CB Watch 3 User Manual
v6.3 - May 2019
Page 49 of 100
CONTACT WEAR MONITORING
General Description
Most circuit breakers use special arcing contacts specifically designed to withstand the
high energy that occurs during arcing when opening a circuit breaker. These have a finite
service life and therefore need to be replaced when it has been reached.
With the appropriate sensors installed, the interrupted current during each CB opening
operation can be monitored by enabling
“Primary current measurement” in the setu
p:
HMI: Settings / Operations monitoring
Figure 68
–
Setting primary current measurement
The arcing time is measured and used to highlight any increase, showing degradation in
the current interruption performance and
possibly even a “non
-
interruption”.
If we measure the current interrupted, square it and multiply it by the arcing time, we get
the “I2T” measure of the energy that the contact has been subjected to. By keeping a
cumulative total of this energy throughout the life of the contact, we can estimate the
“contact wear” that has occurred due to electrical deterioration.
Interrupted Current
The AC current flowing through the breaker is monitored for each pole using the
breaker’s own primary current CTs
(or nearby independent CTs) which are connected
through further CTs to the CBW3 so that an image of its value can be recorded.
Figure 69
–
Current before and during interruption