Contrec 414L, Руководство по введению, Страница: 74 / 82

414L-M-V3
Overall shields should be connected to the case earth at the instrument
end only.
This connection should be as short as possible and connected to the
earthing lug on the side of the case.
In order to comply with the requirements for Electromagnetic
Compatibility as per EMC-Directive 89/336/EEC of the Council of
European Community, this wiring practice is mandatory.
Although it is also possible to connect shields to the signal ground
(terminal 2) this is not in accordance with EMC directives.
RC Networks for Interference Suppression
When driving highly inductive loads with the relay outputs, it is
recommended that RC suppression networks (often called "Snubbers")
are used for two reasons:
Ÿ To limit the amount of electrical noise caused by arcing
across the contacts which may, in extreme cases, cause
the microprocessor to act erratically.
Ÿ To protect the relay contacts against premature wear
through pitting.
RC suppression networks consist of a capacitor and series resistor
and are commonly available in the electrical industry. The values
of R and C are dependent entirely on the load. However, if the
user is unsure of the type of snubber to use, values of 0.25uF and
100ohms will usually suffice. Note that only mains approved RC
suppression networks should be used.
The basic principle of operation is that the capacitor prevent a series of
sparks arcing across the contact as the contact breaks. The series
resistor limits the current through the contact when the contact first
makes.
72 Installation