of the directive 2004/40/EC or the EU recommendation 1999/519/EC the selection of adequate measurement equipment is imperative Several international standards exist that can be applied to perform exposure evaluation Examples of fundamental requirements on adequate measurement procedures are reliable documentation, uncertainty budgets, well trained personnel, adequate calibration of the equipment.. 33

34 Thank you for your attention! 34