BS EN 62489-2:2014 - 8 - IEC62489-2:2014@IEC2014 c)However,staff can come into close proximity of theloop conductorwhile the system is workingunlessstepsaretakentomaintainaminimumseparation. NOTEThese loops often have more than one turn, so that the loop current can be kept reasonably small. 5.5 Small-arealoops For these, again, there are three considerations. a) The separation for portable loops is very much greater than the loop dimensions, but for maintain a minimum separation. Both users and staff can come into close proximity of the loop, even that of a portable C) system. NOTE These loops usually have many turns, so that the actual current is not so large 5.6 Solenoid antennas in Figure 1. A very small solenoid is incorporated in the stem of the device. IEC Figure1-An ear-hook inductiontransducer,with a BTE (behind the ear) hearing aid body for scale 6 Calculations 6.1 General Calculation of the field strength can be reliably made using Equation (1) in almost all cases except where the loop is very small or is a solenoid of length which is not very small compared with its plan dimensions, such as for the ear-hook device. It is necessary to calculate the current required in the loop to produce a field strength of 4oo mA/m at the hearing-aid position, taking into account the orientation of the pick-up coil in the hearing-aid relative to the plane of the loop. In general, this calculation is not easy,but simple, approximate methods give sufficiently accurate results when used with insight. Proprietary calculation software,based on published mathematical analyses,exists. General-purpose mathematicssoftwarecanalsobeused. Translating the calculated field strengths into a form comparable with exposure guidelines or limits is considered in Clause 8. BS EN 62489-2:2014 IEC62489-2:2014IEC2014 -9 6.2 Solenoid antennas There is no simple expression for the field strength at a point outside a cylindrical solenoid. A solenoid maybe treated as a stackof loops,or as amagneticdipole,orthefield strength can be calculated by means of a rather complex equation (see [2]). Measurements 7 7.1 General In the audio-frequency range, exposure time is irrelevant, because the predominant physiological effect, if it occurs, is nerve stimulation, which operates over a time-scale of a fewmilliseconds.Itisthereforeappropriatetousea quasi-peak measurement of field strength. Furthermore, exposure limits and guidelines are given in r.m.s. values, so the quasi- peak meter should be scaled to read r.m.s. values with a sinusoidal signal. This type of meter. thepeakprogrammemeter(PPM),is furtherdescribedinIEC60118-4andIEC60268-10 (type Il) already. It is also necessary to consider the type of magnetic field pick-up coil or sensor. Sensors may be single-axis,with just one coil,or three-axis,with three orthogonal coils.For use with a PPM, the single-axis sensor is most convenient, and if it is properly constructed, it is not difficult to orient it for maximum reading, especially as the likely direction of the field can usuallybepredicted from text-book field patterns. The first measurement that shall be made is that the field strength is correct at the point or points where it is intended to be 4oo mA/m (or the agreed lower value if adjusted to reduce loudness, as specified in IEC 60118-4). NOTE IEC 60118-4 specifies the use of either a PPM or an r.m.s. meter with a 125 ms integration time for the measurement of magnetic field strength.However, T, for the purpose of this standard,the125 ms integration time is incompatible with the requirement to measure field strengths over times of the order of a few milliseconds. The instrument specified for measurements on other equipment and systems, such as in IEC 62233 [3].has an averaging time specified only as an upper limit of 1 s, which is also too slow for the assessment of fields due to audio-frequency signals. 7.2 Input signal The input signal for the amplifier shall be the simulated programme signal described in IE