ZEN-LOG-MAN-16V03 (0930)
Copyright © 2016 Define Instruments
8
2.4 - EMC installation guidelines
The Zen has been designed to cope with
large EMC disturbances.
This has been
achieved by continual testing and improve-
ment of filtering and layout techniques over
many years.
The Zen meets CE noise requirements, and
even surpasses them in many tests. (For
full details and test results, see Appendix
A.) However in some applications with less
than optimum installations and large power
switching, the EMC performance of the Zen
can be further improved, by:
A
Installing the unit in an earthed
Metal
Enclosure
. This is particularly useful
if the control box is mounted close to
large power switching devices like con-
tactors. Every switching cycle there is a
possibility of generating a large amount
of near field radiated noise. The
Metal
Enclosure
, acting as a faraday cage,
will shunt this radiation to ground and
away from the Zen.
B
Increasing the physical distance from
the power devices. For example, in-
creasing the control box distance from
6" to 12" from the noise source will re-
duce the noise seen by the control box
by a factor of 4. (Probably the cheapest
and best results in this situation could
be obtained by adding RC snubbers to
the contactors or power switches.)
C
Using shielded cable on sensitive input
and control signal lines. Good results
can be obtained by grounding the
shields to the metal enclosure close to
the entry point. All cables act as aeri-
als and pick up unwanted R.F. radiated
signals and noise; the earthed shield
acts as a faraday cage around the ca-
bles, shunting the unwanted energy to
ground.
Shields can also help with capacitively
coupled noise typically found in circum-
stances when signal cable is laid on top
of noisy switching power cables. Of
course in this case you are better off to
keep separate signal and power lines.
D
Laying cable on earthed cable trays
can also help reduce noise seen by the
Zen. This is particularly useful if there
are long cable runs, or the unit is close
to radiating sources such as two way
radios.
E
The relay outputs of the Zen have built
in MOV's to help reduce EMI when
switching inductive loads. EMI can fur-
ther be reduced at the load by adding
snubbers for AC signals or a flyback di-
ode for DC coils.
