Page 50 - Energize June 2022
P. 50
TECHNICAL
This contact is burned off and and other factors can adversely affect the operating time of a circuit breaker which has been
becomes shorter at each interruption idle for some time.
of load current. Dynamic resistance The “first trip” will typically represent and reveal the circuit breaker’s true operating
measurements clearly reveal this characteristics, unlike subsequent operations where mechanism components are loosened
shortening of the arcing contact. To obtain or freed up. Comparisons between the “first trip” and second or third breaker operations can
reliable DRM data a high test current is also reveal problems and should be used in determining the breaker’s condition.
required as well as test equipment with The biggest benefit of using first trip testing is to test “real world” operating conditions. If
good voltage measurement resolution. the circuit breaker has not operated for year, first trip testing will show if the circuit breaker
is slower due to problems such as corrosion in the mechanism linkages. With traditional
Switch operation test (timing methods, the testing is carried out after the circuit breaker has been taken out of service
analysis) and has been operated once or twice. The interpretation of circuit breaker operating coil
The trip or operation test is applied to signatures may provide information about the condition of the latching systems.
confirm that the switch operates correctly Referring to Figure 3, when a trip coil is first energised (1), current flows through its
and to measure the state of the operating windings. The coil magnetises the iron core of the armature, in effect inducing a force in the
mechanism. The mechanism operation armature. The current flowing through the trip coil increases, to the point where the force
cannot be observed or measured directly, exerted on the armature is sufficient to overcome combined gravitational and friction forces
so use is made of the operating coil which may be exerted, pulling it through the trip coil core (2).
current as a proxy measurement. By The magnitude of the initial current (1-2) is proportional to the energy required to move
analysing the current consumed by the trip the armature from its initial rest position. The movement of the iron core through the trip coil
coil during the circuit breaker’s operation, generates a back emf in the coil, which in turn has an effect on the current flowing through it.
it is possible to determine whether there The rate of rise of current depends on the change in the inductance of the coil.
are mechanical or electrical issues present. The armature operates the trip latch (3-4), which in turn collapses the trip mechanism
In many cases, such issues can be localised (4-5). More energy is required for the armature to resume motion and overcome the
and help to find the root cause. Optionally, additional loading of the mechanism. The anomaly at (4) may be caused by degradation of
monitoring the tripping supply’s voltage the mechanism bearings, lubrication, changes in temperature, excessive opening spring force
during the operation can detect issues or mechanism adjustment. The armature completes its travel (4-5) and hits a stop (5).
arising with tripping batteries. Of interest is the curve (4-5). As the armature moves from the point where the trip
mechanism is unlatched (4) to the stop (5) the inductance of the coil changes. The curve is an
First trip test indication of the speed of the armature. The steeper the curve the faster the armature is moving.
A first trip test is important to determine After the armature has completed its travel and has hit the stop (5), there is a change in
the condition of the coil operating the current pattern. The magnitude of the current (7) is dependent on the DC resistance of
mechanism and give information how the the coil. The auxiliary contact opens (8) to de-energise the trip coil and the current decays
circuit breaker would perform in a real-life to zero.
fault situation. Therefore, capturing the
first trip operation is essential to effective
circuit breaker condition-monitoring. A
circuit breaker spends most of its lifetime
conducting current without operating.
Once the protection relay detects a
problem, the circuit breaker, which was
idle for a long time, has to operate as
quickly as possible. However, if the circuit
breaker has not been operated for a long
time, the latch friction may increase.
Frequently, this problem is cleared after
the first operation of the breaker, and thus
the original cause cannot be detected in
subsequent testing.
Information about the latch friction
can be obtained from the coil current
waveform recorded during the first trip
operation. When a fault occurs, it is
expected of the circuit breaker to operate
correctly. Unfortunately, environmental
contaminants, hardened grease, vibration Figure 3: Operating coil current (Megger)
energize | June 2022 | 48