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When selecting a metal oxide surge arrester for a medium-voltage power system, two voltage ratings deserve particular attention: rated voltage (Ur) and continuous operating voltage (Uc), also known as MCOV in many ANSI/IEEE applications.
These values are closely related, but they serve different purposes. Confusing them can lead to an unsuitable arrester selection, especially when the system experiences temporary overvoltages during earth faults or other abnormal operating conditions.
For engineers, EPC contractors, electrical distributors and procurement teams, understanding the difference is an important step toward selecting the right surge arrester for a specific
Continuous operating voltage, designated as Uc in IEC terminology, is the maximum permissible RMS power-frequency voltage that can be continuously applied between the terminals of a surge arrester under its specified operating conditions.
In many ANSI/IEEE-based specifications, the corresponding concept is described as Maximum Continuous Operating Voltage (MCOV). Although the terms address the same general selection concern, the applicable standard and product specifications should always be checked before comparing ratings.
Why does Uc matter?
A metal oxide surge arrester is designed to remain in a high-resistance state during normal operation. If the continuous system voltage places excessive electrical stress on its metal oxide varistor elements, the arrester may experience increased leakage current, additional heating and accelerated ageing.
For this reason, Uc must be selected according to the actual operating voltage at the arrester's installation point, rather than simply matching the nominal voltage printed on a system diagram.
The system's highest voltage, neutral grounding arrangement and normal operating conditions all influence this decision.
Rated voltage, designated as Ur, is another important electrical parameter defined in IEC 60099-4.
It is a reference voltage associated with the arrester's operating-duty performance and its ability to withstand specified temporary power-frequency overvoltage conditions. Under the relevant IEC test procedure, the rated voltage is used in a defined operating-duty test.
Unlike Uc, Ur should not be interpreted as the voltage that the arrester can withstand continuously under normal operating conditions.
Temporary overvoltage capability also depends on the magnitude and duration of the overvoltage, the arrester's thermal condition and the applicable test requirements. Therefore, engineers should consult the manufacturer's temporary overvoltage (TOV) characteristic rather than assume that Ur alone describes every possible operating condition.
Parameter | Ur — Rated Voltage | Uc — Continuous Operating Voltage |
|---|---|---|
Main purpose | Reference for rated and operating-duty characteristics | Defines permissible continuous power-frequency voltage |
Typical application consideration | Temporary overvoltage withstand and operating-duty requirements | Long-term voltage stress during normal operation |
Selection question | Can the arrester withstand the specified temporary overvoltage conditions? | Can the arrester operate continuously at the voltage present at the installation point? |
What to verify | Rated voltage and TOV characteristics | Continuous voltage rating and applicable standard |
The two ratings should be evaluated together. Choosing a surge arrester solely because its Ur appears suitable does not establish that its Uc is adequate for continuous operation. Likewise, an appropriate Uc does not, by itself, confirm that the arrester provides the required protection level.
DGG's published technical data for the YH5WZ-51/134 power-station surge arrester lists the following values for its stated application:
Standard system voltage: 35 kV
Arrester rated voltage (Ur): 51 kV
Continuous operating voltage (Uc): 40.8 kV
This example illustrates why Ur and Uc should not be treated as interchangeable numbers.
These ratings belong to a specific product configuration. They are not universal selection values for every 35 kV network. A different neutral grounding arrangement, temporary overvoltage condition or project specification may require a different arrester rating.
When reviewing a manufacturer's datasheet, confirm the complete model designation and the relevant technical parameters before making a purchasing decision.
Voltage ratings are only part of the selection process.
The first question is the electrical condition at the installation point. Engineers need to establish the system's highest operating voltage, the neutral grounding method and the temporary overvoltages that may occur during earth faults. Fault duration and protection-system clearing time can be particularly important when evaluating TOV capability.
The next consideration is insulation coordination. The arrester's residual voltage at the specified discharge current must be evaluated against the insulation withstand level of the equipment being protected. Its nominal discharge current, energy-handling capability and installation arrangement must also suit the project requirements.
For outdoor medium-voltage applications, environmental conditions matter as well. Pollution severity, required creepage distance, altitude and mounting configuration may influence the appropriate product design.
This is why a reliable selection process starts with the project specifications rather than a single voltage number.
DGG supplies metal oxide surge arresters for power distribution and transmission applications, with product configurations selected according to system voltage, installation conditions and project requirements.
For selected models, relevant type-test documentation is available. The applicable report and its scope should be confirmed against the exact product model and technical specification.
If you are sourcing a surge arrester for a utility distribution network, transformer protection, an outdoor substation or an industrial power system, providing the actual application details will help establish the appropriate ratings.
Contact DGG to discuss your surge arrester requirements. Share your system voltage, highest system voltage, neutral grounding method, installation location and project specification. Our team can review the requirements and help identify a suitable product configuration for your application.
Explore our Metal Oxide Surge Arrester Products or view the DGG Polymer-Housed Metal Oxide Surge Arrester.
No. Ur is the arrester's rated voltage, associated with its operating-duty characteristics. Uc is the IEC continuous operating voltage; MCOV is the commonly used term for the corresponding continuous operating voltage concept in many ANSI/IEEE specifications. Always check the applicable standard and datasheet.
No. Selection should consider the highest system voltage, voltage at the installation point, neutral grounding arrangement, temporary overvoltage conditions and insulation coordination requirements.
A TOV curve indicates the overvoltage magnitude that an arrester can withstand for a specified duration under the manufacturer's stated conditions. It helps engineers assess whether the arrester is suitable for abnormal power-frequency overvoltages that may occur during system faults.
Provide the nominal and highest system voltage, neutral grounding method, required rated and continuous operating voltages if already specified, nominal discharge current, installation location and relevant project standards. TOV requirements and the insulation level of the protected equipment are also helpful for technical review.
If you have a project specification or an existing arrester datasheet, send it to DGG for a product and parameter review.
