Technical data

The Problem of Interference
Unfortunately not everything is resolved just because we have succeeded in finding the
right transmission methods and the right interface. The largest irritant to data commu-
nications still remains-Interference. Outside disturbances that result in data loss, trans-
mission errors and in the worst possible scenario knock out equipment. Computer
development has resulted in smaller circuits and components being driven by less
power. This is ideal from an energy standpoint, but regrettably they have also become
more sensitive and more vulnerable to overvoltages. Investigations have shown that up
to 70% of all data disturbances are due to deficiencies in the installations or distur-
bances from the local environment, from neighbouring equipment, machines and
cabling. Only 20% are due to either hardware or software faults. Accordingly, most cul-
prits can be found within our own walls or in the vicinity. The others come from out-
side. Like a bolt from the blue. The largest group is transients. Short yet high voltage
pulses on the network. Computer equipment exposed to transients, 1,000 V and
upwards to 10 kV lasting a few milliseconds, lives dangerously.
Lightning, machinery and fluorescent lamps
We know that a direct stroke of lightning discharges very high voltage and that these
propagate and damage electrical and telecommunication lines, and in worse cases
result in fire. Though you may escape a direct hit, you can be affected by pulses that
propagate over large distances in the cable network or by earth potential differences
between two points. That is why a light can flicker even when a storm seems a long
way off.
www.westermo.com Theoretical and general applications 37
Lightning causes differences
to earth potentials that can
damage electrical equipment.
The potential is
lower at this point
than at the light-
ning strike
Earth potentials
may differ by up
to several hundred
volts
Lightning discharge
generates
extremely high
earth potentials
Copper cables conduct
current better than
the earth
5 to 2500 metres (16.4 ft to 1.55 mi)
Optical isolator
is recommended
Server Machinery with
RS-232
Machinecompany Ltd
B A C K