motivation was (aesthetic, interference/EMC, EM exposure, (or ???) and
whether it's the electric or magnetic field that you're interested in.
separation, height and current) but harder for 3-phase (need to know the
phase sequencing on the wires) and harder still for dual 3-phase - overall
it's probably easier to measure. A 3-axis magnetometer will give you the
magnetic field, the E-field can be estimated if you know the line voltage.
e-field depends on line voltage and height.
If your interest is driven by EM exposure considerations these levels are
way below the exposure guidelines.
If you want more info on this there are a variety of sites: World Health
Organisation EMF Project site (loads of authoritative info), UK National
Radiological Protection Board (recently published a review of research and
concluded no proven risk), International Council for Non-Ionising Radiation
Protection (ICNIRP) and so on. If you search on "EC recommendation 519"
you'll find a huge amount of info.
The bottom line is that there's been a phenomenal amount of research in this
area and the only proven and acknowledged effects are nerve stimulation at
low frequencies and thermal effects at higher frequencies. It's possible to
find loads of sensational scare stories but there is NO independently
replicated and peer-reviewed science that supports them (stands back and
waits for tidal wave).
...if your motivation is interference or EMC - the only common equipment
that's affected by this level of B field are CRT-based VDUs (anything above
0.5 to 1.0 microTesla will cause perceptible jitter. Very little is affected
by LF e-fields but ground shift (especially after a line fault) may cause
damage if you have multiple ground reference points.
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