Why is the rated current of a high-voltage circuit breaker greater than the current-carrying capacity of a cable?

Cable's Role: Passive Bearer. A cable is like a "water pipe," and its current-carrying capacity (e.g., YJV22-8.7/15-3×120, approximately 305A in air) is the maximum safe current that will not cause insulation overheating and aging during long-term operation, based on its insulation material, laying method, ambient temperature, and other conditions. This is a "withstand limit."

"Normal Overcurrent" Situations That Must Be Considered While the current-carrying capacity of cables is a theoretical "red line," in actual operation, the system allows for some brief, harmless overcurrents. Circuit breakers should not malfunction under these conditions, otherwise, it will affect production continuity.

Transformer Inrush Current: When a transformer is closed under no-load conditions, it generates an inrush current that is more than 10 times the rated current, lasting for a very short time. The protection settings must avoid this.

Short-Term Load Fluctuations: The load of some production equipment fluctuates periodically.

If the circuit breaker's rated current is equal to or less than the cable's current-carrying capacity, the circuit breaker may trip erroneously during a normal overcurrent, leading to an unexplained power outage.

Cable heating is also a time-current accumulation process.

Coordination Requirements: Ideally, the circuit breaker's protection curve should be completely below the cable's damage curve. That is, under any overcurrent, the circuit breaker should trip and disconnect the circuit before the cable insulation is damaged by overheating.

To achieve this matching across the entire curve and allow for margin, the starting point of the circuit breaker's rated current (i.e., the threshold value for long-time delay protection) must be higher than the cable's long-term current carrying capacity. Otherwise, the circuit breaker may never trip when the overload current is small, while the cable continues to age slowly.

The cable's current carrying capacity is derived under specific standard conditions (e.g., ambient temperature 30℃, specific laying method). In actual engineering, cables may be multiple in parallel, run in conduits, or be in high-temperature environments, thus reducing their actual allowable current carrying capacity.

During the design phase, derating of the cable under harsh actual operating conditions has been considered, so a circuit breaker with a higher rated current is needed to match its actual safe current carrying capacity after derating.

Therefore, the practice of selecting high-voltage circuit breakers with a rated current exceeding the cable's current-carrying capacity represents a reasonable and necessary engineering approach. This decision is based on a comprehensive assessment of short-circuit protection requirements, equipment standardisation, system selectivity, and engineering margins. The key lies in achieving precise protection for the 'smaller' cable through correct protection settings, enabling this 'larger' circuit breaker to perform its intended function effectively.

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Shaanxi West Power Tongzhong Electrical Co., Ltd.
Contact: Ms.Grace Liu (Director of Sales Department)

Email:xdtz04@westpowerelectric.com

Tel: +86 917 3661109 Fax: +86 917 6739416

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Website:https://www.xdtzelectrical.com

Add: Nanpo Village, Chencang Avenue Jintai District Baoji City, Shaanxi Province, China.