Field Finder
Field Finder
© Enviromentor AB 1998 reg. 98.011/Eng
11
3. Connect the pieces of apparatus one at a time
and measure the magnetic fields in the direc-
tions 0
°
, 90
°
, 180
°
and 270
°
at distances of
30 cm and 50 cm from the outer edge of the
piece of apparatus in question (see figure) or
in the direction the operator is facing. Sum-
marise the measurement readings in a report
form. An example of how to fill out a report
form can be found on page 12. You must not
subtract the background values of the magnetic
fields from the measured values. They should
always be noted as a comparison.
4. Analyse the measurement readings and assess
the need for remedial action, such as rearranging
the furniture in the room and/or moving pieces
of electrical apparatus. The sources of the
magnetic fields may be located some distance
away, and magnetic fields can penetrate almost
all building materials.
5
How magnetic fields arise
Magnetic fields are caused by electrical cur-
rents and always occur in continuous closed
paths around the currents that cause them. A
live conductor gives rise to a magnetic field,
the strength of which is always proportional to
the current in the conductor. Magnetic fields
are usually depicted with the aid of field lines.
The strength of the magnetic field is constant
along the conductor in closed paths around the
live conductor. In the event of other sources,
magnetic fields tend to have a complicated ap-
pearance which usually cannot be calculated
but have to be measured instead. The unit used
to measure the magnetic flux density is called
the tesla [T]. Magnetic fields can be caused by
electrical devices and installation cables. In cer-
tain cases, stray currents can give rise to mag-
netic fields. In Sweden, for example, the elec-
tricity systems generally entail four conductors
leading to each building, which can result in
major problems with currents of this type. The
decay current can pass through the neutral con-
ductor as intended, but it can also pass through
the earth conductor and into the plumbing pipe-
work to the transformer’s earth point. This in-
creases the magnetic field both along the path
of the stray current and along the supply cable.
It is also commonplace for stray currents to ex-
ist in computer networks. As well as causing
magnetic fields, they can also lead to communi-
cation problems. In industrial environments,
common sources include welding equipment,
electric motors and cable clusters.
Straight con-
ductor, I=1A
At 1 m from the conductor, the
magnetic flux density is 0.2
µ
T.
