You press TEST, wait one minute, and the display shows 0.8 MΩ. Is the motor winding wet, aged, or still serviceable? A single-point insulation resistance reading answers none of these questions — which is why the Fluke 1507 includes PI and DAR measurement functions that track how insulation resistance changes over time while test voltage is applied. Interpreting the ratio correctly tells you whether the insulation is clean and dry, moisture-contaminated, or thermally degraded — before you make a $20,000 decision to pull a motor for rewind.
The Fluke 1507 is a compact insulation tester that outputs five selectable test voltages (50 V, 100 V, 250 V, 500 V, and 1000 V) and measures insulation resistance up to 10 GΩ. It is aimed at motor windings, switchgear, cables, and distribution panels — the core workload of an industrial maintenance electrician. Unlike a basic megohmmeter that only captures a spot reading, the 1507 automates the PI and DAR timing and reports the ratio directly, removing the need to manually log readings at 30 seconds and 60 seconds (DAR) or 1 minute and 10 minutes (PI).
DAR: The 60-Second Insulation Diagnostic
DAR is the ratio of the 60-second insulation resistance to the 30-second reading, both taken at the same test voltage. When voltage is first applied to insulation, the measured resistance starts low and rises as the capacitive charging current decays and the absorption current dominates. In clean, dry insulation the resistance climbs rapidly; in moisture-contaminated or carbon-tracked insulation, leakage current swamps the absorption effect and the reading stays flat.
To run a DAR test on the Fluke 1507:
- Disconnect the equipment under test from all power sources and verify with a voltage detector — the 1507 will refuse the test if it detects live voltage above 30 V on the terminals.
- Select the appropriate test voltage based on the equipment's rated voltage. For 480 V motor windings, use 500 V or 1000 V. For 120 V control circuits, use 250 V.
- Connect the black lead to the ground/earth reference and the red lead to the conductor under test. For a motor winding, the ground reference is the motor frame; the red lead goes to one phase lead with the other phases floating.
- Press and hold the TEST button. The 1507 starts a timer and captures the 30-second and 60-second readings automatically, then displays the DAR ratio.
DAR ≥ 1.4: Insulation is clean, dry, and in good condition. The motor or cable can be returned to service. DAR 1.25–1.4: Marginal — the insulation may have surface contamination or early-stage moisture ingress. Consider cleaning and re-testing. DAR < 1.25: The insulation is likely wet or contaminated. Do not energize. Dry the winding (oven or space heater), clean the termination area, and re-test. If DAR remains below 1.25 after drying, the insulation is degraded and the equipment should be scheduled for rewind or replacement.
PI: The 10-Minute Insulation Health Index
PI is defined in IEEE 43 as the ratio of the 10-minute insulation resistance to the 1-minute reading. It serves the same purpose as DAR but with a longer time base that exposes problems DAR misses — particularly in large machines with high-capacitance windings where the absorption current takes minutes to decay completely.
On the Fluke 1507, PI mode is activated by pressing the PI/DAR button before starting the test. The tester runs for a full 10 minutes, captures R1min and R10min, and computes the ratio. A 10-minute hold on the TEST button is impractical — use a test probe with a locking trigger or the Fluke TL224 SureGrip leads.
- PI > 2.0: Excellent. Class A insulation per IEEE 43-2013. The winding is clean, dry, and the insulation system is chemically stable. Return to service.
- PI 1.5–2.0: Adequate. Class B insulation. Acceptable for continued operation but worth trending. If the PI has dropped from a previously higher value, schedule an inspection at the next outage.
- PI 1.0–1.5: Marginal. The insulation may be moisture-affected or thermally aged. Cross-check with a DAR reading — if DAR is also below 1.25, the equipment is wet. If DAR is above 1.4 but PI is low, the insulation has bulk aging (thermal degradation of the binder/resin system rather than surface moisture).
- PI < 1.0: Unacceptable. The insulation resistance is decreasing over time, indicating a developing short path — possibly carbon tracking inside a cable termination or a phase-to-phase crack in a stator winding. Do not re-energize until the fault is located and repaired.
Why did my Fluke 1507 give a PI reading of exactly 1.0?
A PI of exactly 1.0 means the insulation resistance at 10 minutes was identical to the 1-minute reading. This occurs when the absorption current has already decayed to zero before the 1-minute mark — either because the equipment is very small (low capacitance, so absorption finishes in seconds) or because the insulation is so heavily contaminated that leakage current dominates from the start. In small motors under 50 HP, a PI of 1.0 is common and not necessarily a fail — IEEE 43 explicitly notes that for equipment with low geometric capacitance, the PI test is less diagnostic. Switch to a DAR test instead: if DAR is ≥ 1.4 on a small machine, the insulation is fine. If both PI and DAR are near 1.0, the insulation is wet or degraded regardless of machine size.
What test voltage should I use for a 4160 V motor?
IEEE 43 recommends a test voltage proportional to the equipment's rated line-to-line voltage. For a 4160 V motor, use the 1000 V setting on the Fluke 1507. A general guideline: test voltage (VDC) ≥ rated voltage (VAC) × 0.5, but not less than 500 V. For 4160 VAC, the minimum is 2080 VDC — the 1507's 1000 V maximum is below this guideline. In practice, 1000 V is acceptable for a go/no-go screening test because the 1507 is a portable insulation tester intended for maintenance rounds, not a high-potential proof test. For a definitive assessment of a medium-voltage machine, supplement the 1507 reading with a Fluke 1535/1537 which reaches 2500 V and provides a more complete picture of the insulation resistance vs. voltage curve. The 1507 at 1000 V is perfectly valid for trending — take the reading at the same voltage and temperature each month and watch for a downward PI trend, which is more informative than any single absolute value.
Does temperature affect PI and DAR readings?
Yes — and it is the most commonly overlooked variable. Insulation resistance drops roughly 50% for every 10°C rise in winding temperature. However, PI and DAR are ratios, which means the temperature effect partially cancels out, making them less temperature-sensitive than spot resistance readings. But there is a limit: IEEE 43 specifies that PI is only valid when the winding temperature is above the dew point (to avoid surface moisture condensation) and below 40°C. Above 40°C, the absorption current decays so quickly that the PI ratio compresses toward 1.0 even on healthy insulation. If you must test a hot machine, record the winding temperature with a contact thermometer or the Fluke 1507's built-in thermocouple input (via the temperature measurement function, separate from the insulation test mode) and note that the PI was taken above 40°C — the value still has trending utility but should not be used for an accept/reject call. For electrical testing on distribution equipment, the built-in voltage detection and auto-discharge circuit also make the 1507 safer than older hand-cranked megohmmeters that lack these protections.
