Every residential and commercial ELF EMF survey is a new adventure.
Generally, the ELF EMF survey inspector measures the fields in every
room, selected
appliances, and around the outside property. Whether one performs a
comprehensive contour or spot measurement survey, the process is rather
straightforward:
measure and record the magnetic field levels, note significant levels,
identify sources, and recommend mitigation solutions when necessary.
The magnetic fields from nearby electrical power line sources (transmission,
distribution, and service) are easy to measure and identify. Appliance
sources usually jump right out of the display as you walk by. But magnetic
fields from plumbing and ground currents can mystify and perplex the
most
experienced professional. If there is an elevated magnetic field near
a metal water pipe, especially next to the shut-off valve at the point
of
entry, then plumbing currents are probably the source. Horizontally
polarized magnetic fields perpendicular to the water pipe axis radiate
out from the
pipe diminishing in magnitude at a linear 1/r rate. The current on
the pipe can be calculated as follows:
Iamps =
.15(BmG)(Rfeet) where
BmG is magnetic flux density in milligauss.
However, it is much easier to use a clamp-on amp meter around the water
pipe, preferably near the entry point.
In a residence the typical plumbing
current measures between .25-2.5 amps, depending on the local neighborhood
electrical conditions. Place the clamp-on
meter around
the water pipe and monitor the current which normally fluctuates (sometimes
dramatically) over several minutes. Now determine the source and path
by de-energizing the
electrical power. Measure the current on the water pipe, electrical service
(around the sheath, not in the electrical panel), and the grounding conductors
to the
water pipe, ground rod, telephone & CATV. If current is detected
on the water pipe, then the plumbing current source is external, and
usually
exits
via the
neutral conductor (verify by measuring an equal net current on the electrical
service). The local utility should be notified if the external plumbing
current exceeds 3-4 amps.
Next energize all circuits, plug a high current
reference load (hair dyer, heater, etc.) into an outlet, and record
the current on the water
pipe,
electrical service,
and grounding conductors. If under loaded conditions both the water
pipe and net electrical service currents significantly increase (30-100% of
the reference load current), then there is a serious problem with the
neutral conductor.
Fortunately,
the water pipe is providing an alternative conductive path back to
the transformer secondary neutral via a neighbors water pipe and neutral.
Do not disconnect
the grounding conductor to the water pipe, it is energized and potentially
lethal.
Immediately call the local utility-they will inspect the electrical
service
and repair the neutral. If the problem is in the electrical panel,
you must call
a licensed electrician.
Plumbing currents can migrate from the water
pipes into HVAC ductwork, metal conduits, BX metal sheaths, and grounded
equipment housings (sink
disposal, coffee
machine, etc.) emanating magnetic fields everywhere. To eliminate
an external plumbing current source, a licensed plumber must install
a
high-quality dielectric coupler a few inches from the water meter
near the street.
Do
not install the
dielectric coupler inside the house! The National Electrical Code
(NEC) requires a ground conductor from the electrical panel to an underground
water pipe in
direct contract with the earth for a minimum of 10 feet as specified
in section 250-81(a). Remember, safety is paramount when testing,
evaluating, and correcting any known plumbing
current problems.