Technical data

www.westermo.com Theoretical and general applications 39
In the below example, the
earth currents can take the
wrong route, via the computer
network’s signal ground to a
fuse panel, and thereby causing
interference.
There is a large selection of overvoltage protection for signal/telecommunication lines
available on the market as well as for telecom modems, RS-232, 4–20 mA,
RS-485 and other typical signals. The protection consists of primary protection and
secondary protection, where the secondary protection is adapted to the communica-
tion method. The protection is usually maintenance free, when a transient is taken care
of the protection returns to its original state. If not, the protection has gone down due
to one of the following:
… The transient energy was greater than what the protection could handle
(as the stroke of lightning was very close to the installation).
… Damage due to long term overvoltage, for example because of a direct
connection to 230 V.
Earth Loops
Another common causes of data communication errors are differential ground poten-
tials or earth loops. Especially when network equipment is powered from different dis-
tribution panels with different ground potentials when referenced to earth. Any stray
current could take two different routes to ground, either the correct path via the earth
in the distribution panel, or via the signal ground of the serial port to the earth on the
another distribution panel. Ground currents that travel in the network can cause both
disturbances and damage the circuits that power the line. A communication network
consists of many metres of physical cable. Frequently routed with other cables for elec-
tricity and telecommunications. All cables that carry a current create an electromag-
netic field that effects adjacent or crossing cables. Together these form large antennae
that can catch different types of interference. There are recommenda-
tions concerning how different types of cabling should be routed to
minimise electromagnetic interference. The easiest way to
counteract problems with both transients and differential
ground potentials is to use a modem with galvanic isolation that
electrically isolates the cables and the equipment from each
other yet does not affect the signals. This prevents transients,
lightning and ground currents from reaching the equipment.
Zero
Protective ground
Zero
Protective ground
B A C K