ET online
November 2007
Circuit breaker tester now available

Circuit breaker tester now available

14 November 2007

Rickard Jonsson - European sales manager

The TM1800™ is the instrument platform for circuit breaker maintenance, based on more than 20 years’ experience of over 4,000 delivered breaker analysers. The modular construction makes it possible to configure the TM1800 for measurements on all known types of circuit breakers in operation on the world market.

The robust design contains powerful technology that streamlines circuit breaker  testing. Sophisticated measurement modules enable great time savings as many parameters can be measured simultaneously, eliminating the need for new setup each time. DualGround™ testing using the new DCM module makes the testing safe and time saving, by keeping the circuit breaker grounded on both sides throughout the test. The DCM module uses a measuring technology called Dynamic Capacitive Measurement. Timing M/R uses Active Interference Suppression to obtain correct timing and accurate PIR values in high voltage substations. An adaptive, easy-to-use software suite supports activities from timing (simply turning a knob without the need for presetting) to advanced help functions for hooking up to the test object. A full keyboard and 8” colour screen is the front end of the high-level user interface. The Select-Connect-Inspect workflow guides users to fast results in three steps. Testing is made easier to learn and perform.
The system also offers full connection capability to the local network, printers etc.

Timing measurements
Simultaneous measurements within a single phase are 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 differences 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 3.33 ms between zerocrossovers. 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 synchronised breaking must meet more stringent requirements in both of the previously stated situations.
There are no generalised time limits for the time relationships between main and auxiliary contacts, but it is still important to understand and check their operation. The purpose of an auxiliary contact is to close and open a circuit. Such a circuit might enable a closing 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 closing 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 information about this cycle.

Motion measurements
A high-voltage breaker is designed to interrupt a specific short-circuit current, and this requires operation 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 sufficiently to interrupt the current at the next zero-crossover. 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 motion 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 contactclosure or contact-separation point. The time that elapses between these two points ranges from 10 to 20 ms, which corresponds to 1-2 zerocrossovers.
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 acceleration 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 function 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 function 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. When the coil armature (which actuates the latch on the operating mechanism’s energy package) starts to move, the electrical relationship changes and the coil current drops. When the armature hits its mechanical end position, the coil current rises to the current proportional to the coil voltage). 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.
The peak value of the first, lower current peak is related 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 between the two current peaks varies, particularly with temperature. This also applies to the lowest tripping voltage.

For further information about the TM1800 click here