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CT Thermal Rating Factor and Short-Time Current Withstand
CT selection is not only about ratio and accuracy. CTs must also survive thermal and mechanical stress during faults. For MV switchgear, you should verify short-time current withstand and ensure the CT is suitable for the operating temperature range and enclosure conditions.
This article explains what these ratings mean and what to specify in procurement documents.
Why CT Fault Duty Matters
During short circuits, primary current can rise to many times rated current. The CT must withstand heating during the fault and mechanical forces related to peak current.
Short-Time Withstand (Concept)
Short-time withstand is the CT’s ability to survive a specified current for a specified duration. This rating should align with the network short-circuit level at the CT location and the switchgear fault duty.
Dynamic (Peak) Withstand (Concept)
Peak current creates electromagnetic forces. CT mounting and mechanical design must withstand that stress. In switchgear assemblies, this is part of overall mechanical verification.
Environmental Assumptions
Provide realistic environment conditions:
- indoor MV room vs outdoor kiosk
- ambient temperature range
- ventilation and continuous loading duty
Procurement Checklist
- Provide: maximum fault level at CT location
- Specify: required short-time withstand and duration
- Specify: any peak withstand requirement used in your standard
- Define: installation environment and temperature assumptions
Internal Links
CT products: Current Transformers.
Switchgear where CTs are installed: Switchgear.
Conclusion
CT withstand ratings ensure CTs survive fault duty and keep protection signals dependable. Align withstand ratings with network fault level and provide realistic environment conditions so the delivered CT matches real operating stress.
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