Cummins/Onan — Insulation Resistance (Megger) & Polarization Index (PI) Testing
Testing & diagnosis · Generator windings
From a Cummins/Onan service manual — check it applies to your exact model before use.
Applies to: Cummins/Onan DFAA, Cummins/Onan DFAB, Cummins/Onan DFAC, Cummins/Onan DFBF, Cummins/Onan DFCB, Cummins/Onan DFCC, Cummins/Onan DFCE, Cummins/Onan DFE
Before you start
- The windings of medium-voltage (601 through 15,000 volts) generator sets must be dry before the generator is operated.
Procedure
- Perform the Generator Set Grounding Procedure.
- Disconnect plug J10 from A37 and AC control leads 4,5,6,7,8.
- If RTD option installed, ground all six RTD temperature leads (18 leads).
- Perform Insulation Resistance Test and PI Test for desired windings.
- Megger test: apply voltage for up to one minute.
- PI test: apply voltage for ten minutes; record 1-minute and 10-minute readings.
- PI ratio of 2 or greater is good for new and in-service sets.
- Investigate and correct low readings before returning 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 = ten minute reading (megohms) divided by one minute reading (megohms). Ratio of two or greater is considered good.
Full service-parts list: Cummins/Onan DFBF →, Cummins/Onan DFCC →, Cummins/Onan DFCE →, Cummins/Onan DFE →
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.