UL60950 Leakage Current Test: Requirements, Limits, Methods, and Applications

The UL60950 leakage current test, formally known as the touch current test under UL/IEC 60950-1, is a critical safety evaluation for information technology equipment that verifies the current accessible to users cannot cause electric shock under normal and single-fault conditions. This article explains the technical basis of the UL60950 touch current requirements, applicable sample types, measurement circuits, current limits, test procedures, and typical applications in product safety certification.

Technical Overview

UL 60950-1, the Standard for Safety of Information Technology Equipment – Safety – Part 1: General Requirements, is the North American adoption of IEC 60950-1. It applies to mains-powered and battery-powered information technology and telecommunications equipment rated at supply voltages up to 600 V, including computers, servers, monitors, printers, network devices, and associated power supplies.

Clause 5.7 of the standard addresses touch current, which is historically referred to as leakage current. Touch current is the electric current that flows through a person when touching one or more accessible parts of the equipment. The primary sources are the Y-capacitors of the EMI filter, parasitic capacitance across insulation barriers, and insulation leakage. Excessive touch current can cause perception, involuntary startle reactions, muscle contraction, or electric shock, so the standard restricts it to safe values measured through a calibrated network that simulates human body impedance.

• UL 60950-1 is harmonized with IEC 60950-1 and CAN/CSA-C22.2 No. 60950-1, allowing a single test program to support UL Listing, cUL certification, and IEC CB Scheme reports.
• The test is performed under both normal operating conditions and single-fault conditions, such as an open neutral conductor or an interrupted protective earth connection.
• Permissible touch current depends on the equipment protective class: 0.25 mA for Class II equipment, 0.75 mA for hand-held Class I equipment, and 3.5 mA for other Class I equipment.
• Although UL 60950-1 has largely been superseded by UL/IEC 62368-1 for new certifications, leakage current testing remains essential for maintaining legacy certifications and for products still referencing the standard in many global markets.

Testing Samples

Class I Information Technology Equipment

These are earthed products relying on a protective earth conductor for safety, such as desktop computers, workstations, servers, rack-mounted network switches, large monitors, laser printers, and uninterruptible power supplies. Samples must be production-representative units, complete with all accessories, cables, and power cords, and tested in all normal operating positions.

Class II Information Technology Equipment

These are products with double or reinforced insulation and no protective earth requirement, such as external power adapters, chargers, small printers, and battery-operated devices. The accessible enclosure surfaces are evaluated, and metal foil may be applied to insulating surfaces where accessible parts must be simulated.

Permanently Connected Equipment

Large systems intended for permanent wiring or industrial connectors, such as PBX systems, mainframe equipment, and telecom rectifier systems, may be evaluated with higher permissible touch current values of up to 5 percent of the rated input current, provided installation and marking requirements are met.

Components and Subassemblies

Power supplies, power supply modules, and other subassemblies intended for incorporation into end products are tested under defined installation conditions so that the end-product manufacturer can verify compliance of the complete system.

Testing Items

The UL60950 leakage current evaluation comprises the following measured quantities, each compared against the applicable limit:

• Touch current under normal operating conditions, measured between accessible parts and earth.
• Touch current under the single-fault condition of an interrupted neutral conductor, measured between accessible parts and each pole of the supply.
• Touch current under the single-fault condition of an interrupted protective earth conductor, for Class I equipment.
• Measurements with both polarities of the supply connection, since reversing line and neutral can produce the worst-case capacitive current.
• Verification that touch current does not exceed 0.25 mA for Class II equipment, 0.75 mA for hand-held Class I equipment, and 3.5 mA for other Class I equipment, all measured with the Annex D body impedance network.

All measurements are recorded at the most unfavorable normal operating condition, and the highest observed value is compared against the limit. A test result is considered acceptable only when every condition and configuration complies.

Testing Methods

Test Conditions and Sample Preparation

The sample is energized at its rated voltage, or at the upper limit of its rated voltage range, at the rated frequency, and operated under normal load until it reaches normal operating temperature. The supply is provided through an isolated, adjustable AC power source so that fault conditions and polarity reversal can be applied safely and reproducibly.

Measuring Network

Touch current is measured using the human body impedance network defined in Annex D, Figure D.1 of UL/IEC 60950-1. The network presents a frequency-compensated impedance representative of the human body, and the touch current is derived from the voltage U2 measured across the internal body resistance, calculated as U2 divided by 500 ohms and expressed in milliamperes. For equipment containing circuits operating above 1 MHz, additional high-frequency measurement considerations apply to address capacitive heating effects.

Normal Condition Measurement

For Class I equipment, the measuring network is connected between the accessible conductive parts and the protective earth conductor, and the current is recorded under normal operation. For Class II equipment, the network is connected between accessible parts and earth, verifying that no hazardous current flows during intended use.

Single Fault Condition Measurement

The test is repeated with the neutral conductor interrupted, measuring between accessible parts and each pole of the supply in turn, because the open-neutral fault can raise the EMI filter midpoint toward line potential and significantly increase capacitive current. For Class I equipment, the protective earth conductor is also interrupted, and touch current from accessible parts is measured to simulate a user touching the equipment during an earth fault. Measurements are taken with both supply polarities, and the highest reading is reported.

Instrumentation

A dedicated touch current or leakage current tester incorporating a compliant Annex D measuring network is used, together with a true-RMS voltmeter of sufficiently high input impedance, an adjustable AC power source, metal foil and probes for accessible parts, and temperature conditioning facilities. The instrumentation must be calibrated and verified to ensure traceable, accurate results.

Applications

UL60950 leakage current testing is applied throughout the product lifecycle of information technology and telecommunications equipment, from design verification to certification and production surveillance:

• Safety certification of IT equipment for the North American market, including UL Listing, cUL Mark, and ETL certification programs.
• Generation of IEC 60950-1 CB Scheme test reports supporting international market access and CE conformity under the Low Voltage Directive for legacy declarations.
• Design verification during product development, particularly for optimizing EMI filter Y-capacitor values, grounding architecture, and insulation coordination.
• Periodic factory surveillance and conformity testing required to maintain existing certifications issued under UL 60950-1.
• Comparative engineering evaluation when products are redesigned or when component substitutions, such as filter capacitors or power supplies, may affect touch current performance.

FAQ

What is the difference between leakage current and touch current?

Leakage current is the older term used in previous editions of safety standards to describe current escaping from live parts to accessible parts or earth. UL/IEC 60950-1 replaced this concept with touch current, which is the current that would actually flow through a person modeled by the Annex D body impedance network, providing a more physiologically accurate assessment of electric shock risk.

What are the maximum permitted touch current values?

Class II equipment is limited to 0.25 mA, hand-held Class I equipment to 0.75 mA, and other Class I equipment to 3.5 mA. Permanently connected Class I equipment with appropriate installation provisions may be permitted up to 5 percent of its rated input current.

Why must the open-neutral fault condition be tested?

When the neutral conductor is interrupted, the junction point of the EMI filter Y-capacitors can be driven toward full line potential, maximizing the capacitive current delivered to accessible parts. Testing this fault ensures that even a wiring fault in the installation cannot expose the user to hazardous current.

At what voltage and temperature is the test performed?

The test is performed at the rated voltage, or the upper limit of the rated voltage range, at the rated frequency, with the equipment operating under normal load at its normal operating temperature. Both supply polarities are evaluated to capture the worst case.

What equipment is required to perform the test?

A leakage current tester with a built-in measuring network compliant with UL/IEC 60950-1 Annex D, an adjustable isolated AC power source, calibrated true-RMS measurement capability, and appropriate probes and metal foil for accessing parts are required. All instruments must be within calibration for the results to be accepted by certification bodies.

Has UL60950 been replaced, and does the leakage current test still matter?

UL 60950-1 has been superseded by UL/IEC 62368-1, which governs new certifications for audio/video, information, and communication technology equipment. However, UL60950 leakage current testing remains relevant for maintaining existing certifications, supporting legacy products, and serving markets where the standard is still referenced, and the fundamental measurement principles carry over into the successor standard.

How can designers reduce leakage current?

Reducing the total Y-capacitance of the EMI filter, using balanced filter topologies that cancel line-to-earth and neutral-to-earth currents, minimizing parasitic capacitance across isolation barriers, and ensuring a low-impedance, reliable protective earth connection are the most effective design measures for meeting the UL60950 touch current limits.