70 TM1700-SERIES ZP-BL16E BL1448GE
5 PERFORM TESTING
The peak value of the first, lower current peak is re-
lated to the fully saturated coil current (max current),
and this relationship gives an indication of the spread
to the lowest tripping voltage. If the coil was to reach
its maximum current before the armature and latch
start to move, the breaker would not be tripped. It is
important to note, however, that the relationship be-
tween the two current peaks varies, particularly with
temperature. This also applies to the lowest tripping
voltage.
Dynamic Resistance Measurements
(DRM)
The DRM test method is very suitable for diagnostic
testing. Tests are conducted by injecting high DC
current, approximately 250 A or higher, through the
breaker and measuring the voltage drop and current
while the breaker is operated, not to be confused
with static resistance measurement (microohm mea-
surement), which measures contact resistance when a
breaker is closed.
The breaker analyzer then calculates and plots resis-
tance as a function of time.
DRM has a number of applications. On circuit break-
ers with an arching contact design (mainly SF6 types)
DRM can be used to measure the arcing contact wear.
When contact movement is recorded simultaneously,
you can read the resistance at each point of contact.
Since there are a significant difference in resistance
between the main- and arching contacts the resis-
tance graph and the motion graph will indicate the
length of the arching contact. In some cases, circuit
breaker manufacturers can supply reference curves for
the type of contact in question.
Other applications e.g. timing measurements can with
DRM be performed on circuit breakers grounded on
both sides. Either the circuit breaker has to be ground-
ed during test due to safety regulations or the circuit
breaker design does not allow removing one ground.
If a sufficiently high current is used (about 250 A or
higher), there will be a noticeable step in the voltage
change when the breaker contact closes or opens in
spite of the parallel ground connections. Similarly,
DRM can be used when a breaker has parallel main
contacts.
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