Leakage current testing is a fundamental electrical safety evaluation that measures the unintended current flowing through or across the insulation of electrical equipment under normal and single-fault conditions. This article explains the internationally recognized leakage current test methods, including applicable standards such as IEC 60990, IEC 60335-1, IEC 62368-1, IEC 60601-1, and GB 4706.1, together with sample types, test items, measurement procedures, and typical limit values used by accredited third-party laboratories and manufacturers worldwide.
Technical Overview
Leakage current is the current that flows unintentionally through the insulation system, across creepage surfaces, or via capacitive coupling (for example, through Y-capacitors in an EMI filter) between live parts and accessible parts or the protective earth conductor. Although small in magnitude, leakage current can pose a risk of electric shock to users, interfere with sensitive electronic circuits, and indicate degradation or design defects in insulation. Leakage current testing therefore forms a mandatory part of product safety certification for most electrical and electronic products.
Modern leakage current measurement is based on IEC 60990, which defines the standardized measurement circuits that simulate the frequency-dependent impedance of the human body. Product family standards then specify the applicable measurement network, test conditions, fault conditions, and limit values. Key reference standards include:
- IEC 60990 – Methods of measurement of touch-current and protective conductor current (the base measurement standard)
- IEC 60335-1 / GB 4706.1 – Safety of household and similar electrical appliances
- IEC 62368-1 / IEC 60950-1 / GB 4943.1 – Safety of audio/video, information and communication technology equipment
- IEC 60601-1 / GB 9706.1 – Medical electrical equipment
- UL 1310, UL 60950 and related UL standards – North American requirements
- JIS C 1011 – Japanese measurement method for leakage current
It is important to distinguish between the main categories of leakage current. Touch current is the current that would flow through a person bridging accessible parts and earth. Protective conductor current (earth leakage current) is the current flowing in the protective earth conductor under normal operation. Enclosure leakage current and patient leakage current are additional categories defined specifically for medical electrical equipment.
Testing Samples
Household and Similar Electrical Appliances
Refrigerators, washing machines, air conditioners, electric kettles, microwave ovens, electric heaters, fans, and kitchen machines are tested per IEC 60335-1 and its part 2 standards. Both Class I (earthed) and Class II (double-insulated) constructions are covered, and the samples must be tested in their delivered configuration with normal accessories attached.
Information Technology and Audio/Video Equipment
Computers, monitors, printers, power adapters, chargers, servers, televisions, and amplifiers are tested per IEC 62368-1 or IEC 60950-1. Equipment connected to a communication network or with multiple earthing paths may require additional leakage measurements at telecommunication ports.
Medical Electrical Equipment
Devices with applied parts, such as patient monitors, electro-surgical units, infusion pumps, and diagnostic imaging systems, are evaluated per IEC 60601-1. These products demand the strictest limits because current may flow directly through the patient's heart.
Power Tools, Luminaires, and Industrial Equipment
Components and Sub-assemblies
Switch-mode power supplies, EMI filters, safety capacitors, transformers, and heating elements are frequently tested individually, because component-level leakage characteristics determine the leakage behavior of the end product.
Testing Items
Depending on the product standard, the following leakage current quantities are measured:
- Touch current at accessible parts and at the protective earth terminal under normal conditions
- Earth leakage current (protective conductor current) in the earth connection
- Enclosure leakage current between accessible parts and earth, applicable mainly to medical equipment
- Patient leakage current and patient auxiliary current through applied parts of medical devices
- Leakage current under single-fault conditions, such as open neutral conductor, reversed supply polarity, interruption of the protective conductor, or loss of one supply pole
Typical acceptance limits stated in common standards are summarized below. The applicable standard and product class always govern the final judgment.
- IEC 60335-1 household appliances: Class II appliances 0.25 mA; Class I portable appliances 0.75 mA; Class I stationary appliances 3.5 mA (heating appliances may be rated per kW of rated input, subject to a maximum)
- IEC 60950-1 / IEC 62368-1 IT and AV equipment: touch current 0.75 mA for Class I equipment and 0.25 mA for Class II equipment
- IEC 60601-1 medical equipment: patient leakage current down to 0.01 mA for Type CF applied parts and 0.1 mA for Type BF applied parts in normal condition, with higher values permitted under single-fault conditions
Testing Methods
Test Instrumentation and Measurement Network
Leakage current must be measured with a dedicated leakage current tester incorporating the measuring device (MD) network defined in IEC 60990. The network converts the actual current into a weighted value that reflects the physiological effect of frequency on the human body, and the instrument reads the voltage across the defined resistor, calibrated directly in milliamperes or microamperes. For earth leakage current, a low-impedance ammeter circuit is inserted in series with the protective conductor. The instrument must be periodically calibrated and traceable to national standards.
Sample Conditioning
Leakage current is strongly dependent on temperature and humidity, so samples are preconditioned before measurement:
- Operate the appliance under normal working conditions until thermal steady state is reached, because insulation resistance decreases as temperature rises
- Apply humidity preconditioning where required by the standard, for example storage in a humidity cabinet at approximately 91 to 95 percent relative humidity, followed by testing in the prescribed sequence
Measurement Procedure
The general measurement sequence is as follows:
- Place the sample on an insulating surface and connect it to an isolated, adjustable supply set to rated voltage and rated frequency, or to the factor specified by the standard (for example 1.06 or 1.10 times rated voltage in some standards)
- Connect the measuring network between each accessible part (or the earthed metal enclosure) and earth, using a test probe or test finger where specified
- Operate the equipment in all relevant operating modes, including heating, standby, and maximum-load conditions
- Actuate the polarity-reversing switch and record the higher reading for both supply polarities
- Apply the single-fault conditions specified by the standard, such as opening the neutral conductor or interrupting the protective earth conductor, and repeat the measurement
- Record the highest value obtained, correct for instrument accuracy if required, and compare it against the limit in the applicable standard
Influencing Factors and Good Practice
Measurement results can be distorted by stray capacitance of test leads, grounding of auxiliary equipment, ambient temperature and humidity, and supply waveform distortion. Test leads should be short and routed away from the sample, the bench must be non-conductive, and the supply source should provide a clean sinusoidal waveform at the declared tolerance. Repeatability checks and instrument zeroing before each series of measurements are essential for reliable results.
Applications
Leakage current testing is applied across the entire product lifecycle:
- Safety certification of new products for CE marking, national certification marks, and market access under IEC, EN, UL, GB, and JIS systems
- Design verification during research and development, particularly when selecting Y-capacitors, EMI filters, and insulation coordination
- Routine factory testing at the end of production lines, where a simplified 100 percent leakage or touch-current check is often required before shipment
- Periodic quality control, incoming inspection of components and sub-assemblies, and root-cause analysis of field failures or customer complaints
- Verification after design changes, component substitution, or relocation of production, ensuring continued compliance of the insulation system
FAQ
What is the difference between leakage current testing and an insulation resistance or dielectric strength test?
They are complementary but distinct tests. Insulation resistance tests apply a DC voltage to verify the resistance of the insulation barrier, and dielectric strength tests apply a high voltage to detect breakdown. Leakage current testing measures the actual current at operating voltage and frequency, including capacitive and frequency-weighted components, and therefore best represents the real shock hazard experienced by a user.
Why must leakage current be measured with the equipment energized and at working temperature?
Insulation properties deteriorate as temperature rises and humidity increases, and capacitive coupling only appears when the equipment is powered at its rated supply. Testing at thermal steady state under rated voltage reproduces the worst-case normal operating condition defined by the standards.
Why is polarity reversal applied during the test?
The internal position of switches, fuses, and single-pole components relative to the supply poles changes the leakage path. Reversing the supply polarity can significantly alter the measured current, so both polarities must be evaluated and the higher value recorded.
Can a sample pass a dielectric strength test but fail leakage current testing?
Yes. High values of capacitive leakage, for example from oversized Y-capacitors, may exceed touch-current limits even though the solid insulation withstands the dielectric test without breakdown.
Where can leakage current testing be performed?
Manufacturers can perform routine tests in-house with calibrated IEC 60990 compliant instruments, while type testing for certification should be carried out by accredited third-party laboratories with the environmental chambers, calibrated measurement networks, and traceability required by the relevant IEC, EN, UL, GB, or JIS standards.