Technical data

Examples of overvoltage protection
Interface Rated voltage
RS-232 12 V
RS-422/RS-485 12 V
W1 24 V
4-20 mA 24 V
Telecom modem leased line 24 V
Telecom modem dialled-up 170 V
38 Theoretical and general applications www.westermo.com
It is not just storms that create external transients. Your lamps may also flash when a
neighbouring industry starts or shuts down its machinery, this also causes transients
and voltage peaks on the network.
As a rule most transients are created within your own premises. Machines, equip-
ment and fluorescent lamps cause voltage pulses on the network. A fluorescent lamp
that is switched off can, for example, emit stored energy in the form of a transient of
up to 3000 V. A stroke of lightning close to an electric cable can cause a transient of
between 6–10 kV. A standard communication circuit card in a computer is designed
for ±12 V. Transients are usually the reason why computer equipment is unexplainably
knocked out or communications are temporarily disturbed. Transients are the most
common cause of disturbances. Only in about 10% of cases are the disturbances due
to a mains fault, i.e. long term undervoltage or overvoltage or a power failure.
Overvoltage protection and lightning protection
As overvoltages or lightning discharges can damage communications equipment we
are often asked what the most effective protection is.
To fully control the effects of lightning is extremely difficult; however, many problems
can be avoided by installing suitable protective equipment. When discussing lightning
protection it is for two categories, direct hits and induced overvoltages.
Protection against direct hits requires the ability to divert several hundred thousand
amperes. It is easier to protect yourself against induced voltages; these do not have
such a rapid transient time and the current that occurs when diverting is no where
near as severe. Induced overvoltages as the name implies are transferred through
induction, thus no contact with the lighting is necessary. These overvoltages are the
most frequent as they occur in connection with each stroke of lightning.
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