Rated current and rated short-circuit breaking current are two core parameters of high-voltage switchgear. Many purchasers and engineers only focus on voltage level while misunderstanding these two key indicators. Undersized parameters lead to overheating and failure to clear faults; over-specification brings unnecessary extra cost.
Rated Current (In)
Rated current is the maximum continuous working current the switchgear can carry long‑term. Under specified ambient temperature, all components shall not exceed allowable temperature rise limits. It represents capability for normal load operation.
Key point: It describes normal current carrying capacity, not short-circuit fault handling.
Rated Short-circuit Breaking Current (Isc)
When short-circuit fault occurs in power system, huge fault current appears instantly. Rated short-circuit breaking current is the RMS value of maximum fault current that circuit breaker can safely interrupt.
Key point: It applies only to short-circuit faults, representing the capability to clear severe fault energy, a critical safety-related parameter.
Supplementary concept
Rated peak withstand current (dynamic stability): withstand instantaneous peak electromagnetic force during fault;
Rated short-time withstand current (thermal stability): withstand thermal burn within short-circuit duration.
These parameters are normally matched together once short-circuit breaking current is defined.
Core Comparison
1.Rated current: for normal continuous operation, focus on long-term temperature rise; Unit: A, e.g. 630A, 1250A, 2000A, 3150A.
2.Short-circuit breaking current: for instantaneous shortcircuit fault, focus on fault interruption; Unit: kA, e.g. 20kA, 25kA, 31.5kA, 40kA.
Common Selection Pitfalls
Pitfalls 1: Only consider the voltage class, ignoring the short-circuit breaking current.
Voltage class is correct but short-circuit breaking current is undersized. During real short-circuit fault, breaker cannot interrupt fault current, resulting in arc explosion and cabinet fire.
Typical case: Incoming cabinet close to substation with high fault level, people casually select 20kA while system actually requires 31.5kA.
Pitfalls 2: Directly equate the actual operating current to the rated current with no margin reserved.
Actual operating current 800A, select 800A device directly. High ambient temperature and poor cabinet cooling will cause overtemperature and contact burning.
Suggestion: Keep at least 1.25 times margin between rated current and maximum actual working current.
Pitfalls 3: Confuse the short-circuit breaking capability of load switches, fuses and circuit breakers.
Load break switch can carry rated current and switch load, but it has NO short‑circuit breaking capacity. Some customers mistakenly treat load switch as fault‑clearing device only because rated‑current data exists. Only circuit breakers can interrupt short‑circuit faults.
Pitfalls 4: Directly adopt parameters from ordinary distribution networks for new-energy power stations without calculating short-circuit current
PV and wind inverter sources produce asymmetric short‑circuit DC component. Simply adopt conventional 25kA without verifying asymmetric breaking capability may lead to interruption failure under real fault.
Pitfalls 5: Confuse parameter expressions between IEC standards and national standards in overseas projects.
For short-circuit breaking current, confirm it is RMS value. Some legacy overseas tender documents mix peak value and RMS value, leading to non-compliant delivery.
Pitfalls 6: Only increase the rated current without upgrading the short-circuit breaking current.
Some users upgrade rated current from 1250A to 2000A while keeping short‑circuit breaking current at 25kA. If system fault level rises, fault‑clearing risk still exists. Both parameters shall be calculated separately.
Quick Selection Logic
1.Rated current: calculate maximum continuous loop current, multiply by ≥1.25 safety margin, choose standard grade (630/1250/2000/3150A). Take ambient temperature and cabinet cooling into consideration.
2.Short-circuit breaking current: refer to system short-circuit calculation report, select standard kA grade (20/25/31.5/40kA) higher than maximum RMS fault current. For new-energy projects, verify asymmetric breaking performance.
3.Reminder for load break switch: only rated-current parameter applies. It has no short-circuit breaking capacity; short-circuit protection relies on fuse.
FAQ
Q1:The customer's inquiry specifies 1250A, but the short-circuit breaking current is not provided. How should we handle this?
A: 1250A is only rated current. Short-circuit breaking data is mandatory for proper selection. Ask customer for system short-circuit level; quotation without this information will carry major risk.
Q2: What additional conditions shall be considered for short-circuit breaking current under IEC standards?
A: Under IEC 62271-100, pay attention to DC component percentage, especially for inverter-based new-energy projects. Verify whether breaker performance meets DC-component requirements in tender specification.
Conclusion
Rated current governs long-term continuous load and temperature rise; short-circuit breaking current governs fault-interruption capability under short-circuit accident. One for normal operation, one for emergency fault. Major pitfalls: insufficient margin for rated current, undersized short-circuit breaking current. Load break switch cannot replace circuit breaker for fault clearing. Short-circuit calculation is critical for new-energy farms and substation incoming feeders. Both parameters are indispensable.