Megger TM1700 Series, User Manual

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5.8 Evaluate the results
you can evaluate the results by viewing the record-
ings. in the following are some guidelines for how to
interpret the graphs. However these interpretations
are not valid for all circuit breakers. Contact the circuit
breaker manufacturer for specific information.
Timing measurement
simultaneous measurements within a single phase is
important in situations where a number of contacts
are connected in series. Here, the breaker becomes a
voltage divider when it opens a circuit. if the time dif-
ferences are too great, the voltage becomes too high
across one contact, and the tolerance for most types
of breakers is less than 2 ms.
The time tolerance for simultaneous measurements
between phases is greater for a 3-phase power
transmission system running at 50 Hz since there is
always 10 ms between zero-crossovers. still, the time
tolerance is usually specified as less than 2 ms, even
for such systems. it should also be noted that breakers
that perform synchronized breaking must meet more
stringent requirements in both of the previously stated
situations.
There are no generalized time limits for the time
relationships between main and auxiliary contacts, but
it is still important to understand and check their op-
eration. The purpose of an auxiliary contact is to close
and open a circuit. such a circuit might enable a clos-
ing coil when a breaker is about to perform a closing
operation and then open the circuit immediately after
the operation starts, thereby preventing coil burnout.
The A contact must close well in advance of the clos-
ing of the main contact. The B contact must open
when the operating mechanism has released its stored
energy in order to close the breaker. The breaker
manufacturer will be able to provide detailed informa-
tion about this cycle.
Motion measurement
A high-voltage breaker is designed to interrupt a
specific short-circuit current, and this requires opera-
tion at a given speed in order to build up an adequate
cooling stream of air, oil or gas (depending on the
type of breaker). This stream cools the electric arc suf-
ficiently to interrupt the current at the next zero-cross-
over. it is important to interrupt the current in such a
way that the arc will not re-strike before the breaker
contact has entered the so-called damping zone.
speed is calculated between two points on the mo-
tion curve. The upper point is defined as a distance in
length, degrees or percentage of movement from a)
the breaker’s closed-position or b) the contact-closure
or contact-separation point. The time that elapses
between these two points ranges from 10 to 20 ms,
which corresponds to 1-2 zero-crossovers.
The distance throughout which the breaker’s electric
arc must be extinguished is usually called the arcing
zone. From the motion curve, a velocity or accelera-
tion curve can be calculated in order to reveal even
marginal changes that may have taken place in the
breaker mechanics.
Damping is an important parameter for the high-
energy operating mechanisms used to open and close
a circuit breaker. if the damping device does not func-
tion satisfactorily, the powerful mechanical strains that
develop can shorten breaker service life and/or cause
serious damage. The damping of opening operations
is usually measured as a second speed, but it can also
be based on the time that elapses between two points
just above the breaker’s open position.
Coil currents
These can be measured on a routine basis to detect
potential mechanical and/or electrical problems in
actuating coils well in advance of their emergence as
actual faults. The coil’s maximum current (if current is
permitted to reach its highest value) is a direct func-
tion of the coil’s resistance and actuating voltage.
This test indicates whether or not a winding has been
short-circuited.
When you apply a voltage across a coil, the current
curve first shows a straight transition whose rate of
rise depends on the coil’s electrical characteristic and
the supply voltage (points 1-2). When the coil arma-
ture (which actuates the latch on the operating mech-
anism’s energy package) starts to move, the electrical
relationship changes and the coil current drops (points
3-5). When the armature hits its mechanical end posi-
tion, the coil current rises to the current proportional
to the coil voltage (points 5-8). The auxiliary contact
then opens the circuit and the coil current drops to
zero with a current decay caused by the inductance in
the circuit (points 8-9).