Cummins/Onan — Testing Winding Insulation Resistance
Testing & diagnosis · Generator windings
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Disconnect starting battery cables (negative first) before performing tests.
Procedure
- Apply the test potential between the winding and ground (winding laminations) for 10 minutes.
- Record resistance at 1 minute and again at 10 minutes.
- For a new generator with dry windings, obtain at least 5 megohms.
- Polarization index should be at least 2.
- For a set in service, resistance should not be less than 1 megohm.
Tools required
- 500 VAC megger Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- At least 5 megohms new; not less than 1 megohm in service; polarization index at least 2
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.