MA-037 - CB Watch 3 Operating Manual
v7.0 - May 2020
Page 61 of 118
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 setup:
HMI: Settings / Operations monitoring
Figure 88
–
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 “I
2
T” 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.
While keeping track of the I
2
T cumulative total for normal opening operations is important,
especially for frequently operating breakers, what is more damaging to the contacts are the
opening under fault current triggered by the protection relays. In these cases the fault
current and therefore the corresponding i2T values can be 10 or up to 100 times larger and
the erosion of the contacts magnified. Some breakers may only be able to operate a few
times under fault current before the contacts need to be replaced.
Interrupted Current
The AC
current flowing through the breaker is monitored for each pole using the breaker’s
own primary HV CTs (or nearby independent HV CTs) which are connected through further
CBW3 CTs to the CBW3 so that an image of its value can be recorded.
A simplified current curve is displayed and archived with each operation (1 point every 400
micro seconds).
It is important that the CBW3 CTs used have plenty of range to not only measure everyday
currents but also fault currents.
The ANSI standard calls for “The CT
[primary] ratio shall be
chosen so that the maximum symmetrical fault current on the CT secondary is limited to
100A”.
So for everyday currents, the graphs obtained can be a bit noisy. This is because we
are at the low-end of what the CTs can measure in order to still be able to measure a much
higher fault current without saturating.
