Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 151)

In order to ensure the proper functioning of the protection system modern trip devices monitor
their measuring loops for short-circuits and interruption.
750
1650
Release
Reset
Monitor measuring loop
for short-circuit
Fig. 4.2-15
Resistance-temperature characteristic of a Type A PTC sensor and threshold values of the tripping
devices in accordance with IEC 60947-8 ed. 1.1. (TNF = rated operation temperature)
Copyright © IEC, Geneva, Switzerland. www.iec.ch
4.2.4.3.2 Relays for NTC sensors
The characteristics of NTC sensors (Negative Temperature Coefficient) show decreasing
resistance with increasing temperature. They are used in special cases and require tripping
devices with an adjustable response threshold. With one set of sensors, early warning and
tripping can be realized.
4.2.4.3.3 Metal resistance sensors
These sensors – for example Pt 100, Ni 100, Ni 120, Cu 10 – are especially suitable for medium
and high voltage motors. They are used for measuring temperatures of windings and other
machine parts, such as bearings. Tripping devices have a correspondingly larger number of
inputs. The operation and warning levels can be adjusted. Frequently important temperature
values are also shown on displays.
LVSAM-WP001A-EN-P - April 2009 4-43