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cations system. Therefore, surge arresters are installed in this is attenuated (above 3 dB). In order to keep the reaction of
case. BLITZDUCTOR XT combined arresters for protecting ter- BLITZDUCTOR XT to the transmission system within permis-
minal devices can also be used (Figure 8.2.13). sible limits, the signal frequency of the signal circuit must be
below the cut-off frequency for BLITZDUCTOR XT. The cut-off
Case d frequency is indicated for sine-shaped signals. However, sine-
The difference between case d and case c is that the relevant shaped signals are not very common in data transmission.
building has no external lightning protection system and no In this context, it must be observed that the maximum data
measuring and control / telecommunications cables are in- transmission rate of BLITZDUCTOR XT is higher than the trans-
stalled outside the building. Therefore, only surge arresters mission speed of the signal circuit. When transmitting pulse-
must be installed to protect the devices. As in cases b and c, shaped signals for which the rising or decreasing edge of the
combined arresters of the BLITZDUCTOR XT family can be used pulse is evaluated, it must be considered that this edge switch-
(Figure 8.2.14). es from “Low” to “High” or from “High” to “Low” within a
certain period of time. This time interval is important for the
2. Which interference phenomena have to be identification of an edge and for crossing “restricted areas“.
prevented? Therefore, this signal requires a frequency bandwidth which is
Interference is basically classified into common-mode and significantly wider than the fundamental wave of this oscilla-
differential-mode interference. Common-mode in- tion. Thus, the cut-off frequency for the protective device must
terference always occurs between the signal line and be correspondingly set high. As a general rule, the cut-off fre-
earth, whereas differential-mode interference only oc- quency must not be less than five times the fundamental wave.
curs between two signal lines. Most of the interference
occurring in signal circuits is common-mode interference. 5. How high is the operating current of the system
Therefore, protective devices with overvoltage fine limita- to be protected?
tion between the signal core and earth should be selected Due to the electrical properties of the components used in the
(type …E). Some input stages of devices such as isolating protective circuit of BLITZDUCTOR XT, the operating current
transformers do not require overvoltage fine limitation be- which can be transmitted via the protective device is limited.
tween the signal core and earth. In this case, only gas dis- For the application, this means that the operating current of
charge tubes provide protection against common-mode inter- a signalling system may be less than or equal to the nominal
ference. Due to their different tripping characteristics, tripped current of the protective device. In this context, possible short-
gas discharge tubes may develop differential-mode interfer- circuit currents must be considered and limited by adequate
ence into common-mode interference. Therefore, a fine protec- measures (e.g. backup fuses) in the circuit of the installation.
tion element is integrated between the signal cores in this case
(type ...D). 6. What is the maximum operating voltage which
can occur in the system to be protected?
3. Are there special requirements for adopting the The maximum operating voltage which occurs in the signal
protective circuit to the input circuit of the device circuit must be less than or equal to the maximum continu-
to be protected? ous operating voltage U c of BLITZDUCTOR XT so that the pro-
In some cases, it is necessary to protect the device inputs tective device has no limiting effect under normal operating
against common-mode and differential-mode interference. The conditions.
input stages of such electronic devices requiring protection The maximum operating voltage occurring in a signal circuit
are normally fitted with their own protective circuit or contain is typically the nominal voltage of the transmission system
optocoupler inputs to isolate the potential of of the signal cir- when tolerances are taken into account. If current loops (e.g.
cuit and internal circuit of the automation device. This requires 0 – 20 mA) are used, the open-circuit voltage of the system
additional measures to decouple BLITZDUCTOR XT from the can always be employed as maximum operating voltage.
input circuit of the device to be protected. For this purpose,
additional decoupling elements between the fine protection 7. Which reference has the maximum operating volt-
elements and output terminals of BLITZDUCTOR XT are used. age?
Different signal circuits require different types of signal trans-
4. How high is the signal frequency / data trans- mission (balanced / unbalanced). On one hand, the operating
mission rate which must be transmitted? voltage of the system can be provided as core / core volt-
The protective circuit of BLITZDUCTOR XT has low-pass char- age and on the other hand as core / earth voltage. This must
acteristics. The cut-off frequency indicates the frequency value be considered when choosing the protective devices. Due to
above which the amplitude of the signal to be transmitted the different circuits of the fine protection elements in the

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