BS 6656:2002 pdf download

BS 6656:2002 pdf download.Assessment ofinadvertent ignition of flammable atmospheres by radio-frequency radiation—Guide.
6.5.3.2 Speech and musw
When speech and music are transmitted, the power quoted in the technical documentation is that of the unmodulated or carrier L ansmission but the instantaneous field strength at the modulation peaks may be up to two times greater when the speech or music is at its loudest volume. Since the duration of the modulation peaks may exceed the thermal initiation time olthc gas, it is necessary to make some allowance for the power increase (modulation) when making an assessment. As a result of experience gained with amplitude modulated broadcast transmitters. a modulation factor. rn. of 1.4 should be used for calculations of effective field strengths for RF ignition effects (see (61 and 171).
6.5.3.3 Coded transmissions
A version of amplitude modulation used for Morse and other coded transmissions is known as modulated continuous wave (MCW). For modulated continuous wave, whereby the carrier is fully modulated by a continuous tone, a modulation factor. m, of 2 should be used.
6.5.3.4 Tekeizion transmissions (TV)
For television transmissions the peak power is usually quoted in the technical documentot ion hut in this casc no allowance for modulation is necessary. because the mean power is approximately equal to the peak power.
6.5.4 Single sideband (SSB) operation
For single sideband (SSB) operation the peak envelope power is usually quoted in the technical documentation and a value of 0,7 for the modulation factor, m, should be used.
6.5.5 Pulsed radar
Pulsed radar consists of pulses transmitted at frequent intervaLs. The transmissions are characterized by a peak power P.., a mean power P., a pulse duration I (in a) and a pulse repetition rate of ,i pulses per second. These are related by the equation:
The peak power. P,… should be used for assessment purposes and no additional allowance for modulation should be considered,
7 Structures and spark-making mechanisms
7.1 Structures
Voltage and current may be induced in a met-al object in an electromagnetic field: when two such conducting objects are in intermittent contact an RF discharge may result. The source of RF power for ignition by discharge is, in effect, an antenna situated in the incident electromagnetic field. The antenna configuration maybe formed by any metallic object or assembly of objects, such as pipelines. storage tanks bridged by walkways, tanker loading facilities and vent stacks. However. not. all the metallic objects on a site are capable of delivering significant energy into a discharge and some classification of structures can be achieved in order to identify possible hazard situations.
For analogy with simple antenna types, structures may he divided into loop types or vertical (monopole) types for consideration at frequencies tip to and including 30 MHz and may be easily identified in a survey of the plant. At higher frequencies, sections or small parts of the structure, whose physical dimensions are comparable with a wavelength of the transmitter frequencies. may behave as efli.cient antennas. There is no requirement to identify these since due allowance for their characteristics is made in the assessment procedure (see Clause 10 where it. is assumed that their behaviour is not significantly different from that of a long dipole (see 1101).
7.2 Loop-type structures
At frequencies up to and including 30 Mlix. structures which form ioops (see Figure 1) are more efficient as receiving antennas than other types of structure (see L21. 141. 1111 and 112J). Whcn the internal loop
– perimeter is about one half-wavelength, the structure is self-resonant and its efficiency is at a maximum.
– Loop structures having a smaller loop perimeter than the optimum can, however, bc brought into resonance by the presence of stray capacitance across the discontinuity, The efficiency decreases rapidly when the ratio of the loop perimeter to the wavelength is below 0.5 and remains approximately constant for values of this ratio greater than 0,5.