In message
<beda313b3e2b304ca6a08001bd61c9c01a2a7...@ukmb01.mottmac.group.int>,
dated Fri, 31 Jan 2014, "McCallum, Andy" <andy.mccal...@mottmac.com>
writes:
It's a UK based Tram - single conductor wire. Traction current return
via the running rails.
I have now checked and the 15 μT value at 10 m from the track in EN
50121-3 is reasonable. There is a 2013 edition of this standard either
published or nearing publication. I have taken the conductor height as 6
m. There is a question about the length of the current path; I have
assumed that a vehicle drawing 1000 A and the feed point are 1 km apart,
but this dimension is so large that it hardly affects the way the field
strength varies with distance from the track.
But this 15 μT is DC: as such it can't affect an audio-frequency
induction loop. There is obviously a fluctuation as the vehicle current
varies, but this is too slow to be an 'audio frequency'. There is ripple
on the DC supply, which is probably produced by a six-pulse rectifier,
so the frequency is 300 Hz with rich harmonics. This is certainly 'audio
frequency'.
The ripple voltage might be 5% of the DC voltage but we don't know the
ripple current because we don't know the impedance of the vehicle at
these frequencies. The writers of the EN probably have test results to
support their 2% figure, so we can say that the 300 Hz field is 2% of 15
μT. To compare with the induction-loop standard, we have to convert to
amps per metre. 1 A/m is 1.2 μT (B = μH, μ=4pi x 10^(-7).
So, 2% of 15 μT = 0.3 μT = 360 mA/m, which is practically a
full-volume field strength (the standard value is 400 mA/m; this is the
r.m.s. value of the 'loudest bit of the programme', not a peak value).
Since the interfering signal is 'tone-like', it needs to be 40 dB or
more down in order to be tolerable. The inverse-square law doesn't
apply; the field strength falls with distance in a more complex way. My
calculations show that the distance has to be quite large, but there are
local factors that can seriously affect the results in practice.
This why the standard (IEC/EN 60118-4) strongly recommends a site survey
for potentially-interfering magnetic fields *before* a loop system is
installed.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Nondum ex silvis sumus
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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