Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 163)

ƒ Avoidance of interfering external influences (foreign particles, chemical effects) at the site of
installation.
Fig. 5.3-10
Double contacts, increased contact pressure (for example by riffling of the contact surface) and gold-
plating are some of the possible approaches to obtain good contact reliability
The attainment of satisfactory contact and hence functional reliability requires appropriate
measures by the device manufacturer and the user. On the user side, the selection of a suitable
contact design for the respective application from the manufacturer’s product range, compliance
with manufacturer specifications and the measures listed above will have a beneficial effect on
contact reliability. Care is required with all kinds of chemical substances in the switching
cabinet. Thus while contact sprays may be good for oxidized sockets – for switching contacts
they are poison!
Universal control contacts can be used over a wide range of voltages and powers. They are
suitable both for switching contactor coils at 230 V or 110 V and for control of PLC’s at 24 V. To
achieve a high degree of contact reliability, contacts should normally not be connected in series
at the small control voltages as common with PLC control. With contacts that are specially
designed for low signal levels it should be noted that even single switching operations at higher
power levels can destroy the surface structures and hence the electronic-compatibility will
completely be lost or at least strongly reduced.
Universal
control contacts
Num
ber of fai
lure
s
per 1 million operations
Special low-level
control contacts
Number of contacts in series
Fig. 5.3-11
Typical contact reliability values at 15 V/5 mA of universal control contacts and special low-level control
contacts
LVSAM-WP001A-EN-P - April 2009 5-11