12-17. CIRCUIT INTERRUPTER (OVERLOAD CURRENT).
General. A circuit interrupter is connected between the generator voltage reconnection system and the
generator output terminals to disconnect the generator output from the load and to protect the generator from
sustained overload current. The circuit interrupter is operated from a current sensor external to the
Apparatus. Instrumentation for measuring load conditions and field voltage and current shall be as illustrated
in Figure 12-5. In addition, a stopwatch or an oscillograph with galvanometer matching network and a
noninductive shunt, and galvanometers having a flat frequency respond (within plus or minus 5 percent) from
DC to 3,000 Hz are required.
Preparation for test. Connect the load and field instrumentation in accordance with Figure 12-3 and
Figure 12-5, for one voltage and frequency.
If the circuit Interrupter falls to operate within the time specified in Table 12-3 at any time during the
performance of this method, manually open the circuit Interrupter, and reduce the load Impedance to rated value
before reclosing the circuit Interrupter. Record on the data sheet the failure of the Interrupter to operate and the
total elapsed time the overload was on the set.
Start and operate the generator set at rated voltage, rated frequency, and rated load.
Allow the generator set to stabilize at rated load, voltage, and frequency. During this period,
readings of the load and field instrumentation shall be recorded at minimum intervals of 10
minutes. If necessary, adjustments to the load, voltage, and frequency may be made to
maintain the rated load at the rated voltage and frequency. Adjustments to the load, voltage, or
frequency shall be noted on the data sheet Stabilization will be considered to have occurred
when four consecutive voltage and current readings of the exciter field either remain unchanged,
or have only minor variations about an equilibrium condition with no evident continued increase
or decrease in value after the last load, voltage, or frequency adjustment has been made.
In one step, increase the load current to the overload current value specified in Table 12-3 (the
increase in current may be accomplished by any practical means; for example, reactively or
using reduced voltage levels).
The frequency shall be maintained at rated conditions, and the load current shall be kept
constant and balanced equally among the phases.
Record all load instrumentations and the time, in seconds, required for the circuit interrupter to
The generator set contains an overload malfunction indicator. Check and record its indication.
Allow the generator set to cool at rated load for a minimum of 15 minutes.
Repeat steps (c) through (f), above, except that the load current is increased to the overload
current value in phase A only. Phases B and C remain at the rated load current value.
Repeat step (g), above, except that the load is increased to the overload current value in phase
B only. Phases A and C remain at the rated load value of current (i) Repeat step (g), above,
except that the load is increased to the overload current value in phase C only. Phases A and B
remain at the rated load value of current.
Results. The data sheets shall show, as a minimum, whether or not the circuit interrupter operated, the times
required for the interrupter to operate, the indication of the malfunction indicator, the overload conditions and the
stabilization data. Compare this data with the requirements of Table 12-3.