BS 50121-2:2017 pdf download – Railway applications — Electromagnetic compatibility Part 2: Emission of the whole railway system to the outside world Values of measurement
The values measured are expressed as:
— dBµA/m for magnetic fields,
— dBµV/m for electric fields.
These are obtained by using the appropriate antenna factors and conversions. Antenna position and orientation
The plane of the loop antenna shall be positioned to measure the horizontal component of the magnetic field perpendicular to the track respectively to the wall of the substation. The biconical dipole shall be placed in the vertical and horizontal axis. The log periodic antenna shall be arranged to measure the vertical and horizontal polarization signal, with the antenna directed towards the track respectively to the wall of the substation. The test locations should whenever possible avoid objects with changing of field characteristic like turnouts, walls and under bridges. Figures 3, 4 and 5 show the positions and vertical alignments of the antennas as an example for measurements at the track. Ambient noise
At the beginning and at the end of the test series the ambient noise shall be recorded. If at specific frequencies or in specific frequency ranges the ambient noise is higher than the limit values less 6 dB, the measurements at these frequencies need not be considered. These frequencies shall be noted in the test report.
5.1.2 Measurement parameter for moving trains This subclause summarizes the specific conditions for the measurement of moving rolling stock. — It is not considered necessary to carry out two tests to examine both sides of the rolling stock, even if it contains different apparatus on the two sides, as in the majority of cases the level of fields is due to the radiation of catenary and not to the direct radiation from the train. For systems with a third rail, measurements shall be performed at the same side of it.
— The peak measurement method is used. The duration at selected frequency shall be sufficient to obtain an accurate reading. This is a function of the measuring set and the recommended value is 50 ms.
— The noise may not attain its maximum value as the traction vehicle passes the measuring point, but may occur when the vehicle is a long distance away. Therefore, the measuring set shall be active for a sufficient duration before and after the vehicle passes by to ensure that the maximum noise level is recorded.
— In the case of elevated railway systems, if the antenna heights specified above cannot be achieved, the height of the antenna centre can be referenced to the level of the ground instead of to the top of the rail. The conversion formula in shall be employed where D is the slant distance between the train and the antenna. The train shall be visible from the location of the antenna and the axis of the antenna shall be elevated to point directly at the train. A measurement distance of 30 m from the track centreline is preferred for highly elevated railway systems. Full details of the test configuration shall be noted in the test report.
— If tests are being carried out on a railway system with overhead electrified supply, the measuring point shall be at midspan between the support masts of the overhead contact line and not at a discontinuity of the contact wire. It is recognized that resonance can exist in an overhead system at radio frequencies and this may require changes in the values of frequency chosen for measurement. If resonance exists, this shall be noted in the test report.
The radio frequency emission will be affected by the state of the railway system supply system. Switching of feeder stations and temporary works will influence the response of the system. It is therefore necessary to note the condition of the system in the test record and, if possible, all similar tests should be carried out within the same working day. Where the railway system has a track-side conductor rail power supply, the test location should be at least 100 m from gaps in the rail, to avoid inclusion of the transient fields associated with the make and break of collector contact. The conductor rail and the antennas shall be on the same side of the track.