Cummins/Onan — Megger and Polarization Index (PI) Test
Testing & diagnosis · Generator windings
From a Cummins/Onan service manual — check it applies to your exact model before use.
Applies to: Cummins/Onan DFHA, Cummins/Onan DFHB, Cummins/Onan DFHC, Cummins/Onan DFHD, Cummins/Onan QST30 engine
Before you start
- Medium voltage (601-15,000 V) presents hazards of severe personal injury or death
Procedure
- Perform the Generator Set Grounding Procedure.
- Disconnect plug J10 from the voltage regulator output module and disconnect AC control leads 4, 5, 6, 7, 8.
- If RTD option is installed, ground all six RTD temperature leads (18 leads total).
- Perform the Winding Resistance Test for the desired windings.
- Perform the Insulation Resistance Test and PI Test for the desired windings.
- Investigate and correct any low readings before returning the set to service.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- PI ratio of two or greater is considered good
Full service-parts list: Cummins/Onan DFHA →, Cummins/Onan DFHB →, Cummins/Onan DFHC →, Cummins/Onan DFHD →
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.