Electromagnetic interference (EMI) and radio frequency interference (RFI) from the switch can adversely affect other devices,
such as radio and television (TV) receivers, operating near the switch. Radio frequencies that emanate from the switch can
also interfere with cordless and low-power telephones. Conversely, RFI from high-power telephones can cause spurious characters
to appear on the switch monitor.
RFI is defined as any
EMI with a frequency above 10 kHz. This type of interference can travel from
the switch to other devices through the power cable and power source or through
the air as transmitted radio waves. The Federal Communications Commission (FCC)
publishes specific regulations to limit the amount of EMI and RFI that can be
emitted by computing equipment. Each switch meets these FCC regulations.
To reduce the possibility of EMI and RFI, follow these guidelines:
When wires are run for any significant distance in an electromagnetic field, interference can occur between the field and
the signals on the wires with the following implications:
Bad wiring can result in radio interference emanating from the plant wiring.
Strong EMI, especially when it is caused by lightning or radio transmitters, can destroy the signal drivers and receivers
in the chassis and even create an electrical hazard by conducting power surges through lines into equipment.
To predict and
prevent strong EMI, you might need to consult experts in radio frequency
The wiring is unlikely
to emit radio interference if you use twisted-pair cable with a good
distribution of grounding conductors. If you exceed the recommended distances,
use a high-quality twisted-pair cable with one ground conductor for each data
signal when applicable.
If the wires exceed
the recommended distances, or if wires pass between buildings, give special
consideration to the effect of a lightning strike in your vicinity. The
electromagnetic pulse caused by lightning or other high-energy phenomena can
easily couple enough energy into unshielded conductors to destroy electronic
switches. You might want to consult experts in electrical surge suppression and
shielding if you had similar problems in the past.