What Is Surge Withstand Capability (SWC)?

Surge Withstand Capability (SWC) is a measure of the ability of relays, relay systems, and control devices associated with electric power apparatus to maintain correct operation when subjected to repetitive electrical transients. The transients of concern arise from switching operations in nearby high-voltage switchgear, from inductive coupling through control wiring, and from other sources within substations and switchgear installations. Unlike a single overvoltage event addressed by a surge arrester, SWC testing applies repeated high-amplitude oscillatory bursts and fast transient pulses to every relevant terminal of the equipment under test, verifying that the device continues to function within specification throughout and after the test sequence. The principal IEEE standard governing SWC requirements is C37.90.1, which is maintained by the IEEE Power and Energy Society and aligns with IEC 61000-4 immunity requirements for electromagnetic compatibility.

SWC testing is distinct from basic insulation level (BIL) tests. BIL verifies that equipment can withstand a single high-voltage impulse without insulation breakdown. SWC tests instead focus on the functional immunity of electronic circuits and digital logic to rapidly repetitive transients at amplitudes that do not cause insulation failure but can cause false tripping, data corruption, memory reset, or communication errors in protective relay and control systems.

Test Waveforms and Methodology

IEEE C37.90.1 specifies two complementary test waveforms. The oscillatory surge test applies a damped sinusoidal burst at 1 MHz or 100 kHz, with an open-circuit peak voltage of 2.5 kV and a short-circuit peak current of 200 A, for a minimum of 10 bursts with defined polarity. The electrical fast transient (EFT) burst test applies 5/50 nanosecond pulses at 4 kV peak in 15 millisecond repetitive bursts, representing the switching transients generated by vacuum and gas-insulated switchgear. Both tests are applied to power supply terminals, input/output ports, and communication interfaces of the equipment. The IEEE C37.90.1-2012 Standard for Surge Withstand Capability Tests documents the test setup geometry, generator output impedance, and the criteria for determining terminal selection.

Performance Requirements and Acceptance Criteria

For an equipment submission to pass SWC testing, the device must satisfy all of the following conditions during and after the test sequence: no hardware damage, no change in calibration beyond normal tolerances, no loss or corruption of stored data or memory, no system resets requiring manual intervention, and no communications errors that jeopardize protective functions. These criteria reflect the operational demands placed on protective relays and control devices in power system service, where a false operation during a switching transient could cause an unnecessary outage, and a failure to operate during an actual fault would leave power equipment unprotected. The IEEE Standards Association page for C37.90.1 describes the current revision, which updated both the oscillatory and EFT test parameters relative to the 2002 edition.

Role in Power System Protection Design

SWC requirements shape the hardware and firmware design of modern protective relays. Relay manufacturers use shielded enclosures, optically isolated input circuits, transient-absorbing components at terminal interfaces, and software watchdog timers to meet SWC criteria. Substation automation systems, which combine protection, control, and communication functions in integrated electronic systems, must demonstrate SWC compliance for the entire system as well as for individual components. The ANSI/IEEE C37.90.1 compliance testing service offered by accredited laboratories verifies that relay and control device designs meet the SWC requirements before they enter power system service.

Applications

Surge withstand capability standards apply to a wide range of power system protection and control equipment, including:

• Numerical and electromechanical protective relays in transmission and distribution substations
• Supervisory control and data acquisition (SCADA) remote terminal units
• Programmable logic controllers used in substation automation
• Metering and power quality monitoring equipment
• Digital fault recorders and sequence-of-events recorders