Relay Protection Tools
CT Saturation · Overcurrent Coordination
Configuration
Inputs
IEEE C/T uses the class voltage (example C400) at 20× Is. IEC P uses ALF and rated burden. PX uses Vk from a test certificate.
Optional. One value per device, added in series to external burden.
Advanced. Tool uses the equivalent resistance of these parallel branches. In real circuits, relay currents split, and protection CTs should usually avoid parallel loads.
Label only, both use the same voltage-number concept.
Example: 5 A CT, C400 implies 400 V at 100 A secondary (20×).
Used only in IEC P mode for ALF extraction.
Typically 1 A or 5 A.
IEC P uses this for the ALF capability estimate.
Recommended for all modes.
Required for PX mode. Optional elsewhere.
Applies to IEEE class voltage or Vk.
External burden and leads
Equivalent resistive burden at rated current.
Include links, terminals, test block.
Loop uses 2× length (go and return).
If not copper, set leads to 0 and use “Extra series R”.
Lead resistance correction.
Add any other series components not listed.
Fault and transient assumptions
IEEE class envelope is defined up to 20×, tool can still compute beyond with warning.
Default is steady-state (kT=1.0). Enable this only if DC offset or transient saturation is a concern.
Manual uses the kT selector. Auto uses an exponential DC decay approximation, not a full transient CT model.
Used only when transient check is enabled and method is Manual.
Used only when transient check is enabled and method is Auto.
Auto method only.
Auto method only. Use approximate time from fault inception to the point you care about (e.g., relay operate or breaker clear).
IEEE class voltage is at the secondary terminals. Including Rct is conservative.
Separates judgment from arithmetic.
HV curve
Curve
Pickup (A)
TD 0.660
LV curve
Curve
Pickup (A)
TD 0.470
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Overcurrent Relay Curve Constants

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Below is a table of curve constants for different overcurrent relay characteristics aligned with IEEE, IEC, and SEL relay documentation.

Curve TypeABPLDescription

For more details, refer to IEEE C37.112 and IEC 60255.

Info

CT Saturation and Overcurrent Coordination Check

This is a browser-based current transformer saturation and coordination check tool. It evaluates CT burden, knee-point voltage, and saturation margin, then relates them to overcurrent protection coordination. It supports IEEE and IEC CT representations, with modes for IEEE C or T class, IEC Class P, and PX style knee point Vk.

Who it is for

Protection and system engineers, along with commissioning and testing teams who need to confirm CT sizing for protection and check saturation behavior under fault conditions.

Inputs and outputs

Typical use cases

What it does not do

It does not replace manufacturer CT test data or a detailed transient CT analysis. The results support engineering judgment rather than substitute for it.

Privacy: This tool runs entirely in your browser. Data you enter is processed locally on your computer and is not uploaded to a server.

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Frequently asked questions

What is CT saturation?
CT saturation is the condition where a current transformer can no longer reproduce primary current faithfully on its secondary, typically when the required secondary voltage exceeds the CT capability under high fault current or burden. Saturation distorts the secondary current and can affect protection relay operation.
What does this tool check?
It evaluates CT burden, knee-point voltage, and saturation margin, and relates the result to overcurrent protection coordination. It produces a saturation margin assessment from inputs such as CT ratio, burden, knee point or excitation data, and fault current.
Which CT standards and models are supported?
Both IEEE and IEC current transformer representations are supported, with modes for IEEE C or T class, IEC Class P, and PX style knee-point Vk.
Does this replace manufacturer CT data?
No. It does not replace manufacturer CT test data or a detailed transient CT analysis. The results support engineering judgment rather than substitute for it.
Is my data uploaded?
No. This tool runs entirely in your browser. Data you enter is processed locally on your computer and is not uploaded to a server.
How does it relate to overcurrent coordination?
Checking CT sizing and saturation margin under fault conditions supports overcurrent coordination by indicating whether the CT can reproduce fault current accurately enough for the relays to operate and coordinate as intended.