Engineering calculator
CT knee-point check
Required knee-point voltage of a protection CT to avoid saturation for maximum through-fault.
Inputs
Results
- Secondary fault current
- 25A
- Required knee-point
- 255V
Vk ≥ K × (If / N) × (Rct + 2Rlead + Rrelay)
Safety-critical result — professional verification required
This is a screening estimate. It must not be used for design, equipment selection, protection settings or PPE without verification by a suitably qualified and competent engineer using a full study.
Assumptions
- 2Rlead assumes earth-fault loop; use Rlead for three-phase faults
- K depends on X/R and relay manufacturer guidance
Limitations
- Relay-specific requirements (e.g. differential, distance) override this generic check
- Remanence and transient performance (IEC 61869-2 TPX/TPY) not modelled
References: IEC 61869-2 · AS 60044.1 (legacy)
Provided for general information. Gridlinks Solutions accepts no liability for decisions made using this calculator without independent engineering verification.
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