ESA VT130W, Руководство по оборудованию

Страница: 30 / 976

CHAPTER 1
General information on electromagnetic compatibility (EMC)
405.1200.037.2 - Rel.: 2.21 of 25/02/2008
1-2
Electronic devices are being increasingly used in automated systems. pro-
grammable controllers (like the PLC) belong to this category, as do
Man-machine-interface systems (e.g. the Video Terminal), control systems
(e.g. diagnostic terminals), interface elements (e.g. interface boards) and
AC-DC drives. Together with this type of electronic apparatus, you find
installed classical electro-mechanical devices like contactors, elec-
tro-valves, motors etc.
Electrical disturbances caused by the operation of these devices can com-
promise the smooth functioning and the length of the working life of the
electronic devices present on the same switchboard or the same plant. To
get the best out of both the electrical and the electronic devices it is neces-
sary to reduce electrical disturbance.
Laying cables Remember to lay measurement, monitoring and communication cables so
that they are kept apart from power cables. Power cables laid close and par-
allel to communication cables can cause coupling voltages that are strong
enough to disturb or destroy electronic components.
Shielding of
cables It is essential to use suitably shielded cables for communication signal con-
nections (total shielding is recommended). The shielding must be con-
nected to the zero potential.
Earthing of
shielding and
electronic
circuits
With many devices the "0V" is connected to the earth. The signal ground
must be earthed but it is best to separate the ground of the shields and cir-
cuits from that of the power circuits. Note that the earth can only perform
its function if the "Resistance of the earth circuit" is within the max. limits
prescribed.
Switching of
capacitive
loads The current peaks which occur when capacitive loads are switched on can
damage or destroy control devices. Furthermore, the high-frequency com-
ponent of the current peak can cause serious disturbance in electronic cir-
cuits as a result of inductive coupling between the connection cables.
Switching off
inductive
loads When an inductive load is switched off, the magnetic energy stored tends to
oppose this, discharging a voltage peak down the line which can damage or
destroy the control device. Furthermore, the high-frequency component of
the voltage peak can cause disturbance caused by capacitive coupling
between the connection cables.
The physical structure and characteristics of an inductive load make it
impossible to carry out switching without disturbance unless suitable mea-
sures are taken. The disturbance can be at least partially suppressed by fit-
ting a suitable disturbance-suppression module in parallel with the
inductive load. The disturbance-suppression module must not constitute an
additional load during the work phase. Electrical disturbance is propagated
both through the connection cables and electromagnetic transmission. If the
disturbance is propagated by cable or electromagnetic transmission, it is
much more difficult to suppress at the inputs to the units in the danger zone
than it is to suppress the disturbance at its source. If the disturbance is cre-