The insulation performance of high-voltage power equipment directly affects the reliable operation of the power grid and the personal safety of operators. To comprehensively evaluate the health condition of the insulation medium, the power industry generally adopts insulation resistance testing and withstand voltage tests as complementary assessment methods. However, in practical applications, the concepts of these two tests are often confused, especially in terms of the sequence of tests and the selection of standards. Clarifying the functional positioning and standard basis of the two is the prerequisite for standardizing the conduct of high-voltage electrical tests.
The essential differences between technical principles and assessment objectives
1. Insulation Resistance Test: “Quantitative Physical Examination” of Insulation Status
The insulation resistance test generally uses megohmmeters or insulation resistance testers, applying low-voltage direct current voltage. Common settings include 500V, 1000V, 2500V, and even 5000V, which are non-destructive tests. Its core purpose is to measure the MΩ-level direct current resistance value of the insulation structure, qualitatively determining whether the insulation is overall damp, aged, or has a penetrating conductive channel.
In the high-voltage power field, this test should not merely stop at reading a single resistance value. For large-capacity capacitive equipment such as transformers, bushings, and cables, the measurement of absorption ratio (R₆₀s / R₁₅s) and polarization index (R₁₀min / R₁min) is more diagnostic, as they can effectively distinguish insulation moisture from resistance changes caused by surface dirt.
2. Withstand Voltage Test: “Ultimate Verification” of Insulation Strength
The withstand voltage test, also known as dielectric strength test, applies voltages far higher than the equipment’s rated working voltage, including alternating current, direct current, or impulse voltage. This test has two attributes: under ideal controlled conditions, it can avoid damaging the equipment, but itself has the potential risk of damaging insulation weaknesses.
Its assessment objective is extremely clear: to test whether the insulation structure will undergo breakdown or flashover under the specified overvoltage conditions, such as system operation overvoltage or lightning overvoltage scenarios. This test does not focus on the specific resistance value, but uses “whether breakdown occurs” as the core determination basis, while closely monitoring whether the leakage current surges.
Common types of withstand voltage tests:
Power frequency AC withstand voltage: assesses the insulation’s tolerance under power frequency voltage, which is the closest to the actual operating state of the equipment.
DC withstand voltage: equipment is relatively lightweight, and is often used for insulation tests of long-distance power cables. This method has certain limitations in testing the electric field distribution of cross-linked polyethylene cable insulation.
Lightning impulse withstand voltage: simulates atmospheric overvoltage, assessing the insulation’s ability to withstand instantaneous high-amplitude impulses.
The hierarchical correspondence of the standard system
For different test purposes, the high-voltage power industry implements a clearly hierarchical standard system.
Reference for insulation resistance test:
DL/T 474.1‑2018 “Field Insulation Test Implementation Guidelines: Insulation Resistance, Absorption Ratio, and Polarization Index Test”;
For commissioning and acceptance, it follows GB 50150 “Electrical Installation Engineering: Electrical Equipment Commissioning Test Standard”;
For preventive tests, it follows DL/T 596 “Power Equipment Preventive Test Procedures”.
Reference for withstand voltage and impulse withstand voltage tests:
DL/T 474.4‑2018 “Field Insulation Test Implementation Guidelines: AC Withstand Voltage Test”;
For commissioning and acceptance, it also follows GB 50150 “Electrical Installation Engineering: Electrical Equipment Commissioning Test Standard”;
For preventive tests, it follows DL/T 596 “Power Equipment Preventive Test Procedures”;
General high-voltage technical reference: GB/T 16927.1 “High Voltage Test Technology – Part 1: General Definitions and Test Requirements”, IEC 60060‑1;
General requirements for high-voltage switchgear: GB/T 11022 “High Voltage Switchgear and Control Equipment Common Technical Requirements”, DL/T 593;
The basis for insulation coordination is referred to as GB 311.1 “Insulation Coordination – Part 1: Definitions, Principles and Rules”.
Note: The qualified threshold for insulation resistance testing and the specific test voltages for each device should be referred to in the detailed provisions of the above standard.
Unbreakable sequence of on-site tests
In high-voltage power engineering, the execution sequence of two tests has a mandatory logic: it is necessary to strictly follow the process of conducting the insulation resistance test first, followed by the withstand voltage test.
This requirement stems from the potential destructive nature of the withstand voltage test. If insulation has already suffered severe moisture ingress or contains penetrating defects, the measurement results of the insulation resistance test will show a significant decrease, serving as an early warning. If the step of conducting the insulation resistance test is skipped and high voltage is applied directly, it will expand the scope of equipment failure, cause irreversible physical damage such as insulation breakdown and carbonization, and may also trigger the protection trip of the test equipment, posing a potential safety hazard to personnel.
The insulation resistance test is a prerequisite condition for conducting the withstand voltage test. Only when the insulation resistance, absorption ratio, and polarization index all meet the regulations can the withstand voltage test assessment be carried out.
Post time: Sep-15-2026