Yokogawa 414 Batch Controller - Manual (Page 51)

Installation 49
It is good practice to use shielded cable for all signal connections to the Model 414.
Care must be taken to separate signal cables from power cables so as to minimise
interference.
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:
♦ To limit the amount of electrical noise caused by arcing across the
contacts which may, in extreme cases, cause the microprocessor to act
erratically.
♦ 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.