In the field of electronics, EMI and RFI
are synonymous—they both arise from elec-
tromagnetic radiation, which is energy made
up of electromagnetic waves. These waves are
classified by their frequencies, aligned on a
spectrum ranging from low to high: radio
waves, microwaves, infrared radiation, visible
light, ultraviolet radiation, X-rays, and gamma
rays. The nature of electrical current is to emit
electromagnetic radiation. This energy becomes
interference when it causes unwanted signals
or noise to occur in other devices or circuits
that are operating in or close to its frequency
range.88 electrical systems troubleshooting
Standards and Regulations
Most countries require that electronic and
electrical hardware work correctly even when
subjected to specified amounts of RFI, and
that they should not emit RFI that could
interfere with other equipment. The U.S.
Federal Communications Commission (FCC)
has several applicable regulations addressing
these emissions. Manufacturers must test and
label digital devices to inform end users of the
maximum emission level from a product when
it’s used in accordance with its instructions.
The FCC has established two classes of
devices. Class A equipment is intended for use
in industrial or commercial areas. Class B,
which is more stringent, includes residential
applications as well as industrial applications. As
shown in the sidebar, the regulations include a
disclaimer that must be stated in the instruction
manual, essentially stating that harmful interfer-
ence, while prohibited, may occur anyway.
The electronics installation standards of
the National Marine Electronics Association
(NMEA) mirrors the FCC’s advice.
What the equipment manufacturers aren’t
telling you is that the FCC requirements were
intended to deal with issues related to industrial
and residential interference migrating from one
business or household to another, and not for
the relatively close proximity of gear installed
on boats! So even though these standards exist,
we still run into problems on boats due to RFI
noise and/or strong magnetic fields.Common Sources and Victims of Interference
Following are some of the most common
sources of EMI/RFI emissions on boats:- AC generators
- air conditioners
FCC-REQUIRED VERBIAGE
FOR CLASS B DEVICESThis equipment has been tested and found to comply with
the limits for a Class B digital device, pursuant to part 15 of
the FCC Rules. These limits are designed to provide rea-
sonable protection against harmful interference in a resi-
dential installation. This equipment generates, uses, and
can radiate radio frequency energy and, if not installed
and used in accordance with the instructions, may cause
harmful interference to radio communications. However,
there is no guarantee that interference will not occur in a
particular installation. If this equipment does cause harmful
interference to radio or television reception, which can be
determined by turning the equipment off and on, the user is
encouraged to try to correct the interference by one or
more of the following measures:- Reorient or relocate the receiving antenna.
- Increase the separation between the equipment and
receiver. - Connect the equipment into an outlet on a circuit dif-
ferent from that to which the receiver is connected. - Consult the dealer or an experienced radio/TV
technician for help.