5 CB CONDITION MONITORING
The device records various statistics related to each circuit breaker trip operation, allowing an accurate
assessment of the circuit br
eaker condition to be determined. The circuit breaker condition monitoring counters
are incremented every time the device issues a trip command.
These statistics are available in the CB CONDITION column. The menu cells are counter values only, and cannot be
set directly. The counters may be reset, however, during maintenance. This is achieved with the setting Reset CB
Data.
Note:
When in Commissioning test mode the CB condition monitoring counters are not updated.
5.1 APPLICATION NOTES
5.1.1 SETTING THE THRESHOLDS FOR THE TOTAL BROKEN CURRENT
Where power lines use oil circuit breakers (OCBs), changing of the oil accounts for a significant proportion of the
switchgear maintenance costs. Often, oil changes are per
formed after a fixed number of CB fault operations.
However, this may result in premature maintenance where fault currents tend to be low, because oil degradation
may be slower than would normally be expected. The Total Current Accumulator (I^ counter) cumulatively stores
the total value of the current broken by the circuit breaker providing a more accurate assessment of the circuit
breaker condition.
The dielectric withstand of the oil generally decreases as a function of I
2
t, where ‘I’ is the broken fault current and
‘t’ is the arcing time within the interrupter tank. The arcing time cannot be determined accurately, but is generally
dependent on the type of circuit breaker being used. Instead, you set a factor (Broken I^) with a value between 1
and 2, depending on the circuit breaker.
Most circuit breakers would have this value set to '2', but for some types of circuit breaker, especially those
operating on higher voltage systems, a value of 2 may be too high. In such applications Broken I^ may be set
lower, typically 1.4 or 1.5.
The setting range for Broken I^ is variable between 1.0 and 2.0 in 0.1 steps.
Note:
Any maintenance program must be fully compliant with the switchgear manufacturer’s instructions.
5.1.2 SETTING THE THRESHOLDS FOR THE NUMBER OF OPERATIONS
Every circuit breaker operation results in some degree of wear for its components. Therefore routine maintenance,
such as oiling of mechanisms, may be based on the number of operations. Suitable setting of the maintenance
thr
eshold will allow an alarm to be raised, indicating when preventative maintenance is due. Should maintenance
not be carried out, the device can be set to lockout the autoreclose function on reaching a second operations
threshold (No. CB ops Lock). This prevents further reclosure when the circuit breaker has not been maintained to
the standard demanded by the switchgear manufacturer’s maintenance instructions.
Some circuit breakers, such as oil circuit breakers (OCBs) can only perform a certain number of fault interruptions
before requiring maintenance attention. This is because each fault interruption causes carbonising of the oil,
degrading its dielectric properties. The maintenance alarm threshold (setting No. CB Ops Maint) may be set to
indicate the requirement for oil dielectric testing, or for more comprehensive maintenance. Again, the lockout
threshold No. CB Ops Lock may be set to disable autoreclosure when repeated further fault interruptions could
not be guaranteed. This minimises the risk of oil fires or explosion.
Chapter 14 - Monitoring and Control P14x
320 P14xEd1-TM-EN-1
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