The overspeed protective device shall shut down the generator set each time an engine speed of 2070 to
2250 rpm is obtained, and the generator set shall not restart until the reset button is pressed, and both air
If the generator set does not shut down at the prescribed rpm, or can be started after a shut down without
pressing the reset button, troubleshoot the overspeed protection circuit in accordance with Chapter 2.
12-16. CIRCUIT INTERRUPTER (SHORT CIRCUIT).
General. A circuit interrupter is connected between the generator voltage reconnection system and the
generator set output terminals to disconnect the generator output from the load and also to protect the
generator from a short circuit. The circuit interrupter is operated from a current sensor external to the
Apparatus. Instrumentation for measuring load conditions shall be as illustrated in Figure 12-5. In addition, a
noninductive shunt, a short-circuiting switch, a galvanometer matching network an oscillograph, and
galvanometers having a flat frequency response (flat within plus or minus 5 percent) from DC to 3,000 Hz
will be required.
Preparation for Test.
Connect the load and instrumentation in accordance with Figure 12-5 and Figure 12-6, for one
voltage and frequency.
Connect the shunt, galvanometer matching network, oscillograph, and short-circuiting switch
across the load terminals of the phase being tested.
Start and operate the generator set at rated voltage, rated frequency, and rated load.
Set the oscillograph time marker to a minimum of 0.01 second or use a 60 Hz timing trace. Set the
chart speed so that the individual peaks of the current waveform are clearly visible, and adjust the
peak-to-peak rated current amplitude to a minimum of 0.5 inch (13 mm).
Before dosing the short-circuiting switch, record a portion of the steady-state load for calibration. With
the same load conditions record all instrument readings.
If the circuit Interrupter fails to operate within the specified time, open the short
circuit switch to prevent damage. Note the failure to operate on the data sheet.
With the oscillograph still recording the steady-state current, dose the short-circuiting switch.
The generator set contains an overload malfunction indicator; check and record its indication.
Restart the generator set, and test the time delay by momentarily dosing and opening the short circuit
switch. When performing this test, the short circuit switch shall not be dosed longer than 9 seconds.
Repeat steps (a) through (e), above, for each possible short circuit (L1-L0, L2-L3, L1-L2-, L3, etc.)
Repeat steps (a) through (g), above, for all voltage connections.
From the oscillograms taken in step (2)(d), above, determine the time between the indicated closure of the
short-circuiting switch and the opening of the circuit interrupter. See Figure 12-7.
Calculate the short-circuit current using the peak-to-peak amplitudes of the current trace and the steady-
state ammeter reading prior to application of the short circuit.
Use the oscillograms taken in step (e), above, and determine if the circuit interrupter actuated upon
application of the momentary short circuit.
Tabulate the above results for each line connection, and compare the results with the requirements in Table