Section 3
Installation
19
FT702LT/D Flat Front Wind Sensor Manual
3.3.2
Protection against indirect lightning effects and electromagnetic interference
Objects within the protection zone described above can still be subject to very high electromagnetic field and
partial lightning surge currents. It is therefore critical that appropriate shielding and termination is used
throughout the system to reduce these effects. A shielded signal cable will offer some protection however it is
recommended that double shielding is employed between the sensor and the chassis of the data acquisition
and power supply cabinet.
A metal conduit surrounding the shielded cable is a good way of providing this additional protection and will also
help to prolong the life of cables and connectors. The impedance of this metal conduit needs to be as low as
possible since a substantial proportion of the lightning current will flow in it. An example of metal conduit could
be
’s HelaGuard steel conduit with plastic coating and steel overbraid.
All cable shielding must be continuous and terminated at both ends using EMC glands or cable clamps with a
direct connection to the cabinet chassis. There must also be a direct connection from the cabinet chassis to the
grounding reference. Any metal conduit used must also be continuous and terminated at both ends with
appropriate fittings. Figure 18 below shows the principle of the protection scheme.
①
Preferably this
connection is established
using structural steel
parts or alternatively by
use of a copper cable
with a cross section of
min. 50mm
2
②
Any shielding conduit
must be terminated at
both ends
③
Shielded cable must
either be terminated
using an EMC cable
gland in the cabinet wall -
or alternatively using a
cable clamp in direct
connection with the
cabinet chassis
④
The chassis of the
cabinet must have a
direct connection to
ground
Figure 18: Protection of equipment against indirect effects
3.3.3
Surge protection
All connections from the wind sensor to any data acquisition equipment and power supply should run through
Surge Protection Devices (SPDs). This will suppress any unwanted overvoltage transients present on the signal
or power lines. The surge suppression devices are to be UL 1449 listed.
The ratings of the SPDs must be suitable for the surge conditions. Assuming that appropriate shielding and
termination has been used throughout, then the SPDs used with our sensor should have a minimum surge
current rating of 20kA (8/20µs) and be capable of clamping the output below the maximum input voltage
accepted by the electronic systems they are connected to. This will prevent any surges or large voltage
differences being present at the inputs to the wind sensor, data acquisition electronics or power supply.
