FT Technologies acures FT722 - Page 34

Section 3 Mechanical Installation & Lightning Protection
34 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 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.
This can be in the form of continuous pipe between the Pipe Mount Adapter and the data acquisition and power
supply cabinet. If the pipe is not continuous, then metal conduit surrounding any exposed sections of the shielded
cable is a good way of providing this additional protection and will also help to prolong the life of the cable. 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 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 metallic 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 29 below shows the principle of the protection scheme.
① Preferably this
connection is
established using
aluminium parts or
alternatively by use of
a copper cable with a
cross section of min.
50mm
2
② Any pipe or
shielding conduit
must be terminated at
both ends
③ Shielded cable
must either be 360°
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 29: Protection of equipment against indirect effects