Automotive wiring harness electrical withstand voltage evaluation, also known as dielectric withstand or hipot testing, verifies the insulation integrity of low-voltage and high-voltage harnesses used in conventional, hybrid and electric vehicles. This guide explains the applicable standards such as ISO 6469-3, ISO 16750-2, GB/T 28046.2 and QC/T 29106, typical test samples, test items and procedures, acceptance criteria, and answers frequently asked questions for engineers and quality teams working with accredited third-party laboratories.
Technical Overview
Electrical withstand voltage evaluation is one of the most fundamental safety assessments performed on automotive wiring harnesses. During the test, a voltage significantly higher than the normal operating voltage is applied between current-carrying conductors, between conductors and shielding or braid, and between conductors and the external environment. The purpose is to confirm that the insulation system, including wire insulation, connector housings, seals, tape and protective sleeving, can withstand transient overvoltages, switching surges and long-term electrical stress without breakdown.
The importance of this evaluation has grown sharply with the electrification of vehicles. Modern electric vehicles carry high-voltage harnesses operating between 60 V and 1500 V DC (or 30 V to 1000 V AC), where an insulation defect can lead to leakage current, short circuits, thermal events or electric shock hazards. Dielectric withstand testing is therefore a mandatory element of design validation, type testing and routine production control for both low-voltage (12 V / 24 V) and high-voltage automotive wiring systems.
The evaluation is typically conducted in accordance with international and national standards, including ISO 6469-3 for electrical safety of electrically propelled vehicles, ISO 16750-2 (and its Chinese equivalent GB/T 28046.2) for electrical loads on electronic equipment, the ISO 19642 series for automotive wires and cables, GB/T 25085 for 60 V / 600 V cables, QC/T 29106 for automotive wiring harness technical requirements, UL 2251 for electric vehicle charging cables, and relevant IEC and JIS test method standards. Accredited third-party laboratories routinely perform these tests as part of homologation and supplier qualification programs.
Testing Samples
Low-Voltage Wiring Harnesses
Body, chassis and comfort-system harnesses operating at 12 V or 24 V nominal voltage. These include main body harnesses, engine bay harnesses, door harnesses, roof harnesses and instrument panel harnesses. Although the operating voltage is low, the insulation must still withstand transient overvoltages and the harness must not exhibit breakdown between adjacent circuits or to ground.
High-Voltage Wiring Harnesses
Harnesses used in hybrid, battery electric and fuel cell vehicles for the traction battery, inverters, electric drive units, DC-DC converters, power distribution units and electric heating or air-conditioning compressors. These harnesses are typically shielded, use high-voltage connectors with interlock circuits, and are subjected to the most stringent dielectric requirements, often derived from ISO 6469-3 and OEM specifications.
Charging Harnesses and Cables
On-board charger input harnesses and Mode 2 / Mode 3 charging cable assemblies rated up to 250 V or 480 V AC single- or three-phase. These products are evaluated under UL 2251, IEC 62196-related requirements and regional regulations, with dielectric withstand tests performed on the complete cable assembly including connectors and couplers.
Harness Sub-Components and Material Specimens
In addition to complete harnesses, evaluation may cover individual components that determine system insulation performance: high-voltage connectors, terminals, splices, grommets, conduit and corrugated tubing, heat-shrink tubing, and wire specimens with defined insulation wall thickness. Cable specimens are frequently tested in a water-immersed condition to assess bulk insulation quality as required by the ISO 19642 and GB/T 25085 test methods.
Testing Items
A complete withstand voltage evaluation usually consists of several complementary test items:
- AC dielectric withstand voltage test (50 Hz or 60 Hz sinusoidal voltage applied between conductors and between conductor and shield or ground).
- DC dielectric withstand voltage test, preferred for high-capacitance harnesses and for HV systems where DC stress better represents service conditions.
- Insulation resistance measurement, typically at 500 V DC or 1000 V DC, to quantify the ohmic quality of the insulation prior to and after voltage stress.
- Leakage current measurement during withstand testing, monitored against a defined limit to detect weak insulation before catastrophic breakdown.
- Partial discharge testing for shielded high-voltage cables and connectors, verifying that the partial discharge extinction voltage exceeds the maximum operating voltage with the required margin.
- Post-conditioning dielectric retest, in which withstand voltage and insulation resistance are repeated after thermal aging, temperature cycling, damp heat, salt spray, vibration, flexing or fluid exposure, in line with ISO 16750-2 and GB/T 28046.2 test sequences.
- Water immersion dielectric tests on wire specimens, in which a specified AC voltage is applied between the immersed conductor and the water electrode.
Testing Methods
Sample Preparation and Conditioning
Harnesses are visually inspected for mechanical damage and are conditioned in a standard laboratory atmosphere, typically 23 °C ± 5 °C with controlled relative humidity, for a defined stabilization period. For high-voltage harnesses, shields and exposed conductive parts are connected to ground potential, and all circuits not under test are bonded together and grounded. Test electrodes are attached to stripped conductor ends, terminals or dedicated test contacts without damaging the insulation.
Test Voltage Selection
For high-voltage systems, a widely used rule derived from IEC-based insulation coordination and adopted in ISO 6469-3 and most OEM specifications sets the AC test voltage at approximately 2 × U + 1000 V RMS, where U is the maximum working voltage. Low-voltage harnesses are commonly tested with a fixed AC voltage in the range of several hundred volts to 1000 V RMS or an equivalent DC voltage. DC test levels are typically 1.414 times the specified AC value to achieve equivalent stress. The exact voltage, waveform, ramp rate and discharge requirements are defined by the applicable product standard or customer specification.
Test Procedure
The test voltage is applied starting from zero or a low value and raised smoothly to the specified level to avoid false failures from inrush charging current. The voltage is then held for the specified duration, commonly 60 seconds for type testing and 1 to 5 seconds for routine production testing. The test is repeated for each required measurement path: conductor to conductor, conductor to shield or braid, and conductor to external ground or a conductive wrapper placed around the harness. After testing, the sample is discharged through a suitable resistor before handling.
Acceptance Criteria
A sample passes if no dielectric breakdown or flashover occurs during the voltage application and if the monitored leakage current remains below the specified limit, which is typically set by the OEM based on harness capacitance and test duration. Insulation resistance acceptance values commonly require 100 MΩ or more for high-voltage harnesses measured at 500 V or 1000 V DC, with lower thresholds such as 10 MΩ applied in some low-voltage specifications. For partial discharge testing, the extinction voltage must exceed the maximum system operating voltage by the margin defined in the applicable standard. Any breakdown, disruptive discharge, or out-of-limit leakage current constitutes a failure and triggers failure analysis of the insulation system.
Applications
Withstand voltage evaluation is applied throughout the automotive product lifecycle:
- Design validation and type testing of new harness families prior to production release, often performed at accredited third-party laboratories for impartial evidence.
- Regulatory compliance and homologation support for electric vehicle high-voltage components and charging systems, where dielectric strength is a core electrical safety requirement of ISO 6469-3 based approvals.
- Serial production quality control, using automated hipot testers integrated in end-of-line test benches to screen every high-voltage harness for insulation defects.
- Incoming inspection of purchased harnesses by vehicle manufacturers and tier-one integrators.
- Requalification after design or process changes, supplier changes, or relocation of manufacturing lines.
- Failure analysis and root-cause investigation of field returns, warranty claims and insulation degradation caused by aging, moisture ingress, abrasion or chemical exposure.
- Research and development of new insulation materials, thinner wall cables and high-voltage connectors for 800 V and higher platform architectures.
FAQ
What test voltage is applied to an EV high-voltage wiring harness?
Most specifications based on ISO 6469-3 and IEC insulation coordination principles use an AC test voltage of approximately 2 × U + 1000 V RMS, where U is the maximum working voltage. For example, a harness operating at 600 V DC is typically tested at roughly 2200 V AC RMS or the equivalent DC voltage of about 3100 V. The exact value must always be confirmed against the applicable standard or OEM requirement.
Should AC or DC withstand voltage be used?
AC testing stresses the insulation in both polarities and is the traditional reference method, but it produces higher capacitive leakage current on long harnesses. DC testing eliminates the capacitive component, allows lower test current limits and is preferred for production testing of HV harnesses with significant capacitance. Both are accepted when the specified equivalence factors are applied.
How long is the test voltage maintained?
For laboratory type testing the hold time is typically 60 seconds. For routine 100 percent production screening, durations of 1 to 5 seconds are common, with a correspondingly defined voltage and leakage current limit.
What are the most common failure modes found during testing?
Typical failure causes include insulation damage from routing or assembly, connector sealing defects allowing moisture ingress, terminal mis-crimping or bent terminals reducing creepage and clearance, contamination by conductive particles, inadequate insulation wall thickness, and aging or chemical degradation of the cable jacket.
Can withstand voltage testing damage the harness?
When performed at the specified levels, the test is non-destructive and places only a controlled overstress on a healthy insulation system. However, excessive voltage, too-fast ramping, or failure to discharge the sample after DC testing can degrade the insulation, so test parameters must follow the standard strictly.
How is insulation resistance related to dielectric withstand testing?
Insulation resistance measurement is usually performed before and after the withstand test. It provides a quantitative indication of insulation quality, while the withstand test verifies dielectric strength under voltage stress. A harness may show acceptable insulation resistance yet fail the withstand test if a localized defect exists, which is why both items are required in a complete evaluation.