Section 3 Mechanical Installation & Lightning Protection
22 FT722 & FT742 (RS485) – FF & PM Sensors – User Manual
Protection Against Indirect Lightning Effects & Electromagnetic Interference
Objects within the protection zone described above can still be subject to very high electromagnetic fields 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 computer 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 the 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
HellermannTyton‘s HelaGuard steel conduit with plastic coating and steel overbraid.
All cable shielding must be continuous and 360° 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 13 below shows the principle of the protection scheme. The cabinet should be
protected with a metallic grounded housing.
① Preferably this
connection is established
using structural
aluminium 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 360° terminated
using a 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
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