Dielectric Absorption Ratio (DAR) Explained: The 60-Second Insulation Diagnostic

You don’t always have 10 minutes for a full polarization index test. Sometimes you need a quick answer: is this insulation healthy or not?

That’s where the dielectric absorption ratio comes in. The DAR gives you a meaningful diagnostic in just 60 seconds. It’s not as thorough as the PI, but it’s far more useful than a simple spot reading — and it takes a fraction of the time.

This guide covers how the DAR works, how to interpret it, when it’s the right test to use, and where it falls short. All interpretation values are cross-referenced against IEEE 43-2013 and common industry practice.

What Is the Dielectric Absorption Ratio?

The dielectric absorption ratio is a ratio of two insulation resistance readings taken during the same test:

DAR = IR at 60 seconds ÷ IR at 30 seconds

That’s it. You apply test voltage, record the resistance at 30 seconds, keep the voltage applied for another 30 seconds, record the resistance at 60 seconds, and divide.

The result tells you whether the insulation is absorbing electrical charge normally. Good insulation absorbs charge steadily — resistance rises over time. Bad insulation is dominated by leakage — resistance stays flat or drops.

A DAR of 1.4 means the 60-second reading is 1.4 times the 30-second reading. The insulation is absorbing charge. A DAR of 1.0 means the readings are identical — no absorption happening. That’s a problem.

How DAR Works (The Physics)

When you apply DC voltage to insulation, three types of current flow:

Capacitive charging current — Charges the insulation’s natural capacitance. Dies out in a few seconds. By 30 seconds, it’s gone.

Absorption (polarization) current — Flows as the molecules in the insulation material align with the electric field. This current decays slowly over minutes. It’s the current that makes good insulation show rising resistance over time.

Leakage (conduction) current — The steady current that flows through and across the insulation. It doesn’t decay. This is the “bad” current — the one that indicates degradation, moisture, or contamination.

What happens between 30 and 60 seconds

At 30 seconds, the capacitive current is already gone. What remains is a mix of absorption current and leakage current.

At 60 seconds, the absorption current has decayed further. The leakage current stays the same.

In good insulation: Absorption current is still significant at 30 seconds but has decayed noticeably by 60 seconds. Total current drops. Resistance rises. DAR is above 1.0.

In bad insulation: Leakage current dominates from the start. The absorption current is either very small or masked by the high leakage. Total current barely changes between 30 and 60 seconds. Resistance stays flat. DAR is near 1.0 or below.

Why the 30/60 second window works

The 30-to-60-second window captures the absorption behavior at its most active phase. Most of the capacitive charging is done, but absorption is still decaying significantly. This gives the best signal-to-noise ratio in the shortest time.

If you measured at 10 and 20 seconds, the capacitive current would still be affecting the readings. If you measured at 5 and 10 minutes, you’d have a PI test. The 30/60 second window is the practical sweet spot for a quick diagnostic.

How to Perform a DAR Test

Equipment needed

Any insulation resistance tester (megohmmeter) that can hold test voltage for at least 60 seconds. Most modern digital megohmmeters calculate DAR automatically — you just need to run a 60-second test and the instrument displays the ratio.

If your megger doesn’t calculate DAR automatically, you can do it manually by recording the 30-second and 60-second readings.

Step-by-step procedure

1. Prepare the equipment. De-energize, isolate, lockout/tagout. Disconnect cables at the equipment terminals.

2. Discharge stored energy. Short all terminals to ground for at least 60 seconds.

3. Record conditions. Note ambient temperature, equipment temperature (if possible), and relative humidity.

4. Connect the megger.

• LINE lead → conductor or winding terminal
• EARTH lead → equipment frame / ground
• GUARD lead → surface insulation (if available)

5. Select test voltage. Per the applicable standard for your equipment:

• Motors ≤1 kV: 500V DC (IEEE 43-2013, Table 1)
• LV installations up to 500V: 500V DC (IEC 60364-6)
• Machinery: 500V DC (IEC 60204-1)
• Protection relays: 500V DC (IEC 60255-5)

6. Apply voltage for 60 seconds. Record the readings at:

• 30 seconds → this is your IR₃₀ value
• 60 seconds → this is your IR₆₀ value

7. Calculate DAR.

DAR = IR₆₀ ÷ IR₃₀

8. Discharge the equipment. Short all terminals to ground for at least 4 minutes (4× test duration).

Interpreting DAR Results

DAR interpretation table

DAR Value	Condition	Action
Below 1.0	Dangerous	Do not operate. Resistance is actually decreasing — severe moisture saturation, contamination, or insulation failure.
1.0 – 1.25	Poor	Investigate immediately. Very little absorption occurring. Likely moisture or heavy contamination.
1.25 – 1.6	Questionable	Monitor closely. Insulation shows some absorption but below normal levels. Schedule cleaning or drying.
1.6 – 2.0	Good	Insulation is absorbing charge normally. Healthy condition. Continue regular testing.
Above 2.0	Excellent	Clean, dry insulation in very good condition.

Important context

These DAR values are widely used in field practice and referenced in NETA maintenance testing specifications. Unlike the PI, the DAR is not explicitly defined in the main tables of IEEE 43-2013 — the standard focuses on the 10-minute PI test for rotating machinery. However, IEEE 43-2013 does discuss dielectric absorption as a diagnostic concept, and the DAR is a practical application of that concept.

The DAR values above are consistent with the PI interpretation: a DAR of 1.6 corresponds roughly to a PI of 2.0 on the same insulation, and a DAR of 1.0 corresponds to a PI near 1.0.

DAR vs PI: When to Use Which

Feature	DAR	PI
Duration	60 seconds	10 minutes
Formula	IR₆₀ / IR₃₀	IR₆₀₀ / IR₆₀
Temperature independent	Mostly yes	Yes
Size independent	Mostly yes	Yes
Depth of diagnosis	Moderate	High
Standard reference	NETA, field practice	IEEE 43-2013, Table 3
Best for	Quick checks, smaller motors, routine screening	Large motors, generators, critical equipment

When to use DAR

• Routine screening of multiple motors during a maintenance outage — you need speed
• Smaller motors (below 100 kW) where the absorption curve is shorter
• Quick check after a concerning spot reading to decide if further investigation is needed
• Field situations where 10 minutes per test is impractical (testing 50 motors in a day)

When to use PI

• Large motors above 100 kW
• Generators of any size
• Medium and high-voltage machines (above 1 kV)
• Critical equipment where failure causes significant downtime or safety risk
• When a DAR result comes back questionable and you need a more definitive answer
• Baseline testing on new or rewound motors

My rule of thumb

If I’m testing a critical motor, a generator, or anything above 100 kW — I run the full PI test. Ten minutes is worth the diagnostic certainty.

If I’m screening a batch of 30 small motors during a shutdown, I run DAR on all of them. Any motor with a DAR below 1.6 gets a follow-up PI test.

DAR vs Spot Reading: Why DAR Is Better

A spot reading (60-second IR value) gives you one number. That number is heavily influenced by temperature, humidity, and equipment size. A 400V motor reading 15 MΩ could be perfectly healthy at 35°C or failing at 15°C.

The DAR eliminates most of these variables. Because it’s a ratio of two readings taken 30 seconds apart under the same conditions, temperature effects largely cancel out. Equipment size effects cancel out. What you’re left with is a measure of the insulation’s absorption behavior — which directly reflects its condition.

Example:

Motor A reads 50 MΩ at 60 seconds. DAR = 1.8. Good insulation — clean and dry.

Motor B reads 50 MΩ at 60 seconds. DAR = 1.05. Bad insulation — the identical spot reading masks the fact that this motor has almost no absorption. Leakage dominates.

Without the DAR, both motors look the same. With it, you can see that Motor B needs attention.

Limitations of the DAR Test

The DAR is a useful quick diagnostic, but it has limitations you should know about.

Short absorption window

Some insulation materials — especially modern epoxy-mica systems — have absorption curves that develop slowly. The 30-to-60-second window may not capture enough of the absorption to give a clear signal. The PI test’s 1-to-10-minute window catches these slower-developing materials better.

Low-capacitance equipment

Equipment with very low capacitance — short cable runs, small switchgear, protection relays — may reach steady state so quickly that there’s no meaningful difference between the 30-second and 60-second readings. A DAR of 1.0 on a small relay doesn’t necessarily mean bad insulation — it may just mean the insulation has very little capacitance to absorb charge.

Very high IR values

If the insulation resistance is very high (above 5 GΩ), the leakage currents are so tiny that measurement noise can affect the ratio. The same limitation applies to the PI — IEEE 43-2013 Clause 12.2.2 notes that the PI may not be meaningful above 5 GΩ. The same principle applies to DAR.

Not a standalone pass/fail

The DAR tells you about absorption behavior, but you still need the absolute IR value to complete the picture. A DAR of 1.8 is great — but if the 60-second reading is only 2 MΩ, the insulation still has a problem. Always evaluate DAR alongside the absolute IR value.

DAR on Different Equipment Types

Motors

DAR works well for motors because motor windings have enough capacitance and insulation mass to show clear absorption behavior. For random-wound motors below 1 kV, a DAR above 1.4 generally indicates acceptable insulation. For form-wound motors above 1 kV, aim for DAR above 1.6.

Transformers

DAR on oil-filled transformers can be misleading because transformer oil has a very flat absorption curve (close to 1.0). This means a healthy oil-filled transformer may show a low DAR even when the solid insulation is fine. For transformer diagnostics, the PI is more useful, and DGA is more important than either.

For dry-type transformers, DAR works the same as for motors.

Cables

DAR is useful for cable insulation testing, especially on longer runs where the capacitance gives a clear absorption signal. On very short cable runs (a few meters), the capacitance may be too low for meaningful DAR results — use a spot reading instead.

Switchgear and protection relays

These typically have low capacitance. DAR results may not be meaningful. A spot reading at 500V DC per IEC 60255-5 (≥100 MΩ for relays) or per IEC 62271 is usually sufficient.

Common Mistakes

Comparing DAR values across different equipment types. A DAR of 1.2 on a motor is concerning. A DAR of 1.2 on a small relay is meaningless — the relay just doesn’t have enough capacitance for absorption to develop in 60 seconds.

Not recording the 30-second reading. Some technicians only record the 60-second value and miss the DAR entirely. Always record at 30 seconds, 60 seconds, and (if running a PI test) 10 minutes.

Testing without discharging first. Residual charge from a previous test or from recent operation can skew the initial current and distort the ratio. Discharge all terminals to ground for at least 1 minute before testing.

Relying on DAR alone for critical equipment. DAR is a screening tool. For critical motors, generators, and transformers, the full PI test gives a more reliable diagnostic. Use DAR for screening, PI for confirmation.

Confusing DAR with PI. They measure the same phenomenon (dielectric absorption) but at different time scales. DAR uses 30/60 seconds. PI uses 1/10 minutes. Different time windows, different interpretation tables. Don’t apply PI minimums to DAR values or vice versa.

FAQ

Key Takeaways

• DAR = IR at 60 seconds ÷ IR at 30 seconds. It measures dielectric absorption in 60 seconds.
• DAR above 1.6 is good. Below 1.25 is poor. Below 1.0 is dangerous.
• DAR eliminates temperature and equipment size variables — it’s a ratio, not an absolute value.
• Use DAR for quick screening of multiple motors. Use PI for diagnosis of critical equipment.
• DAR works best on equipment with enough capacitance to show absorption — motors, cables, large windings. It’s less useful on low-capacitance equipment like relays and small switchgear.
• Always evaluate DAR alongside the absolute IR value. A good ratio on a bad absolute number is still a problem.
• DAR is not a substitute for the PI test on critical equipment. It’s a faster, less thorough alternative for routine checks.

Standards Referenced in This Article

Standard	Key Content
IEEE 43-2013	Dielectric absorption concept, PI test methodology, PI invalid above 5 GΩ (Clause 12.2.2)
IEC 60204-1	Test voltage: 500V DC, minimum IR ≥1 MΩ (Clause 18.3)
IEC 60364-6	LV installation test voltages: 250/500/1000V DC (Clause 6.4.3.3)
IEC 60255-5	Protection relay IR: 500V DC, ≥100 MΩ (Clause 6.2.2)
NETA MTS	DAR interpretation values referenced in maintenance testing specifications