Cummins/Onan NTA855 — Megger and PI Test
Testing & diagnosis · Generator windings
For the Cummins/Onan NTA855, from the Cummins/Onan service manual.
Applies to: Cummins/Onan DFAB, Cummins/Onan DFAC, Cummins/Onan DFBF, Cummins/Onan DFCC, Cummins/Onan DFCE, Cummins/Onan NTA855, Cummins/Onan LTA10 engine, Cummins/Onan NT855 engine, Cummins/Onan NTA855 engine
Before you start
- Medium-voltage, 601 to 15,000 volts, present special hazards of severe personal injury or death.
- Windings of medium voltage generator sets must be dry before the generator is operated.
Procedure
- Perform the Generator Set Grounding Procedure.
- Disconnect plug J10 from voltage regulator output stage module and AC control input leads marked 4, 5, 6, 7 and 8.
- If RTD option installed, ground all six RTD temperature leads (18 leads total).
- Remove and separate neutral leads from generator load terminal marked N.
- Connect megger between one stator phase and ground while grounding other two phases; conduct test.
- Repeat for the other two phases.
- Disconnect main rotor and voltage suppressor leads from terminals F1+ and F2- and isolate from ground.
- Connect megger between one rotor lead and ground and conduct the test.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Megger Test applies voltage for up to one minute
- PI Test applies voltage for ten minutes; ratio of ten-minute to one-minute reading; ratio of two or greater is good
Full service-parts list: Cummins/Onan DFBF →, Cummins/Onan DFCC →, Cummins/Onan DFCE →
Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.