Why Is not There a Unified Global Circuit Breaker Voltage Standard?

The root cause is that the global power industry historically developed two parallel, independent mainstream technical standard systems:

IEC System (International Electrotechnical Commission)

• Regions: Europe, China, Middle East (except US-standard projects), Africa, Southeast Asia, Australia, most of South America.
• Equipment rated voltage sequence (Ur): A standardised set of values used globally by IEC manufacturers:7.2 kV, 12 kV, 24 kV, 36 kV, 40.5 kV, 72.5 kV, 123/126 kV, 145 kV, 170 kV, 245/252 kV, 363 kV, 420 kV, 550 kV, 800 kV, etc.
• Core logic: Manufacturers produce equipment to this standardised sequence, which is then matched to each country's "nominal system voltage." For example, a 110 kV IEC system would use a circuit breaker with a rated voltage of 123 kV or 126 kV. The equipment voltage (Ur) is deliberately set slightly higher than the system voltage to provide a safety margin.

ANSI/IEEE System (American National Standards)

• Regions: United States, Canada, Mexico, Philippines, Saudi Arabia, parts of the Caribbean and South America.
• Equipment rated voltage sequence:4.76 kV, 8.25 kV, 15 kV, 27 kV, 38 kV, 48.3 kV, 72.5 kV, 121 kV, 145 kV, 169 kV, 242 kV, 362 kV, 550 kV, etc.
• Core logic: Similar to IEC but with different numerical values. For instance, a 138 kV ANSI system in North America would use a circuit breaker with a rated voltage of 145 kV.

Historical Special Cases

Some countries (e.g., Russia, Australia, South Africa) adopted 330 kV as their nominal system voltage during the development of their power grids. Under the IEC system, the corresponding equipment rated voltage is 363 kV. These are sovereign national choices — they are not "non-standard" and are fully supported by the IEC 363 kV equipment supply chain.

Are the Two Standards Just Different Voltage Numbers?

Absolutely not. The numerical differences are only the surface-level symptom. Behind them lie entirely different technical requirements, including:

• Insulation levels (BIL, power-frequency withstand): Different definitions and test values.
• Breaking and making capability verification: Different requirements for DC component of short-circuit current and transient recovery voltage (TRV).
• Design and testing procedures: Completely different governing standards.

Conclusion: A circuit breaker certified to IEC 145 kV cannot be directly used in a system requiring ANSI 145 kV compliance. The two are "similar in form but different in substance" — they are not interchangeable.

Edited From: Garca