Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 162)

0,5 mA 2 mA 15 mA
U L min.
U L max.
I L max.
U
L max. or
U
T min.
U H min. or U T max.
I
H min.
I
H max.
U H max.
0
-3 V
5 V
15 V
24 V
30 V
I T min. I T max.
U
e min.
U
e max.
U
e
U [V]
On region
Off region
I mA
Fig. 5.3-9
Operational range of PLC inputs in accordance with IEC 61131-2 (Programmable controllers – Part 2:
Equipment requirements and tests) and IEC 60947-1 Annex S (Digital inputs and outputs) for contact-
inputs (digital input type 1) at a rated control voltage of 24 V. The contact load for ON can be between 30
and 15 V and 15 and 2 mA.
Copyright © IEC, Geneva, Switzerland. www.iec.ch
While at switching higher voltages and loads, a cleaning process by the arc takes place with
every switching operation, with small signals special measures are required to ensure a high
quality of contact making, that is to guarantee a high degree of contact reliability. At a typical
PLC input resistance of several k
Ω it is not a matter of mΩ as in power contacts. Good contact
making can be for example prevented by
ƒ Films on the contact surfaces, originating from reactions with ambient gases (for example
oxidation, formation of sulphide layers) or from deposits of volatile components of the ambi-
ent atmosphere (e.g. that originate from production processes at the location or effluvia from
plastics in the switchboard cabinet). Such films can usually only be identified with special
devices and the cause is often hard to determine and eliminate.
ƒ Films on the contact surface that are caused by migration of metal material from the contact
base and often interact with the first point and the switching operation.
ƒ Contamination of the contact surface that can emanate from the environment (open switch-
ing cabinet doors during commissioning!), from the interior of the switching cabinet or from
the device itself. A problem that should not be neglected is the generation of foreign particles
by the operation of the devices itself, for example due to abrasion.
Provisions for ensuring good contact reliability include
ƒ Selection of suitable contact materials (basic material and possibly surface coatings such as
gold)
ƒ Avoidance of internal sources (for example materials and/or abrasion) that could have an
adverse effect on contact reliability.
ƒ Use of high contact pressures that are able to break through tarnishing films, e.g. by
appropriate shape of the contact surface.
ƒ Relative movement of the contact surfaces during circuit making that break through tarnish-
ing layers and can remove contamination. It should be noted that this can cause abrasion,
which may have a negative effect on life span and possibly contact reliability .
ƒ Use of multiple contacts (double contacts, H-contacts), with which the likelihood of good
contact making is increased by parallel connection of the contact points.
ƒ Avoidance of too low contact loading and of series connection of a bigger number of
contacts.
LVSAM-WP001A-EN-P - April 2009 5-10