Validating UPS Transfer Time According to IEC 62040-3

Uninterruptible Power Supplies (UPS) provide continuous, conditioned power to critical loads when utility power becomes unstable or unavailable. Installed between the grid and the load, a UPS supplies backup energy from batteries or supercapacitors during outages while regulating voltage and frequency to protect sensitive equipment from disturbances such as sags, surges, transients, and harmonics.

UPS systems are used in applications where even brief interruptions can result in data loss, equipment damage, or operational downtime. Typical examples include data centers, hospitals, industrial automation systems, and telecommunications infrastructure.

UPS Performance and IEC 62040-3

IEC 62040-3 is the international standard that defines performance testing requirements for UPS systems. Rather than specifying internal topology, the standard focuses on verifiable performance through standardized test methods applicable to all UPS classifications:

• VFI (Voltage and Frequency Independent)

• VI (Voltage Independent)

• VFD (Voltage and Frequency Dependent)

The standard establishes measurable criteria for evaluating UPS behavior, including voltage regulation, frequency stability, waveform quality, efficiency, and transfer performance. This enables consistent and comparable evaluation across manufacturers, integrators, and end users.

IEC 62040-3 defines structured test sequences that assess UPS operation under varying conditions, including:

• Different load levels

• Linear and nonlinear load types

• Step load changes

• Battery charge states

These tests verify that the UPS can maintain output stability and protect connected equipment under realistic operating scenarios.

Transfer Time as a Core Performance Metric

Transfer time is a key dynamic performance metric defined in IEC 62040-3. It describes the interval between the onset of a power disturbance and the point at which the UPS output returns to, or remains within, specified limits. This definition applies whether or not a measurable disturbance is observed at the output.

Transfer events occur in two directions. One is the transition from utility power to battery-backed operation when the input supply is disturbed or lost. The other is the transition from battery operation back to utility power once normal input conditions are restored.

During these transitions, brief voltage deviations or waveform disturbances can occur. In some UPS designs, particularly online systems, normal operation may result in little to no measurable transfer time or voltage deviation. In these cases, the lack of a visible disturbance is still an important outcome and reflects the intended performance of the system under test.

IEC 62040-3 therefore requires that transfer-time testing include:

• Measurement of the output voltage waveform to confirm whether any voltage sags, swells, interruptions, or frequency variations occur during the transfer event and that they remain within specified limits

• Evaluation under multiple load conditions and operating modes to characterize UPS behavior across expected operating scenarios

• Capture and documentation of voltage and current waveforms during the transfer event to provide objective evidence of system response and support compliance assessment

These requirements help ensure that UPS transfer performance is characterized in a consistent and repeatable way, regardless of whether the transfer event produces a visible disturbance at the output.

Understanding the Transfer-Time Waveform

Transfer-time evaluation is performed by monitoring the UPS output voltage while using an independent signal to identify when a transfer is initiated.

A common and reliable trigger source is the grid input current. When utility power is lost, the grid current collapses to zero, clearly indicating the start of a transfer. When utility power is restored, the grid current returns to a sinusoidal waveform, marking the transfer back to line power.

In the waveforms shown:

• Grid input current is displayed in RED

• UPS output voltage is displayed in YELLOW

During a transfer from grid to backup operation, the grid current drops to zero while the output voltage remains continuous and within specification, indicating a successful transition without load disturbance.

During a transfer from backup operation back to utility power, the grid current returns to a sinusoidal waveform. The output voltage is monitored to confirm that the transition occurs without excessive voltage dips, overshoot, or waveform distortion.

Measuring Transfer Time with a ScopeCorder

Accurate transfer-time measurement requires a structured setup capable of capturing fast transient events while maintaining long observation windows. A ScopeCorder provides the acquisition speed, memory depth, isolation, and trigger flexibility required for IEC 62040-3 compliance testing.

Setup Procedure

Probe Connections
Isolated voltage probes and current probes are connected to both the AC input and AC output to monitor source and load behavior simultaneously. If required, the DC bus can also be measured to correlate inverter operation during the transfer.

Channel Configuration
Dedicated channels are assigned to input and output signals, with voltage and current ranges selected based on the UPS rating to ensure optimal resolution.

Timebase Configuration
The timebase is configured to capture the full transfer event. For example, to observe a one-second event, a time setting of at least 100 ms/div provides adequate context.

Trigger Configuration

ON-state transfers are captured using a wave window trigger to detect voltage disturbances or dropouts on the input side. This trigger defines an allowable amplitude window around the nominal sinusoidal waveform and activates when the signal deviates beyond that range. Because a sinusoidal input does not present a single, well-defined transition point, a wave window trigger provides more reliable detection than a simple edge trigger for identifying input power loss.

OFF-state transfers are captured using an edge trigger on the restored input voltage when utility power returns, where a clear transition is present and edge-based triggering is effective.

Measurement and Analysis

Built-in measurement and math functions are used to quantify voltage behavior before, during, and after the transfer. Typical parameters include peak value, RMS voltage, and frequency.

Transfer time is evaluated by verifying that the output voltage stabilizes within the limits specified by IEC 62040-3, typically within a 50 ms window. Using the DL950 time window function, parameter measurements can be restricted to this interval to isolate transfer behavior.

Multiple transfer events can be captured and analyzed using statistical functions to calculate averages, maxima, and minima, enabling assessment of measurement repeatability and consistency.

Automation

Automation improves both efficiency and consistency in transfer-time validation. With a ScopeCorder, engineers can:

• Remotely trigger measurements and simulate transfer events under varying load conditions

• Load and switch between saved setup files for different UPS modes or configurations

• Automatically extract and log parameters such as RMS voltage, crest factor, and transfer time

This approach reduces manual intervention and minimizes operator-dependent variability.

Conclusion

Validating UPS performance to IEC 62040-3 requires precise and repeatable measurement of transfer time, voltage stability, and waveform quality under real operating conditions.

The ScopeCorder series combines high-speed acquisition with long-duration recording to capture transient events during source switching. Advanced trigger functions and automated analysis simplify compliance testing and improve confidence in results. When paired with power analyzers and IS8000 software, engineers can perform synchronized evaluation of efficiency, harmonics, and power quality within a single test environment.

With decades of experience in power measurement, Yokogawa provides the accuracy and reliability required to validate UPS performance and ensure continuous, high-quality power delivery.