Electrical strength (dielectric withstand) testing verifies that the solid insulation of a thermal storage electric heater can safely withstand transient overvoltages during long-duration, high-power operation. This article explains the applicable IEC, EN, GB, UL and JIS standards, typical sample requirements, test voltages, conditioning procedures and acceptance criteria used by accredited third-party laboratories and manufacturer quality departments. Understanding these requirements is essential for product certification, factory routine testing and design verification of storage heaters.
Technical Overview
Thermal storage electric heaters (also called storage heaters or heat-storage radiators) charge a high-heat-capacity core, typically composed of magnesite, magnetite or ceramic bricks, during off-peak periods and release the stored heat gradually through natural convection or fan-assisted airflow. Because the heating elements operate at high power for many hours while the appliance remains permanently connected to the mains, the integrity of the solid insulation between live parts and accessible surfaces is a primary safety concern.
Electrical strength testing, often referred to as dielectric withstand or high-potential (hipot) testing, applies a power-frequency alternating voltage that is substantially higher than the rated voltage between live parts and accessible metal parts or foil-covered insulating surfaces. The test reveals insufficient creepage distances, degraded or contaminated insulation, moisture ingress, and assembly defects such as wiring pinched near a hot storage core.
The governing requirements are defined in IEC 60335-1 (general safety of household and similar electrical appliances), together with the product-specific parts IEC 60335-2-30 for room heaters and IEC 60335-2-61 for storage heaters. Corresponding national adoptions include the EN 60335 series in Europe, the GB 4706.1 series and the relevant part-2 standard for room heaters (GB 4706.23) in China, UL appliance standards in North America, and the JIS C 9335 series in Japan.
- Detection of insulation weakness between live parts and accessible surfaces
- Verification of basic, supplementary and reinforced insulation performance
- Assessment of insulation behavior after thermal loading and humidity exposure
- Support for certification schemes and 100 percent factory routine testing
Testing Samples
Complete Appliances for Type Testing
Representative production units, typically rated 220 V to 240 V single-phase with input powers from about 1 kW to 3 kW, are submitted in new and unused condition. The sample should include all standard fittings such as the storage core, charge control device, thermal cutouts, terminal block, supply cord and, where fitted, the fan assembly. Multiple identical samples may be required because the electrical strength test is frequently performed after other sequential tests such as heating, abnormal operation and humidity conditioning.
Production-Line Units for Routine Testing
Every manufactured unit is subjected to a short-duration routine electrical strength test after final assembly, in accordance with the routine-test annex of IEC 60335-1. These samples are tested as delivered, with controls in the operating position and without additional conditioning.
Insulating Components and Sub-Assemblies
Component-level samples may also be evaluated, including mica or ceramic insulators supporting the heating elements, terminal blocks, silicone or fiberglass sleeving used near the storage core, internal wiring, and protective insulating barriers. Component testing supports incoming inspection and supplier qualification programs.
Conditioned Samples
Specific test sequences require samples that have completed preliminary conditioning, such as heating at rated input until thermal equilibrium of the storage core, damp-heat exposure, or endurance and abnormal operation tests. The electrical strength verification is then performed on the stressed sample in the state specified by the standard.
Testing Items
Electrical strength verification for storage heaters is carried out at several defined stages of the safety evaluation, each with its own test point, voltage and condition.
Electric Strength at Operating Temperature
Immediately after the heating test, during which the storage core reaches its maximum temperature, the insulation is subjected for one minute to an alternating voltage while the appliance is still hot. For appliances with a working voltage not exceeding 250 V, the commonly applied values are 1000 V for basic insulation and 2500 V for reinforced insulation. This stage exposes insulation weaknesses caused by sustained thermal stress.
Electric Strength after Humidity Conditioning
After the sample has been held for 48 hours in a humidity cabinet at approximately 93 percent relative humidity and a temperature between 20 °C and 30 °C, the electric strength test of Clause 16.3 is performed. For working voltages above 150 V and up to 250 V, the standard test voltages are 1250 V for basic insulation, 2500 V for supplementary insulation and 3750 V for reinforced insulation, applied for one minute.
Production Routine Tests
For 100 percent production-line verification, the routine-test annex permits a greatly shortened test of approximately one second at a specified voltage. In factory practice, levels such as 1000 V for Class I constructions and 2500 V for Class II constructions are widely used between live parts and accessible metal parts, subject to the exact routine-test provisions adopted by the manufacturer and accepted by the certifying scheme.
Complementary Insulation Assessments
Electrical strength testing is normally combined with related insulation verifications to give a complete picture of safety performance:
- Leakage current measurement at operating temperature
- Insulation resistance measurement as a supporting diagnostic
- Verification of creepage and clearance distances in the insulation coordination assessment
- Ball-pressure and glow-wire testing of insulating parts located close to the hot storage core
Testing Methods
Equipment and Test Setup
The test is performed with an alternating-current withstand voltage tester supplying an essentially sinusoidal 50 Hz or 60 Hz output, adjustable up to at least 5 kV and driven by a high-voltage transformer of adequate rating, typically 500 VA or more. The instrument must provide calibrated voltage indication, an adjustable overcurrent trip, and preferably arc or corona detection. Measuring equipment should be maintained under a documented calibration program consistent with ISO/IEC 17025 laboratory practice.
Sample Preparation and Conditioning
Accessible insulating surfaces are covered with metal foil, and accessible metal parts are connected together to form one test electrode. All live parts, including both supply poles, are tied together to form the other electrode. Switches and controls are placed in the closed or energized position so that the test voltage stresses the complete insulation system. For the humidity-conditioned test, the verification is carried out in the atmosphere of the humidity cabinet, or immediately after removal, so that condensation on the sample is not appreciably changed.
Voltage Application Procedure
- Connect the high-voltage output to the tied live parts and the return to accessible metal parts or foil.
- Apply the voltage starting from a low value, commonly no more than half of the specified test voltage.
- Raise the voltage smoothly to the full specified value, generally within no more than 10 seconds.
- Maintain the full voltage for one minute for type testing, or approximately one second for routine testing.
- Reduce the voltage gradually, disconnect the sample, and safely discharge any stored charge before handling.
For Class II storage heaters, reinforced insulation is tested across the complete insulation system, and where the construction permits, supplementary and basic insulation are tested separately at their respective voltages. Where electronic charge controllers produce working voltages exceeding the mains voltage, the applicable test voltage is selected from the standard tables according to the measured working voltage.
Acceptance Criteria
The sample is considered to pass if no breakdown occurs during the application of the test voltage. Breakdown is judged when the overcurrent protection device in the test circuit operates, indicating a disruptive discharge through or over the insulation. Modern hipot instruments use adjustable trip thresholds, typically in the range of a few milliamperes to tens of milliamperes, and arc-detection functions help distinguish genuine insulation failure from harmless surface effects.
Safety Precautions
- Use an interlocked test enclosure so that high voltage cannot be applied while the operator has access to the sample.
- Ground the test station and use only trained, authorized personnel.
- Verify that the sample is fully discharged after each test before touching terminals or foil.
- Post warning signage and restrict access to the high-voltage test area.
Applications
Electrical strength testing of thermal storage electric heaters serves multiple purposes across the product life cycle:
- Certification and market access under compulsory and voluntary schemes, including the Chinese CCC scheme for household appliances, European conformity marking based on the EN 60335 series, and North American listing under UL appliance standards.
- Factory quality control through 100 percent routine hipot testing of every finished unit and statistical sampling of production batches.
- Design verification during research and development, particularly when new storage core materials, element supports, insulating barriers or electronic charge controls are introduced.
- Supplier and incoming component inspection for insulators, terminal systems, wiring and heating element assemblies.
- Re-testing after design changes, rework, component substitution or relocation of production.
- Market surveillance and complaint investigation, where failed units in the field are analyzed for insulation degradation.
FAQ
What test voltage is applied to a thermal storage electric heater?
It depends on the insulation type and the test stage. After humidity conditioning, common values for 220 V to 240 V appliances are 1250 V for basic insulation, 2500 V for supplementary insulation and 3750 V for reinforced insulation. At operating temperature, the applied levels are typically 1000 V for basic insulation and 2500 V for reinforced insulation.
How long is the voltage applied?
Type tests require the full test voltage to be maintained for one minute. Production routine tests are much shorter, typically around one second, as permitted by the routine-test provisions of the applicable standard.
What counts as a failure?
A failure is a breakdown of the insulation, indicated by operation of the overcurrent device in the test circuit or a disruptive discharge (flashover) through or across the insulation. Surface corona or a small displacement current without breakdown is not itself a failure, which is why calibrated trip settings and arc-detection features are important.
Why is the test performed after damp-heat conditioning?
Storage heaters operate in living spaces where humidity and dust accumulate over years of service. Exposing the sample to 48 hours of high relative humidity before testing reveals insulation systems that are vulnerable to moisture absorption, which is one of the most common causes of long-term insulation failure.
Which standards govern the test?
The core requirements come from IEC 60335-1 together with IEC 60335-2-30 for room heaters and IEC 60335-2-61 for storage heaters. National adoptions include the EN 60335 series, the GB 4706.1 series with the relevant part-2 standard for room heaters, UL appliance standards, and the JIS C 9335 series.
Must every manufactured unit be tested?
Yes. The routine-test annex of the general safety standard requires a short-duration electrical strength test on 100 percent of finished appliances as part of routine factory testing, in addition to the full type-test sequence performed on representative samples.