Insulation Resistance Tester: Key Parameters, Voltage Ranges& EquipmentSelection

4. Six Core Parameters to Check Before Procurement

Figure: Core Parameter Decision Chain for the Insulation Resistance Tester

Once you receive the manufacturer’s technical specification sheet, all of the following 6 parameters must be verified one by one — none can be omitted.

Parameter 1: Test Voltage Levels and Voltage Regulation Accuracy

What to check: which voltage levels the instrument supports, and the voltage regulation accuracy of each level (usually expressed as ±%).

Why it matters: if the test voltage is unstable, the insulation resistance readings cannot be trusted. Especially for large-capacity test objects (such as large transformers), whose equivalent capacitance is large, charging them pulls the test voltage down; if the instrument’s voltage regulation capability is insufficient, the actually applied voltage is far below the nominal value and the measured insulation resistance reads artificially high — a "falsely high" result that misjudges the insulation as being in good condition.

Reference requirement: the voltage regulation accuracy of mainstream qualified products is usually within ±5%, while high-end products can reach ±1% to ±2%. It is recommended to require the manufacturer to provide measured voltage regulation data under rated short-circuit current load, rather than the nominal accuracy under no-load conditions.

Parameter 2: Upper Limit of the Measuring Range

What to check: the highest insulation resistance value the instrument can measure reliably (unit: GΩ or TΩ).

Why it matters: if the range is insufficient, the result shows "overflow" or stops at the upper range limit and the true value cannot be obtained. For large high-voltage equipment (such as 110 kV and above main transformers and high-voltage cables), the insulation resistance in normal condition can reach several hundred GΩ or even above 1 TΩ; an instrument with insufficient range is effectively useless for such objects.

Note the distinction:

• Measuring range (the range within which readings are accurate)

• Display range (the range that can be displayed but with potentially large errors)

These are not the same concept. You should require the manufacturer to specify the accurate measuring range at each voltage level, rather than the maximum display range.

Parameter 3: Short-Circuit Current (Short-Circuit Output Current)

What to check: the maximum output current of the instrument when the test voltage output terminal is short-circuited (unit: mA).

Why it matters: this is an often overlooked but very important parameter, reflecting the instrument’s charging capability for large-capacity test objects. When the test object’s insulation resistance is extremely low (for example, severely moisture-affected equipment may have an insulation resistance of only a few hundred kΩ), the instrument needs to output a relatively large current to maintain a stable test voltage. If the short-circuit current is too small, the measurement result for a low-insulation test object will be distorted by the severe drop in test voltage.

In addition, the short-circuit current is also an important indicator of equipment safety — different types and classes of equipment have different safety limit requirements. Standards such as IEC 60060-3 contain provisions on this (please verify against the currently valid version).

Reference range: the short-circuit current of common products ranges from 1 mA to 10 mA. For testing large-capacity objects such as large transformers, products with a short-circuit current ≥ 3 mA are recommended; for low-voltage small-capacity equipment testing, 1–2 mA is usually sufficient.

Parameter 4: Automatic Discharge Function

What to check: after the test ends, whether the instrument can automatically and safely release the static charge stored on the test object, as well as the design of the discharge time and discharge path.

Why it matters: a large-capacity test object accumulates a large amount of charge during the test. If the test leads are disconnected immediately after the test, this charge may discharge through the operator and cause an electric shock accident. The automatic discharge function is a basic safety configuration of modern megohmmeters and should be treated as a red line in procurement: any product without this function should be excluded outright.

A closer look: some products claim to have automatic discharge, but the discharge resistance is too large, resulting in an excessively long discharge time (several minutes to drop to a safe voltage). In actual use, operators may act prematurely because the waiting time is too long, creating a hidden danger. During procurement, you should require the manufacturer to provide discharge time data for different equivalent capacitances of test objects; ideally the discharge should drop below the 50 V safe voltage within 1 minute (subject to the currently valid safety regulations).

Parameter 5: Interference Immunity (Immunity to Electromagnetic Interference)

What to check: the measurement stability of the instrument in on-site electromagnetic interference environments, usually expressed as the "common-mode interference rejection ratio" (CMRR) or a "field interference immunity class".

Why it matters: field environments (substations, power plants, industrial sites) contain a large amount of electromagnetic interference, especially when testing de-energized equipment located next to energized equipment. Interference currents superimpose on the measured leakage current, causing insulation resistance readings to be significantly lower than actual and leading to a false judgment that the insulation is unqualified.

How to identify: require the manufacturer to provide measurement error data under simulated interference environments; or conduct an on-site demonstration before procurement — test equipment known to be in good insulation condition in the actual working environment and observe whether the readings are stable.

Role of the G terminal (guard terminal): high-quality megohmmeters are all equipped with a G (Guard/shield) terminal, used to eliminate the influence of the test object’s surface leakage current on volume resistance measurement. During procurement, confirm that the G terminal can be connected independently, and learn the wiring method recommended by the manufacturer.

Parameter 6: Data Storage and Management Capability

What to check: whether the instrument can store measurement data, how large the storage capacity is, whether data can be exported to a PC, and whether it supports recording measurement curves (R-t curves).

Why it matters: for preventive testing that requires longitudinal historical comparison, a single reading is far less valuable than historical trends. Equipment capable of recording complete R-t curves (i.e., the curve of the insulation resistance change every second during measurement) allows you to see intuitively whether the absorption process is normal, rather than just the ratio at two points in time.

Procurement advice:

• Daily inspection and quick judgment: no data storage function needed; a basic model is sufficient;

• Periodic preventive testing and building equipment insulation archives: data storage and PC export are essential functions;

• High-end comprehensive assessment: R-t curve recording, automatic report generation, and synchronized temperature data recording are required.

5. What Voltage Level Corresponds to Different Test Equipment, and How to Estimate Range and Safety Class

Figure: Voltage Level and Range Strategy for Different Test Objects

This is the most critical technical judgment in procurement and selection, and the part that must never rely on guesswork. The classification below is based on DL/T 596 Preventive Test Regulations for Electric Power Equipment (please refer to the currently valid version) and relevant product standards; specifics shall be verified against the currently valid version.

5.1 Principles for Selecting Test Voltage

Basic principle: the test voltage should match the rated insulation level of the test equipment, generally several times the rated operating voltage of the equipment, but must not exceed the withstand level of the equipment insulation. Specific values are subject to relevant test regulations and equipment manufacturer requirements.

HV equipment (rated voltage ≥ 1 kV) → generally use 2500 V or 5000 V
LV equipment (rated voltage < 1 kV) → generally use 500 V or 1000 V
Extra-low voltage circuits (control circuits, secondary circuits) → generally use 250 V or 500 V

Notes: the above is a general reference; in practice, the technical documents of the test equipment manufacturer, relevant national/industry standards, and test regulations shall prevail.

5.2 Applicable Objects for Each Voltage Level

250 V Level

Typical applications:

• Control circuits and secondary circuits with rated voltage ≤ 250 V;

• ELV equipment and low-voltage communication cables;

• Some low-voltage sensor and instrument circuits;

• Preliminary safety checks of aged equipment (to avoid high voltage damaging already degraded insulation).

Range requirement: the acceptable insulation resistance of such objects is usually in the MΩ range; the range requirement is not high, and 5–20 GΩ is sufficient.

500 V Level

Typical applications:

• Low-voltage motors and LV distribution lines rated 220 V / 380 V;

• Insulation testing of low-voltage switchgear;

• Insulation testing of lighting and socket circuits;

• Low-voltage cables (acceptable insulation value usually ≥ 0.5 MΩ, subject to currently valid standards).

Range requirement: for low-voltage small-capacity motors and short-distance cables, a 500 GΩ range is usually sufficient.

1000 V Level

Typical applications:

• Medium-voltage equipment rated 660 V / 1000 V;

• Equipment rated below 1 kV but with a higher insulation class (such as new high-voltage motors and VFD drive systems of high insulation class);

• In some scenarios, insulation testing of the secondary windings of 10 kV instrument transformers.

Range requirement: a range of 100 GΩ–1 TΩ usually meets the need.

2500 V Level

Typical applications:

• High-voltage motors rated 6 kV / 10 kV (insulation acceptance criteria are subject to equipment technical specifications and currently valid standards);

• Insulation testing of 10 kV distribution transformers (in some scenarios);

• Insulation testing of 10 kV power cable lines;

• Insulation testing of 10 kV switchgear;

• Insulation testing of 10 kV instrument transformers (CT/PT);

• In some scenarios, preliminary checks of 35 kV equipment at a lower voltage level.

Range requirement: the normal insulation resistance of high-voltage motors and 10 kV cables can reach several hundred GΩ; the upper range limit is recommended to be no less than 500 GΩ, preferably 1 TΩ or above.

5000 V Level

Typical applications:

• Winding insulation testing of power transformers at 35 kV and above;

• Insulation testing of power cables at 35 kV and above;

• Insulation testing of CTs and PTs in systems of 110 kV and above;

• Comprehensive insulation assessment of large generator stator windings;

• Insulation assessment of high-voltage switchgear (GIS, etc.).

Range requirement: the normal insulation resistance of large main transformers (110 kV and above) can reach several hundred GΩ to several TΩ; the upper range limit is recommended to be no less than 1 TΩ, and high-end products should reach above 10 TΩ.

5.3 Range Estimation Method

How to estimate the range required by the test object before procurement? You can refer to the following logic:

Step 1: Check the technical standards or factory test reports of the test equipment to learn
the reference insulation resistance range of this type of equipment in normal condition;

Step 2: Take the upper limit of the normal value as the minimum required range, with a margin of 3–5 times;

Step 3: For large equipment (main transformers, generators), it is recommended to take the highest value
in actual operation and historical test records as the upper-bound reference for the range requirement.

Empirical reference (the following values are rough references; actual values are subject to equipment technical specifications and standards):

The values in the table above are reference levels only; in practice, the provisions of the test equipment manufacturer and currently valid standards prevail. New equipment usually performs significantly better than the reference values above, while aged equipment may be close to the lower acceptance limit.