Leakage current testing is a critical safety assessment for storage electric heaters, verifying that thermal storage appliances do not expose users to hazardous touch currents during operation. Performed in accordance with IEC 60335-1, IEC 60335-2-61, and the corresponding GB 4706 series standards, this test evaluates insulation performance at operating temperature. This article outlines the applicable standards, sample categories, testing items, measurement procedures, acceptance limits, and practical applications of leakage current detection for storage heaters.
Technical Overview
A storage electric heater, also known as a thermal storage heater or night storage heater, converts electrical energy into heat during charging periods and stores it in a high-density ceramic, magnetite, or refractory core for gradual release into the room throughout the day. Because these appliances operate unattended for extended hours at elevated core temperatures—often in bedrooms, classrooms, and offices—rigorous verification of electrical safety is essential before they can be placed on the market.
Leakage current is the current that flows from live parts through insulation paths and parasitic capacitance to accessible surfaces or the protective earth conductor when the appliance is energized at rated voltage. Excessive leakage current in a storage heater may indicate degraded element sealing, moisture ingress into the storage core, contaminated wiring compartments, or defective insulation systems, and can result in perceptible electric shock or dangerous touch currents for the user.
Leakage current testing of storage heaters is conducted under the following framework of international and national standards:
- IEC 60335-1 / GB 4706.1 — Household and similar electrical appliances — Safety — Part 1: General requirements, Clause 13 (leakage current and electric strength at operating temperature)
- IEC 60335-2-61 / GB 4706.61 — Particular requirements for storage heating appliances
- EN 60335-2-61 — European harmonized standard for storage heating appliances
- IEC 60990 — Methods for measurement of touch current and protective conductor current, which defines the standardized human-body impedance measuring network
Accredited third-party laboratories and in-house compliance departments rely on these standards to confirm that the insulation system of a storage heater remains safe under real operating conditions, including thermal stress, prolonged energization, and environmental humidity.
Testing Samples
Domestic Storage Heaters
Floor-standing or wall-mounted residential thermal storage heaters, typically rated between 1 kW and 3 kW per module, in both natural convection and fan-assisted designs. These samples are tested as complete appliances with all covers, controls, charge regulators, and dampers installed in the as-delivered configuration.
Commercial and Heavy-Duty Storage Units
High-input storage heaters used in commercial buildings, schools, and public facilities, often comprising multiple heating elements and larger storage cores. Such units may be rated well above 3 kW and frequently require permanent wiring, so protective conductor current measurement is included in the evaluation scope.
Heating Elements and Insulation Sub-assemblies
Sheathed tubular heating elements, mica-supported element assemblies, ceramic core sections, terminal blocks, and lead wires intended for integration into storage heaters. Component-level leakage current and electric strength verification supports incoming quality control and supplier qualification programs.
Testing Items
A complete leakage current evaluation program for storage heaters typically comprises the following items:
- Leakage current at operating temperature, measured after the appliance reaches thermal stabilization under normal operation
- Leakage current after humidity conditioning, applicable to appliances exposed to moist environments during storage or use
- Leakage current following abnormal and endurance test sequences, to confirm insulation integrity after thermal cycling and aging
- Protective conductor current measurement for permanently connected Class I appliances
- Electric strength (dielectric withstand) verification, performed immediately after the leakage current measurement in accordance with Clause 13 of IEC 60335-1
Acceptance limits for leakage current at operating temperature, when supplied at 1.06 times rated voltage, are defined in IEC 60335-1:
- Class II appliances: not greater than 0.75 mA
- Class 0, 0I, and III appliances: not greater than 0.5 mA
- Class I portable appliances: not greater than 0.75 mA
- Class I stationary heating appliances, which include storage heaters: not greater than 0.75 mA per kW of rated input, with a minimum limit of 0.75 mA and a maximum limit of 5 mA
For example, a Class I storage heater rated at 2 kW must exhibit a leakage current no greater than 1.5 mA at operating temperature, while a 7 kW commercial unit is capped at the 5 mA maximum.
Testing Methods
Test Setup and Conditions
The appliance is installed in a test room free from draughts and operated under normal conditions as specified in IEC 60335-2-61 until the storage core reaches charge equilibrium and the appliance attains operating temperature. The supply voltage is set to 1.06 times the rated voltage at rated frequency. The measuring instrument incorporates the perception-current/reactance measuring network defined in IEC 60990, which simulates the impedance of the human body and correctly weights frequency components of the leakage current.
Measurement Procedure
The measurement is carried out in the following sequence:
- Connect the measuring network between accessible metal parts (or metal foil applied to accessible insulating surfaces) and the protective earth terminal
- Energize the appliance at 1.06 times rated voltage and allow it to reach thermal stabilization during normal operation
- Measure the leakage current with the supply polarity reversed, recording the higher of the two values
- For Class I appliances, interrupt the protective earth conductor one pole at a time and repeat the measurement as required by the standard
- Immediately after the leakage current measurement, apply the electric strength test of Clause 13 to confirm dielectric withstand capability
Determination of Compliance
The sample is considered compliant when the highest recorded leakage current does not exceed the limit applicable to its protection class and rated input, and the subsequent electric strength test shows no breakdown. Any exceeding value triggers investigation of element sealing, insulation materials, creepage and clearance paths, and internal wiring layout, followed by corrective action and retesting.
Applications
Leakage current detection for storage heaters serves multiple compliance and quality assurance purposes across the product lifecycle:
- Mandatory product certification, such as CCC certification in China under GB 4706.1 and GB 4706.61, and CE marking support under EN 60335-1 and EN 60335-2-61
- Type testing and safety evaluations performed by accredited third-party laboratories for market access in different regulatory regions
- Factory quality control, including periodic safety audits and production line verification after design or material changes
- Research and development support, validating insulation coordination, heating element selection, and terminal compartment layout during product design
- Failure analysis after humidity, damp heat, or thermal endurance testing, where leakage current serves as a sensitive indicator of insulation degradation
- Market surveillance and import inspection programs verifying continued conformity of appliances already in circulation
FAQ
What is the maximum permissible leakage current for a Class I storage heater?
Under IEC 60335-1, Class I stationary heating appliances such as storage heaters must not exceed 0.75 mA per kW of rated input power, with an absolute minimum limit of 0.75 mA and an absolute maximum limit of 5 mA. Class II appliances are limited to 0.75 mA regardless of rating.
Why must leakage current be measured at operating temperature?
Insulation resistance falls significantly as component temperatures rise, and parasitic capacitance effects increase during energization. Measuring at 1.06 times rated voltage after thermal stabilization reproduces the worst realistic operating condition and reveals insulation weaknesses that a cold, de-energized insulation resistance test cannot detect.
What equipment is required for the test?
A calibrated leakage current tester incorporating the IEC 60990 measuring network, an adjustable AC power source capable of delivering 1.06 times rated voltage, thermocouples or infrared instrumentation for confirming thermal stabilization, and metal foil electrodes for accessible insulating surfaces are the core requirements.
Is leakage current testing different from insulation resistance testing?
Yes. Leakage current testing energizes the appliance at operational voltage and measures actual current flowing through insulation and capacitive paths, while insulation resistance testing applies a DC voltage to de-energized insulation and measures resistance in megohms. Both are complementary parts of a complete insulation safety assessment.
How often should storage heaters be retested?
Type testing is required for initial certification, whenever the design, insulation materials, or heating elements change, and periodically as part of factory supervision in certification schemes. Routine production safety tests, including earth continuity and electric strength, are performed on every unit on the production line.