Brochure/Catalogue

Lightning and surge protection for control and instrumentation signals
Lightning and surge protection for control and
instrumentation signals
Basic classication
The current scope of automation
technology has resulted in a wide range
of applications for surge protection
in the eld of instrumentation and
control engineering. One important
prerequisite is the consistent use of
coordinated surge protection in all
sections of the plant or building. In
industry, instrumentation and control
systems are important areas and
breakdowns or malfunctions can lead
to exorbitant costs. As the standards
covering low control voltages do not
specify many parameters, the use of
surge protection, apart from lightning
protection zoning concepts, has to be
classied according to type of signal,
application circuit and the anticipated
interference voltage phenomena.
Types of interference voltage
Transient surges coupled into a system
via one or more coupling mechanisms
occur as normal- or common-mode
interference. These are measured as
longitudinal or transverse voltages and,
depending on the circuit, designated as
symmetrical or asymmetrical voltages.
(For further information see the
“Principles” chapter.)
common mode interference
I
common mode
U
push-pull
U
common
mode
U
sym.
U
unbalanced. 1
I
common mode
U
unbalanced 2
U
asym.
Z / 2
Z / 2
longitudinal voltage
push pull interference
I
push-pull
U
push-pull
U
common
mode
U
sym.
U
unbalanced 1
I
push-pull
U
unbalanced 2
U
asym.
Z / 2
Z / 2
transverse voltage
Common-mode interference
(asymmetrical interference):
Common-mode voltage between
conductor and reference potential.
(earth)/mainly caused by capacitive
coupling (electrical eld)
Normal-mode interference
(symmetrical interference):
Normal-mode voltage between supply and
return conductor load and interference
source connected in series, e.g. inductive
(magnetic eld) or conductive coupling
(common impedance)
Types of signal
Binary signals SL ≜
(symmetrical loop)
These are two-wire signals with a
common reference potential which are
required by, for example, by switches,
PLC switch outputs, photoelectric
barriers, position sensors, solenoid
valves, warning lamps, PLC inputs,
etc. Normally, these signals have a
common reference potential that can
be either connected or not connected
to earth potential, depending on
the type of protection. The coupled
transient interference is primarily
common-mode interference.
Electrical system
MSR
0
1
2
3
4
5
SURGE PROTECTION
SURGE PROTECTION
SURGE PROTECTION
Analogue signals CL ≜
(current loop)
Measuring circuits are normally
designed as two-wire current loops
or voltage signals without a common
reference potential, like the
0(4)...20 mA current loop.
The coupled tr
ansient interference is
primarily normal-mode interference.
For temperature measurements with
the PT100 measuring shunt in the
three-wire version, the voltage drop
at the shunt is measured via the third
wire. This must be included in the
system of protection.
The PT100 measuring shunt is also
available in a four-wire version in
which the voltage drop at the shunt is
measured via the two additional lines
without additional line losses in the
PT100 measuring circuit. The coupled
transient normal-mode interference
occurs between the various wires.
B
B.6 2028840000
Lightning and surge protection for control and instrumentation signals