ETS-Lindgren HI-3624(A) Survey Meter - Manual (Page 14)

14 | Practical Application and Use
Residential Measurements
Magnetic fields found in home environments are highly variable, depending on
location within the home. This variability is strongly related to the distribution of
the wiring in the home, the location of electrical appliances and occasionally, the
location of plumbing lines or other metallic structures within the ground which
may form low resistance paths for electrical ground return currents. Establishing
what the ambient magnetic field environment is in a home usually requires
numerous measurements throughout the home, with at least one measurement
within each room. Normal practice would include at least one field measurement
taken near the center of each room. A more thorough approach would include
five measurements in each room, one at the center and one near each corner of
the room. A reasonable technique is to position the sensor at a point
approximately one meter from each room corner for the flux density
measurement. This avoids, to an extent, placing the sensor immediately next to
wiring which may be hidden within the walls of the room and which may yield
unrealistically high values of flux density compared to what most individuals
within the room might be exposed.
Surveys of the areas near electrical appliances will usually reveal higher values
of flux density due to the currents flowing within motors or heating elements.
Logical choices would include the location of beds, for example, since this is a
location of extended occupancy. In characterizing the magnetic fields near
obvious sources, such as appliances, it is often helpful to measure and record
the flux density value at intervals of a few inches (or centimeters) beginning near
the surface of the device. These data will help provide a perspective on the
spatial extent of the elevated field levels and the significance of the levels relative
to other values determined elsewhere within the home. Field measurements
should take into account the likelihood that individuals may have access to areas
where measurements are contemplated.
Because 60-Hz magnetic fields produced by the use of electricity within the home
are dependent on the magnitude of current flowing within wires or the operation
of appliances, flux densities will be seen to vary with time, being greater when
more electrical power is being used. For example, when heating or air
conditioning systems turn on or the compressor within a refrigerator cycles on,
the flux density will increase. Measurements must take this condition into
account and it is recommended that, when taking measurements in a room, the
meter be watched for a period of time to observe for fluctuations in the indicated
value of flux density. After some experience, it may be possible to relate the
observed fluctuations to various uses of electricity within the home.